Developing Climate-Resilient Crops authored by Banga & Kang. Journal of Crop Improvement 28:57-87...

Journal of Crop Improvement, 28:57–87, 2014Copyright © Taylor & Francis Group, LLCISSN: 1542-7528 print/1542-7536 onlineDOI: 10.1080/15427528.2014.865410

Developing Climate-Resilient Crops

SURINDER S. BANGA1 and MANJIT S. KANG2

1Department of Plant Breeding and Genetics, Punjab Agricultural, University Ludhiana,India

2Department of Plant Pathology, Kansas State University, Manhattan, Kansas, United States

Genetic mitigation needs require a species-by-species and region-by-region analysis of projected environmental issues. This may bedone through close interactions among breeders, farmers, and cli-mate change scientists. Although adaptation is highly site specific,sets of sites may exist across the globe that face similar adaptationissues or where the current environment may represent projectedfuture environment for some other regions. Such locations maybe extensively utilized to screen available germplasm resourcesand also for raising crops continuously and naturally, developingcrop populations as germplasm conduits for global plant breed-ing efforts. Comparing adaptation experiences between each ofthe world’s geoclimatic groupings and promoting cross-learning isdesirable. Participatory breeding and participatory varietal selec-tion may help fast track the development of climate-resilient cropvarieties and cropping systems aimed at multiple breeding targets.Such efforts may further be optimized by knowledge consoli-dation relative to CO2 assimilation, nutrient dynamics, mixedcropping, vectors, pests, diseases, intercropping, water use, tem-perature responses, gene pools, and genomic resources in plants.Amalgamation of conservation genetics and genomics with breed-ing is essential to involve the whole adaptation process frombio-reserves to genes to cultivars. It is also advantageous to lookfor ways to create a balance between requirements of intellectualproperty rights, access and benefit sharing, and equitable accessfor farmers to breeding materials. Development of decision-support

This article was modified and updated from “Developing Climate-Resilient Crops:A Conceptual Framework” (Chapter 8) in Combating Climate Change: An AgriculturalPerspective, edited by Manjit S. Kang and Surinder S. Banga. Copyright 2013 by Taylor &Francis Group LLC. Reprinted with permission.

Address correspondence to Surinder S. Banga, Department of Plant Breeding andGenetics, Punjab Agricultural University, Ludhiana 141001, India. E-mail: [email protected]

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58 S. S. Banga and M. S. Kang

tools to help prioritize actionable strategies, technologies, and prac-tices and to manage trade-offs is imperative. In addition, inbiodiversity hotspots, both incentives and institutional support mustbe provided to empower indigenous farmers and conservators,particularly women. Present crop management and germplasmimprovement strategies need to be integrated with new results andbest practices from related knowledge domains. Increased invest-ments in climate change research (adaptation and mitigation) areneeded.

KEYWORDS adaptation, biodiversity, climate change, conser-vation genetics, crop improvement, germplasm, mitigation,participatory breeding

INTRODUCTION

Crop production is central to human survival, and production of increasedquantities of food is an ad infinitum necessity to sustain current and futureglobal populations. Agricultural crops provide nearly 70%–80% of caloriesand 60%–70% of all proteins consumed by human beings. Of the approxi-mately 50,000 plant species that are edible, less than 50 are used, of whichonly 15 supply 90% of the world’s food, and just 3—wheat, rice, and maize—supply 60% of human food, making the world agricultural system veryvulnerable and non-resilient. There are also pressures of increased demandfor nonfood usage of agricultural products, for example, biofuels. One of theunderlying causes of the unprecedented increases in food prices in 2008 wasbiofuels that were produced from grains (e.g., maize) and used to substitutebioenergy for fossil fuel energy to mitigate climate change. The increasedfood prices pushed the number of people going to bed hungry to more than1 billion, most of them in developing countries.

Climate change is already impacting all agricultural sectors—agriculture,forestry, and fisheries—by reducing production capabilities as well as byincreasing production risks. Most of the developing countries are highly vul-nerable to climate change because of their specific geographies, relativelylarge populations, and the predominant role that agriculture plays in theirfood security, growth, and employment generation. Millions of people inthese countries are projected to face climate-induced agricultural losses thatare unprecedented in the history of mankind (see Figure 8.1 in Banga andKang 2013).

Climate vulnerability, poverty, and food insecurity are stronglyinterlinked. Lack of access to food has sparked riots in coun-tries on every continent (http://www.time.com/time/world/-article/0,8599,1717572,00.html). Only a significantly enhanced biological adaptation

Developing Climate-Resilient Crops 59

potential of crops and livestock, and consequently improved resilience ofagroecosystems, can help improve the current agricultural production levelsand meet future food needs (Folke et al. 2004). Even present productionsystems that produce less and have variable yields are acutely vulnera-ble, less efficient, and less resilient to shocks than they normally couldbe. Several studies have examined the sensitivity of specific crops to pro-jected climatic changes to assess the vulnerability of systems based on thosecrops (http://ccafs.cgiar.org/sites/default/files/pdf/CC_for_COP15_Final_LR_2.pdf). Outcomes of some investigations are described below:

Maize in sub-Saharan Africa and Latin America: The International Centerfor Tropical Agriculture (CIAT) and the International Livestock ResearchInstitute have projected an aggregate yield decline of 10% for rainfed maizeby 2055 for smallholder farmers, which represents an annual economicloss of US$2 billion. These projected crop losses were based on climatesimulation models and data from various sources across sub-Saharan Africaand Central and South America.

Wheat in the Indo-Gangetic Plains (IGP): Wheat thrives in temperate climes,and rising temperatures may render many currently important wheat areasclimatically unsuited for its production. Studies at the International Maizeand Wheat Improvement Center have detailed possible climate shifts in thevast areas of very heavily populated IGP of south Asia, a region of 13 mil-lion hectares that extends from Pakistan across northern India, Nepal, andBangladesh. This area produces 15% of the world’s wheat. The study pro-jected that, by 2050, more than half of the IGP area might become, with asignificantly abbreviated crop season, too heat stressed to produce wheat.

Rice in the megadeltas of Asia: The Mekong and the Red River Deltas inVietnam, the Irrawaddy Delta in Myanmar, and the Ganges–BrahmaputraDelta in Bangladesh and India are vital for the world rice production.Projected impacts of climate change—increased flooding and salinity—pose a major threat to rice production in these areas. The InternationalRice Research Institute (IRRI) and partners have mapped the hydrologicalimpacts of projected sea level rise within the Mekong Delta and plan touse this information to define adaptation strategies based on improved ricecultivars and management options.

Potatoes and sweet potatoes: The International Potato Center (CIP) has mod-eled the impact of climate change on potato production with and withoutadaptation strategies. Potato yield reductions in the tropics and subtropicswere 20%–30%; these were mitigated with adaptation strategies, includ-ing stress-tolerant varieties and improved crop management. The CIP andpartners have also mapped drought-prone and high temperature-pronepotato and sweet potato cropping areas and have identified regions withhigh vulnerability to climate change effects.


Adaptation of agriculture to climate change has been broadly definedas any response that improves an outcome (Reilly and Schimmelpfennig2000). Strengthening the adaptive capacities of farmers include diversifica-tion of production systems besides more efficient use of natural resourcesin both irrigated and dryland ecologies. As in the past, plant breedingis expected to play a major role in adapting crops to climate change.The Green Revolution of the 1960s/1970s was triggered by enormous pro-ductivity increases achieved by streamlining crop architecture and farmingmethods. Redesigned crop architecture created an agricultural system thatwas highly responsive to agronomic inputs (e.g., fertilizers, water, and pes-ticides), notwithstanding huge environmental costs. The present situationrequires that superior crop performance be achieved under adverse envi-ronmental conditions. Farmers in many rice-growing areas, for instance inPunjab and Haryana (India), will have to be presented with equally produc-tive but less water-intensive crop options simply because there is not enoughwater in the subsoil aquifers to continue supporting current water-guzzlingcrops. The growing global demand for food and competition for resourcesamong different economic sectors compel future agricultural systems to bedefined relative to targeted agronomics, farming systems, crops, and culti-vars best suited to predicted environments. Improved water managementapproaches, under conservation agriculture, are central to adaptation strate-gies. Substantive benefits can accrue through enhanced synergies betweenknowledge portals from diverse scientific areas, such as agriculture, climatechange, water, energy, environment, and economics. Further downscaling ofclimate change models, and other related global models, is also necessaryfor their outputs to become more relevant to policymakers, service providers,researchers, and farmers.

ISSUES, PRESENT STATE OF KNOWLEDGE, AND GENETICMITIGATION OPTIONS

Crop Biodiversity: Threats, Adaptation, Preservation, and Utilization

Ecosystems are subject to natural selection; flora and fauna interact to createa state of near equilibrium or homeostasis. Without homeostasis, ecosystemsbecome vulnerable and fail to withstand environmental stresses. A resilientagricultural system is the backbone of ecosystem services, such as food,feed, and livelihoods (Altieri 1999; Tilman et al. 2002). With greater cli-mate variability, a range of crop and ecosystem responses may alter nutrientcycling, soil moisture, as well as distribution of pests and diseases. All ofthese can adversely impact food production and food security (Fuhrer 2003;Jones and Thornton 2003). Extinction of species may cause ecosystems tomove from a sustainable equilibrium to inherently turbulent state, therebyconstraining their ability to provide ecosystem services (Folke et al. 2004).


For example, as a result of coevolution, the life cycle stages of pollinatorinsects are in perfect consonance with crop flowering phenologies of targetcrops. Any uncoupling of insect life cycle stage and crop flowering phenol-ogy caused by alterations in annual temperature cycle can strongly impactcross-pollination (Memmott et al. 2007). Under natural ecosystems, this canhave devastating effects on adaptive potential of pollinator insect species aswell as on target host species. Vandermeer et al. (1998) emphasized the roleof diversity in agroecosystems in maintaining the functional capacity andresilience of ecosystems. Biodiversity enhances ecosystem function becausedifferent species or genotypes perform slightly different roles and thereforeoccupy different ecological niches. The most compelling evidence regardingthe importance of stress-induced natural evolutionary forces, such as poly-ploidy, in conferring adaptive advantage seems to come from the timing ofa very large number of whole genome duplication events in many taxa offlowering plants. These mostly date back to about 60–70 million years; theapproximate time of the massive climatic event of the Cretaceous–Paleogene(K–Pg) boundary (previously known as the K–T boundary). The inferenceis that polyploids might have adapted rather quickly to rapidly changing cli-matic conditions, consequently achieving higher tolerance of a wide range ofenvironmental conditions, whereas related diploids became extinct (Fawcettet al. 2009).

The Himalayan Mountains are experiencing an exceptional glacialretreat. With snowlines retreating, tree line and shrubs are moving up. Thisshift in the Himalayan ecosystem is much bigger than that in any otheralpine zones across the globe. Tibetans now grow grapes, which was previ-ously unthinkable because of severity of the Himalayan climate. Some of themost threatened, slow-growing plants, such as the endemic snow lotus in theTibetan highlands, could become extinct, whereas more common “weedy”species could take over and lessen the region’s biodiversity. Five nationalparks in South Africa were projected to lose 40% of their plant species(Rutherford et al. 2000). The same is true of Canada, where 75%–80% ofnational parks were expected to see shifts in dominant vegetation under ascenario of doubled levels of carbon dioxide (CO2) (Scott and Suffling 2000).The United States expects floristic reorganization of unimaginable magnitude(Bartlein et al. 1997). A recent study regarding the impact of rising atmo-spheric CO2 on natural ecosystems in Africa projected that, by 2100, muchof the famed savannahs in Africa will be replaced by forests (Higgins andScheiter 2012). This is because the main players, grasses and trees, differfundamentally in their response to temperature, CO2 supply, and fire andare in an unrelenting struggle for the dominance of the savannah complex.High levels of CO2 support trees. One can just imagine the impact it willhave on the natural food chain and global environment.

Crop wild relatives (CWRs) constitute reservoirs of genetic diversityaccumulated during millions of years of evolution. The CWRs are key


resources for adaptation to climate change, as they provide researchers withgenes and traits for developing plants resistant to biotic and abiotic stresses(Lane and Jarvis 2007; Maxted et al. 2008). The CWRs have historically savedagriculture millions of dollars, both directly and indirectly, by improving cropresilience to biotic and abiotic stresses (Dwivedi et al. 2008). A number ofcrops, such as sugarcane, tomato, and tobacco, would not have been grownon commercial scales without the contributions of CWRs to disease resistance(FAO 1997). The CWRs are now also expected to play a critical role in geneticmitigation processes. The irony, however, is that CWRs themselves are nowunder threat of extinction because of climate change. Based on a cross sec-tion of about 1100 wild plant species, Thomas et al. (2004) predicted that15%–37% were in danger of extinction. Using computer simulations, scien-tists from Bioversity International, CIAT, and IRRI estimated that 16%–22% ofthe wild relatives of the sampled crops, such as cowpea, peanut, and potato,could become extinct by 2055 (Jarvis et al. 2004) and that the distributionof the remainder could be reduced by more than half as a consequence ofclimate change. About half of the 51 peanut-related species studied couldbecome extinct, and the distribution of the remainder could decline bymore than 90%. Under elevated CO2 levels, CWRs produce relatively lessfruit and seed than domesticated crops (Jablonski et al. 2002), increasingtheir risk of extinction. For genetic evolution to occur, genetic diversity andgradual onset of change are prerequisites. Contrarily, wild populations arenow required to adapt fast to a variety of environmental stressors, despitethemselves suffering decline and shifts as a consequence of environmentalchange. Population declines enhance genetic erosion and genetic drift, jeop-ardizing adaptive evolution process. Small population sizes also accentuateinbreeding depression and reduced fitness (Bijlsma and Loeschcke 2012).Although pesticide resistance and heavy metal tolerance (Bishop and Cook1981; MacNair 1997) and a few other developmental traits (Bradshaw andHolzapfel 2006; Franks et al. 2007; Reusch and Wood 2007; Gienapp et al.2008) are known to evolve rather rapidly, not all species or populations showrapid adaptive genetic responses, mostly because of the absence of underly-ing mutations for resistance (MacNair 1997). The development of resistancehas been, in most cases, based on the presence of specific alleles, even priorto the actual onset of stress (McKenzie and Batterham 1998). Thus, evolu-tionary responses to climate change may have to depend considerably onpreexisting genetic variation in natural populations (Blows and Hoffmann2005; Kellerman et al. 2006; Teotónio et al. 2009). The CWRs have, untilnow, received a relatively low priority in germplasm collections becauseof financial and political impediments. Increasing threats to natural habitatsand farming systems make it imperative to collect, conserve, and characterizetraditional varieties (landraces) and wild relatives to have them available foruse in mitigating the effects of biotic and abiotic stresses caused by climatechange (Lane and Jarvis 2007). Maintenance of genetic diversity in global


gene banks and breeder fields is equally important. In situ maintenance ofagrobiodiversity may help in allowing natural selection for genetic traits thatare adapted to climate-related changes in temperature, precipitation, and/orpests.

Developing genetic responses to climate change is incumbent on effec-tive evaluation and exploitation of the existing genetic variability. Use ofmolecular markers to predict adaptive variability is rather ineffective becauselittle correlation exists between molecular genetic diversity and quantita-tive genetic variation (Gilligan et al. 2005). Also, heterozygosity at thesemarkers shows no correlation with level of inbreeding (Pemberton 2004).Availability of complete genomic sequences may help tackle the issues ofinbreeding depression, structure and extent of adaptive variation, the level oflocal adaptation, and genotype-by-environment interactions (Allendorf et al.2010; Kristensen et al. 2010; Ouborg et al. 2010). Genome sequencing ofmany CWRs is under way in several plant families, including Gramineae andBrassicaceae. These are likely to provide a conceptual toolbox for genomicresearch in conservation biology and highlight possibilities for the mechanis-tic study of functional variation, adaptation, and inbreeding (Angeloni et al.2012). Phenotypic change, in response to environmental stimuli, can be theconsequence of either plasticity (nonheritable) or heritable genetic evolution(Bonduriansky et al. 2012). There is also a need to understand the diversityof epigenetic and other transgenerational effects and their role in adaptationand maladaptation. While breeding programs have greatly enhanced pro-ductivity, they have also narrowed genetic diversity. Besides wild relatives,farmers’ fields and bio-reserves hold agrobiodiversity that also representsconstantly evolving gene pool that may already reflect species responsesto changing climate. The gene banks of the world, including ConsultativeGroup on International Agricultural Research (CGIAR), hold large numbersof genetically diverse plant collections—improved crop varieties and tradi-tional landraces and wild crop species. This gene pool should be adequatelycataloged and made available for adapting farming systems to future cli-mates. There is a need to plant sufficiently large “evolutionary” breedingpopulations of different crops at multiple sites across the globe. Such pro-grams can capitalize on recombination and natural selection to evolve andadapt. Breeders can use such breeding populations as germplasm conduits ofcontinuously evolving genetic variation. Data mining through meta-analysisof past and present multilocation trials may also help in analyzing climatesensitivity and tolerance in existing germplasm. As germplasm accessionsare sensitive to climatic variables, such as temperature and photoperiod,georeferencing of data will help elucidate the sensitivity of accessions totemperature and photoperiod. Hannah et al. (2002) have suggested a setof strategies for preserving and utilizing biodiversity with the followingoperative steps:


● Regional modeling of biodiversity response to climate change;● Systematic selection of protected areas, with climate change as an integral

selection system;● Management of biodiversity across regional landscapes, including core

protected areas and their surrounding matrix, with climate change as anexplicit management parameter;

● Mechanism to support regional coordination of management, both acrossinternational borders and across interface between park and non-parkconservation areas; and

● Provision of resources from countries with greatest resources and greatestrole in generating climate change to those where climate change effectsand biodiversity are highest.

That variability is the essence of plant breeding has remained unchangedever since hunter-gatherers first began to domesticate plants for food thou-sands of years ago. With excessive dependence on domesticated plants andanimals, however, vast numbers of other species and ecosystems have beenlost. In the context of climate change, crop breeding may, in fact, also includeredomestication with altered trait emphasis across a much shorter timescale.Present domesticators—the professional plant breeders—have to deal withthe constraints of reduced biodiversity and growing population besides chal-lenges of constantly developing new varieties, declining soil health, climateuncertainty, and changing spectrum of pests and diseases. But today’s plantbreeders also have the advantage of having the very best and latest set of sci-entific tools made available by the sciences of genetics and genomics. Muchcan be learned from species diversity in agriculture and forestry managementsystems of indigenous people in diversity hotspots of the world. Irrespectiveof the technical advances, plant breeding will continue to depend upongenetic resources that nature and evolution have provided. Also needed arepolicy paradigms that create a balance among legitimate ownership rights ofgene-rich geographies, germplasm access, requirements of intellectual prop-erty rights, benefit sharing, and a fair access for farmers to enhanced breedingmaterials.

Improving Plant Tolerance to Heat, Drought, and Submergence

The agricultural sector alone uses 70%–80% of the available freshwater sup-plies of the world; this figure may go up as temperatures rise further (Brookesand Barfoot 2008). In addition, almost 10.12 million hectares of land ispractically lost each year because of salinity caused by unsustainable irri-gation techniques (Ruane et al. 2008). This would translate into 30% landloss within the next 20 years or so and up to 50% land loss by 2050(Wang et al. 2003; Valliyodan et al. 2006). As per the Palmer Drought


Severity Index (PDSI), global area affected by drought has increased fromapproximately 5%–10% during the 1960s (Palmer 1965) to 15%–25% during2008 (Li et al. 2009). There has also been a corresponding increase in meanPDSI (Parry et al. 2007). The affected geographies include bulk of area sownto major crops, such as barley, maize, rice, sorghum, soybean, and wheat.Anthropogenic increases in greenhouse gas and aerosol concentrations havemade a detectable contribution to the observed drying trend as reflected bythe PDSI (Burke et al. 2006). Managing water scarcity is a major concern.Unsustainable water demand is driven in part by underpricing of water,together with subsidies for groundwater pumping to raise crops, such asrice and sugarcane, in ecologically fragile areas of the world. Climate changewill further exacerbate water scarcity through increased evapotranspiration,decreased groundwater recharge, loss of glacial water storage, and salinewater intrusion.

Another negative impact of climate change on agriculture is theincreased exposure of crop plants to heat stress. During 1972, abnormallyhigh summer temperatures in the former USSR contributed to widespreaddisruptions in the world cereal markets and caused food insecurity (Battistiand Naylor 2009). The impact of high temperatures on final yield is gener-ally dependent on the crop growth stage. Wollenweber et al. (2003) foundthat the plants experienced warming periods as independent events andthat critical temperature of 35.8◦C for a short period around anthesis hadsevere yield-reducing effects. However, high temperatures during the veg-etative stage may not have significant effects on growth and development.Changes in short-term temperature extremes can be critical, especially if theycoincide with key stages of plant development. Only a few days of extremetemperature at the flowering stage of most crops can reduce yield drastically(Wheeler et al. 2000). Temperatures as low as 25◦C can reduce grain-fillingperiod in wheat, after which a 1◦C temperature rise shortens the repro-ductive phase by 6% and shortens the grain-filling duration by about 5%;grain yield and harvest index are also reduced proportionately (Lawlor andMitchell 2000). Temperature thresholds for crop plants are well defined andhighly conserved between species, especially for processes such as anthesisand grain filling (Porter and Gawith 1999; Wheeler et al. 2000). Heat stressimpacts key crop metabolic and developmental processes, such as pollina-tion, grain filling, and approach to maturity, which are adversely affectedafter temperature-tolerance thresholds are exceeded.

Crop physiological processes related to growth, such as photosynthesisand respiration, show continuous and nonlinear responses to temperature,whereas rates of crop development often show a linear response to tem-perature to a certain level. In the short run, high temperatures can affectenzymatic reactions and gene expression. From a longer-term perspective,these may reduce carbon assimilation, growth rates, and eventually yield.Although ribulose-1,5-bisphosphate carboxylase–oxygenase (RuBisCo) itself


is quite thermostable, carbon assimilation reactions catalyzed by RuBisCoare directly affected by temperature and the rate of carboxylation contin-ues to increase beyond 50◦C. Decreased discrimination by RuBisCo for itsalternative substrate oxygen and increased solubility of oxygen relative toCO2 with rising temperature inhibit net photosynthesis in C3 plants becauseof increased photorespiration (Ainsworth and Orat 2010). Maize exhibitsreduced pollen viability at temperatures above 36◦C. Rice grain sterility maybe caused by temperatures in the mid-30◦C and similar temperatures canreverse vernalization effects of cold temperatures in wheat.

Agricultural production is also adversely affected by other climate-induced weather extremes, such as flooding and submergence (Parry et al.2007). Excess water in the soil reduces oxygen availability to the plant(Kozlowski 1984). The extended deep submersion can cause plant deathbecause of 1) a lack of oxygen that is required for energy production tosustain plant growth and 2) an accumulation of toxic substances, such asorganic acids, NO2

−, Mn2+, Fe2+, and H2S (Kozlowski 1984; Janiesch 1991).Thus, development of more flood-tolerant cultivars is critical for enhanc-ing sustainable production of crops. Farmers’ livelihoods depend on theircrops producing higher yields in good years and providing a high degreeof stability under adverse environments. Vigorous root systems and reducedevapotranspiration developed through breeding, for example, allow plants tocapture and save scarce water. The past decade has witnessed an increase instudies related to detection of quantitative trait loci (QTL) for drought-relatedtraits, and the first encouraging results in QTL cloning have been reported inSalvi and Tuberosa (2005). The QTL cloning has enabled better understand-ing and effective manipulation of the traits influencing drought tolerance(Tuberosa and Salvi 2006). With the availability of whole genome sequencesof plants, physical maps, and genetic and functional genomics tools, inte-grated approaches using molecular breeding and genetic engineering offernew opportunities for improving stress resistance (Apse and Blumwald 2002;Ruane et al. 2008; Manavalan et al. 2009). Although genetic variation existsfor the plants’ ability to cope with temperatures that are significantly higherthan current adaptation regimes of most crops, it may still be necessaryto identify genes that condition adaptation to much higher levels of abi-otic stresses. This may include extensive gene expression assays to identifygenes in both cultivated varieties and CWRs that are important for survivalin different climatic conditions. The dehydration-responsive element-bindingproteins (DREBs) or C-repeat-binding proteins (CBFs) are among the firstfamilies of transcriptional regulators that are transcriptionally upregulatedby water deficit or low temperature (Morran et al. 2011). The expressionof individual DREBs/CBFs may be regulated by drought, salt, and cold, orby drought and cold only. Most published evaluations of transgenic plantsfor drought tolerance considered survival rather than final productivity orgrain yield. The drought protection offered by overexpression of TaDREB2


or TaDREB3 is typical of the DREBs/CBFs and is largely related to survivaland recovery from severe drought stress. Molecular control mechanisms forabiotic stress tolerance are based on activation and regulation of specificstress-related genes. Hundreds of genes and their products respond to thesestresses at transcriptional and translational levels (Cushman and Bohnert2000; Sreenivasulu et al. 2004; Yamaguchi-Shinozaki and Shinozaki 2005;Umezawa et al. 2006).

Alternative strategies for engineering drought tolerance also includeincreasing the depth or vigor of the root system to facilitate water acquisitionand increase water-use efficiency (WUE). Although adaptation to stress undernatural conditions has some ecological advantages, the metabolic and energycosts may far exceed benefit to agriculture (Wang et al. 2001; Apse andBlumwald 2002). Transgenic plants can be engineered on the basis of differ-ent stress mechanisms: metabolism, regulatory controls, ion transport, antiox-idants and detoxification, late embryogenesis abundant proteins, heat-shockprocesses, and heat-shock proteins (Wang et al. 2001 2003). Salt-tolerantplants also often tolerate other stresses, including chilling, freezing, heat, anddrought (Zhu 2001). A number of abiotic stress-tolerant, high-performance,genetically modified (GM) plants have been developed in tobacco (Honget al. 2000); Arabidopsis thaliana and Brassica napus (Jaglo et al. 2001);tomato (Zhang and Blumwaldt 2001; Hsieh et al. 2002); rice (Yamanouchiet al. 2002); and maize, cotton, wheat, and oilseed rape (Yamaguchi andBlumwals 2005; Brookes and Barfoot 2006). Plants may also be engineeredto reduce the levels of poly(ADP-ribose) polymerase, a key stress-relatedenzyme. Resultant plants were able to survive drought better and showeda 44% increase in yield (Brookes and Barfoot 2008). Another technology,involving the use of “genetic switches” (transcription factors and stress genes)from microbial sources, is currently being researched at the United KingdomAgricultural Biotechnology Council (http://www.-abcinformation.org). Thistechnology has been tested and has resulted in a twofold increase in produc-tivity of Arabidopsis and 30% increase in yield of maize during severe waterstress. Also of interest is the discovery that RuBisCo activase is associatedwith chloroplast GroEL homolog (cpn60b), which suggests that this proteinmay be acting in the way of other hsp60s, providing a mechanism to pro-tect RuBisCo activase and acclimate photosynthesis to heat stress (Salvucci2008). Improvements to the thermal stability of RuBisCo activase achieved bygene shuffling increased rates of photosynthesis and growth and enhancedyield in genetically transformed Arabidopsis exposed to moderate heat stress(Kurek et al. 2007). In addition to the temperature sensitivity of RuBisCoactivase, ionic conductance of thylakoid membrane decreased at leaf tem-peratures as low as 36◦C in cotton (Gossypium hirsutum), which in turnlowered photosynthesis, even though photosystem I cyclic electron trans-port was increased, compensating, in part, for the loss of proton motiveforce needed for adenosine-5′-triphosphate formation (Schrader et al. 2004).


A promising path to improve high-temperature tolerance has been indicatedby a “photorespiratory bypass” engineered in Arabidopsis chloroplasts bythe introduction of Escherichia coli glycolate catabolic pathway that sub-stantially suppresses photorespiratory flux (Kebeish et al. 2007; Maurino andPeterhansel 2010). This photorespiratory bypass in turn would be expectedto increase the temperature optimum of net photosynthesis in transformedplants.

The specific aim must be to identify genes that control the plants’ abilityto tolerate stress and to develop biotechnology-oriented selection and breed-ing procedures (functional analysis, marker probes, and transformation withspecific genes). Further improvement and adaptation of current agriculturalpractices may also be needed (Wang et al. 2003). The use of biotechnologyto improve crops genetically has not yet been widely accepted, but the pres-sures on the world’s agricultural systems may hasten its acceptance, with allthe appropriate biosafety precautions in place.

Specific Tolerance or Resistance: Pests, Diseases, and Nutrient Stress

Rising temperature and atmospheric CO2 are also indirectly affecting cropsthrough their effects on pests and diseases. These interactions are com-plex, and their full impact on crop yield is yet to be fully appreciated.Impacts of warming or drought on resistance of crops to specific diseasesmay be through the increased pathogenicity of organisms or by mutationsinduced by environmental stresses (Gregory et al. 2009). The influence ofclimate change on plant pathogens and their consequent diseases has beenreviewed extensively (Coakley 1995; Manning and Tidemann 1995; Coakleyand Scherm 1996; Chakraborty et al. 1998, 2002; Chakraborty 2005; Eladand Pertot 2013). Different individual parameters associated with climatechange, such as warming, increased levels of CO2, decreased rainfall, anderratic pattern of rainfall, have been studied for their influence on differentaspects of pathogens and diseases across various crops (Chakraborty 2005).These include pathogen life cycle (e.g., Chakraborty et al. 2000a; Chakrabortyand Datta 2003), expression of host resistance (e.g., Hartley et al. 2000;Chakraborty and Datta 2003), disease epidemiology and severity of diseaseepidemics (e.g., Pangga et al. 2004; Evans et al. 2008; Butterworth et al. 2010),and pathogen inoculum production (e.g., Chakraborty et al. 2000b). Changesassociated with climate change have direct effects not only on the ability ofthe crop (host) to yield and on the behavior of the pathogens but also onthe pathogen–host interactions at all levels (Newton et al. 2011; Eastburnet al. 2011). Chakraborty et al. (2000b) reviewed the effects of increasingCO2 levels on plant disease. For biotrophic fungi, an increase in diseaseseverity for 6–10 biotrophic fungi studied and a decrease for the other 4 fungiwere observed. Out of the 15 necrotrophic fungi investigated, 9 exhib-ited an increase in disease severity, 4 showed a decrease, and 2 remained


unchanged. This suggested that predicting effects of climate change onunstudied pathosystems would be challenging. In contrast, mechanisms ofthe effects of elevated CO2 concentration on plants are fairly well under-stood and will have effects on plant diseases. For example, reduced stomatalopening and changes in leaf chemistry are expected to reduce the inci-dence of diseases caused by pathogens that infect through stomata, forexample, Phyllosticta minima (Mcelrone et al. 2005). Von Tiedemann andFirsching (2000) predicted that benefits from elevated CO2 counterbalancednegative effects from ozone but would not compensate for the effects offungal infection. The range of many pathogens is limited by climate require-ments for overwintering or oversummering of the pathogen or vector. Highwinter temperatures of −6◦C versus −10◦C increased survival of overwin-tering rust fungi (Puccinia graminis) and increased subsequent disease onFestuca and Lolium (Pfender and Vollmer 1999). Increasing levels of atmo-spheric CO2 reduced ability of soybeans to withstand infestation of beanleaf beetle, resulting in increased pest population pressure (Zavala et al.2008). Generation turnover in a given season of pathogen reproductiondetermines the rate at which pathogens evolve and temperature governsthe rate of reproduction for many pathogens; for example, the root rotpathogen reproduces more quickly at relatively high temperatures (Waughet al. 2003). Longer growing seasons (especially in higher latitudes), resultingfrom higher temperatures, would allow more time for pathogens to evolve.Increased overwintering rates at higher temperatures would also substantiallyenhance pathogen populations. Climate change might also influence whetheror not pathogen populations reproduce sexually or asexually. In some cases,altered temperatures may favor overwintering of sexual propagules, thusincreasing their evolutionary potential, but most pathogens will have theadvantage over plants because of their shorter generation times and, inmany cases, the ability to spread readily. As an example, during the next10–20 years, diseases affecting oilseed rape could increase in severity withinits existing range as well as spread to more northern regions where at presentthese are not observed (Evans et al. 2008). Changes in climate variability mayalso be significant, affecting the predictability and amplitude of outbreaks.Integrated pest, disease, and weed management becomes even more impor-tant as climate change alters patterns of pests and diseases. Threats includenew and emerging pests and diseases and the spread of existing ones tonew regions because of climate change. Initial results have shown that pests,such as aphids (Newman 2004) and weevil larvae (Staley and Johnson 2008),responded positively to elevated CO2. Increased temperatures also reducedthe overwintering mortality of aphids, enabling their earlier and potentiallymore widespread dispersion (Zhou et al. 1995). In sub-Saharan Africa, migra-tion patterns of locusts are influenced by rainfall patterns (Cheke and Tratalos2007), thus the potential exists for climate change to shape the impacts ofthis devastating pest.


Conventional breeding initiatives have contributed significantly to cropadaptations through the development of strains that are resistant to bioticstresses, such as insects, fungi, bacteria, and viruses (Vallad and Goodman2004; Bianchi et al. 2006). Transgenic Bt crops are proving to be valuabletools for integrated pest management programs by giving farmers new pestcontrol choices. Also, GM cassava, potatoes, bananas, and other crops car-rying resistance to fungi, bacteria, and viruses are in the pipeline; somehave already been commercialized while others are undergoing field tri-als (Mneney et al. 2001; Van Camp 2005; Clive 2011). Studies carried outbetween 2002 and 2005 determined that biotic stress-tolerant GM cropsaccounted for increases in average yield of 11%–12% for canola and maizecompared with conventional crops (Qaim and Zilberman 2003; Gomez-Barbero et al. 2008; Brookes and Barfoot 2008 2009). The significantlyincreased pest population pressure, their rapid evolution, and their abil-ity to move, combined with possible impact of climate change on theexpression of current resistance gene(s), will require a coordinated breedingresponse to source genes from wild relatives of crops and microorganisms.Microorganisms might be a better source of resistance gene(s) because oftheir faster rate of evolution as compared with plants. Transgenic/cisgenicbreeding approaches may be used increasingly to combat stresses thatrequire genetic responses for multitude of stresses at the same time.

Enhanced CO2 Fertilization and Nutrient- and Water-Use Efficiency ofCrop Plants

Between 1958 and 2008, atmospheric concentration of CO2 increased from316 to 385 mg/kg, and continued increases in CO2 concentration areexpected to significantly affect long-term climate change, including vari-ations in agricultural yields (Kirkham 2011). Increased atmospheric CO2,coupled with water and temperature constraints, is likely to impact WUEand nutrient-use efficiency of crops, pastures, and forests; nutrient recom-mendations for a changed climate are also not likely to operate on thesame premise as current recommendations. Changed metabolic responsesof plants resulting from environmental stresses may have to be factoredin. Understanding of metabolic processes associated with such changes isexpected to help with formulation of adaptation strategies under both rain-fed and irrigated agroecosystems. Improved nutrient-use efficiency in plantscan assist with mitigation through reduced carbon intensity of agriculturaloperations. Because nutrient uptake and utilization are genetically con-trolled, a major requirement would be to identify genes/alleles that regulatenutrient-use efficiency.

Projections show that by including increased CO2 fertilization, all butthe very driest regions would show increases in productivity. Global-scalecomparisons of the impacts of CO2 fertilization with those of changes in


mean climate reveal that the strength of CO2 fertilization effects is a criticalfactor for projecting global-scale crop yields (Parry et al. 2004; Nelson et al.2009). North America and Europe may benefit from increased CO2 fertiliza-tion effects, at least in the short run. Other regions, such as Africa and India,are nevertheless projected to experience up to 5% losses in food grain pro-duction by 2050, even with strong CO2 fertilization. These losses increaseup to 30% if the effects of CO2 fertilization are omitted. In fact, withoutCO2 fertilization, all regions are projected to experience a loss in productiv-ity by 2050 because of climate change. However, existing global-scale studies(Parry et al. 2004; Nelson et al. 2009) are based only on a limited sample ofavailable climate model projections.

Higher atmospheric concentrations of CO2 are normally thought toincrease leaf and canopy photosynthesis, especially in C3 plants, with littlealteration in dark respiration. Additional CO2 may increase biomass withoutsignificantly altering dry matter partitioning, but it will reduce transpirationof most plants and improve WUE (Field et al. 1995). However, their overallimpacts on crop productivity and water use will vary as nutrient acquisi-tion is closely associated with overall biomass and strongly influenced byroot surface area. Modest, crop-specific benefits in productivity may be pos-sible only where nutrient availability can be optimized and where climatechange increases temperatures to a species-specific optimum and changesprecipitation patterns to reduce water stress (drought or flooding) days.

The C3 species may also accrue a direct benefit from CO2 fertiliza-tion under such conditions. Plant availability of nutrients in the soil alsodepends on soil chemical properties, nutrient availability, and location ofnutrient ions relative to the root surface and the length or distance the nutri-ent must “travel” in the soil to reach the root surface (Jungk 2002). Soilmoisture and temperature are primary determinants of nutrient availabil-ity and root growth and development. Process outcomes are expected tobe dependent on changed climate. There are also suggestions that climatechange impacts on nutrient-use efficiency will be primarily accomplishedthrough direct effects on root surface area (Itoh and Barber 1983). In theabsence of roots, steady-state solution-phase concentrations of nutrient ionsare controlled by adsorption–precipitation and desorption–dissolution reac-tions between nutrients and the surface complex of soil, mineralization andimmobilization for solutes of organic origin, and additions from fertilizer.Given the importance of C and N cycling to both agricultural productivityand sustainability, root–nutrient interface and plant growth are expected toalter total nutrient needs.

Modifications in regional nutrient requirements of prevalent croppingsystems will be severe if these include shifts in ecozones or alter farmingsystems to capture new uses from existing systems. Therefore, any assess-ment of the impacts of CO2-induced climate change on crop productivityshould be tempered with other associated physiological factors (Brouder


and Volenec 2008). The CO2 physiological response varies with speciesand photosynthesis system (C3 or C4). The difference between these twosystems depends on the level of CO2 saturation of RuBisCo in plant cells.Under prevailing CO2 levels, RuBisCo in C3 plants is not CO2 saturated, sohigher CO2 concentrations increase the possibility of net uptake of carbonand thus promote growth. Experiments under ideal conditions have shownthat a doubling of atmospheric CO2 concentration increased photosynthe-sis by 30%–50% in C3 plant species and 10%–25% in C4 species (Ainsworthand Long 2005). As crop yield increase is lower than the photosyntheticresponse, increases in atmospheric CO2 to 550 mg/kg would, on average,increase C3 crop yields by 10%–20% and C4 crop yields by 0%–10% (Gifford2004; Long et al. 2004; Ainsworth and Long 2005). There are also sugges-tions that crop response to elevated CO2 may be lower than that previouslythought (Long et al. 2004 2009). Plant physiologists and modelers are nowof the opinion that the effects of elevated CO2, as measured in experimen-tal settings and subsequently implemented in models (Tubiello et al. 2007),may overestimate actual field- and farm-level responses. This is because theeffects of many limiting factors, such as pests and weeds, nutrients, com-petition for resources, soil water, and air quality, have not been adequatelytaken into account in the major models. In C4 plants, such as maize, CO2 con-centration is three to six times greater than the atmospheric concentrationsof CO2; thus, RuBisCo is already saturated (von Caemmerer and Furbank2003). Most plant physiologists recognize that both single-leaf and canopyphotosynthesis of C3 plants will increase more than that of C4 plants, asatmospheric CO2 concentrations increase. This is, in part, because of com-petitive inhibition of photorespiration in C3 plants, a process that does notimpact photosynthesis of C4 plants.

Crops may, in general, become more water-use efficient at elevatedCO2 concentrations, as stomata do not need to stay open for longer peri-ods to enable plants to receive the required CO2. As a consequence, theremay be only marginal yield appreciations (Long et al. 2004). Despite thepotential positive effects on yield, elevated CO2 may be detrimental to yieldquality of certain crops. For example, elevated CO2 is detrimental to wheatflour quality through reductions in protein content (Sinclair et al. 2000).Elevated atmospheric CO2 may even reduce plant growth when combinedwith other likely consequences of climate change, namely, elevated tem-peratures, increased precipitation, or increased nitrogen deposits in the soil.The interactive effects of soil moisture and nutrient availability are two keyedaphic factors that determine crop yield (Ziska and Bunce 2007). It is alsowell known that agricultural crops require large quantities of nitrate-rich fer-tilizer to realize optimal yields. Necessity of using higher N fertilizers has tobe balanced with associated increased carbon intensity and also to minimizeavailability of potentially harmful nitrates that can leach into groundwa-ter and surface water. Elevated CO2 also reduces stomatal conductance


in many species to 77%–86% of values found under ambient CO2 con-ditions (Bunce 1995). For cotton, however, both transpiration and growthincreased at high CO2 concentrations. Transpiration of maize was reducedat high CO2 and these plants exhibited only a modest increase in plantbiomass (Samarakoon and Gifford 1995). Wheat transpiration was not con-sistently influenced by high CO2 even though plant growth was much higher.Regardless of stomatal response, WUE of all species was greater at elevatedCO2 and total water use was reduced when compared with ambient CO2.Such shifts in water use might alter mass flow of nutrients to the root sur-face, change soil moisture patterns, and increase foliage temperatures thatcould reduce photosynthesis. Chartzoulakis and Psarras (2005) suggestedthat, although high CO2 might improve plant WUE, reductions in precip-itation and increases in evapotranspiration reduced soil moisture in someparts of southern Europe. They predicted that this would reduce photosyn-thesis and alter soil fertility, including soil organic matter decomposition andnitrate leaching. However, Manderscheid and Weigel (2007) showed that theeffect of drought was negated somewhat by elevated CO2 (550 mmol/mol).When compared with ambient CO2 conditions, high CO2 increased WUE by20% under well-watered conditions, but WUE increased by 42% in responseto high CO2 under drought conditions. Apparently, the negative effects ofclimate change-induced drought would be mitigated by high CO2. Clearly,elevated CO2 may also result in site-specific changes in water availability, butincreases in WUE and reductions in total water use are expected to influencekey plant metabolic processes.

During their long evolutionary history, plants have evolved differentmechanisms of adaptation, for example, changes in ion transporter expres-sion and activity (Ashley et al. 2006; Jung et al. 2009), increase in root growthto explore more soil volume (Jordan-Meille and Pellerin 2008), or root zoneacidification of the surrounding soil for mineral nutrient mobilization (Ryanet al. 2001). Despite these adaptations, the mechanisms involved in sens-ing and signaling low mineral nutrient status are still not fully understood,although some progress has been made (Doerner 2008; Jung et al. 2009;Wang and Wu 2010).

Quantitative trait loci have been detected for growth-related traits, ioncontent, biochemical traits, and their responses to reduced supplies ofpotassium or phosphate in Arabidopsis (Prinzenberg et al. 2010). Improvednutrient uptake through transgenic manipulation of transports has yet to berealized. Low-input cultures may be used to identify traits and genotypesassociated with high nutrient-use efficiency. These traits may include greaterroot biomass, higher root surface area, longer/denser root hairs, more adven-titious roots, smaller root diameter and shallower basal roots in surface soils,more dispersed laterals, and enhanced exudation (Lynch 2007). Breeding forroot traits has just begun, but current availability of rapid root zone screeningtechniques may facilitate rapid screening of global germplasm resources.


REVISITING CROPS AND CROPPING SYSTEMS, REINTRODUCTIONOF HARDY CROP SPECIES TO COMMERCIAL FARMING

Post-Green Revolution, significantly higher productivity, abundant andcheap supplies of nitrogenous fertilizers, and subsidized irrigation facili-ties helped wheat and rice occupy niches that were previously occupiedby environmentally hardy crops, such as sorghum, pearl millet, prosomillet, legumes, and oilseeds. Cropping systems were developed aroundthese highly input-intensive crops at the cost of environment and long-term sustainability. Rice cultivation in northwest India is a prime examplewhere rice, with a high water requirement, replaced more temperature-and drought-hardy millets, legumes, and oilseeds. Similar instances areavailable across the globe where sustainability was disregarded or sacri-ficed in favor of productivity and food security. The major gross virtualwater exporters (i.e., export of agricultural commodities), which togetheraccount for more than half of the global virtual water export, include theUnited States (314 Gm3/year), China (143 Gm3/year), India (125 Gm3/year),Brazil (112 Gm3/year), Argentina (98 Gm3/year), Canada (91 Gm3/year),Australia (89 Gm3/year), Indonesia (72 Gm3/year), France (65 Gm3/year),and Germany (64 Gm3/year) (Hoekstra and Mekonnen 2012). Of these, theUnited States, Pakistan, India, Australia, Uzbekistan, China, and Turkey arethe largest blue virtual water exporters, accounting for 49% of the globalblue virtual water export. All of these countries are partially under waterstress (see Figure 8.2 in Banga and Kang 2013). At some stage, countrieswith limited natural blue water resources will have to reconsider exportof water-intensive agricultural products as these are neither sustainable norwater-use efficient. It will be necessary to factor in water scarcity in the priceof water for agricultural usage as well as WUE of crops being cultivated andcropping systems being followed.

Crop improvement efforts can only hasten the evolutionary and adaptiveprocesses. Reilly et al. (2003) analyzed the geographic centers of productionfor maize, soybean, and wheat over the past 100 years. They found a sig-nificant north and westward shift in centroids for both maize and soybeanproduction, and this shift was accompanied by a 4◦C decrease in temperaturedespite an estimated U.S. warming trend of 0.6◦C. This shift reflects man-agement and genetic technologies, including development of new varietiesof soybean that are adapted to longer photoperiods and earlier maturingmaize hybrids with decreased risk of early frost. Reilly et al. (2003) con-cluded that in the past 100 years, adaptation to the magnitude of temperaturechange expected for the coming century had been seen, albeit, in the oppo-site direction. Cook et al. (1998) studied long-term (>100 years) effects onhigh concentration of CO2 on Nardus stricta plants growing near a natu-ral CO2 spring (concentration of CO2≈ 790 µmol/mol) by comparing withplants growing nearby in an ambient CO2 environment (≈360 µmol/mol).


The plants selected in and adapted to high concentration CO2 exhibited ear-lier senescence and reductions in photosynthetic capacity (≈25%), RuBisCocontent (≈26%), RuBisCo activity (≈40%), RuBisCo activation state (≈23%),chlorophyll content (≈33%), and leaf area index (≈22%) compared withplants growing away from the spring.

It is now important to revisit crops and cropping systems across theglobe with the aim of achieving WUE at both regional and global levels.There is also a need to develop farming system-specific varieties that fit in afarming system context and are freely accessible to farmers. There is also aneed for large-scale reintroduction of hardy crop species, such as sorghumand millets, to commercial farming in water-scarce countries in Asia, LatinAmerica, and Africa. Of special interest are the adaptation techniques, such asno-till agriculture and bed planting. By leaving at least 30% of residue on thesoil surface, no-till agriculture reduces loss of CO2 from agricultural systemsand also plays a role in reducing water loss through evaporation, increasessoil stability, and creates a cooler soil microclimate. Conservation agriculturalpractices that help prevent soil erosion may also sequester soil carbon andenhance methane consumption (West and Post 2002; Johnson et al. 2007).Powlson et al. (2011) have suggested that climate change benefit of increasedsoil organic carbon from enhanced crop growth must be balanced againstgreenhouse gas emissions from production and use of nitrogenous fertilizersrequired for higher growth trajectories. In modern agricultural practices, GMRoundup Ready (herbicide-resistant) soybean technology has accounted forup to 95% of no-till area in the United States and Argentina and led to seques-tration of 63,859 million tons of CO2 (Fawcett and Towery 2003; Brimneret al. 2004; Kleter et al. 2008). Transgenic canola (oilseed rape) and soybeanhave been modified to be resistant to specific herbicides (May et al. 2005;Bonny 2008). These GM crops should allow farmers to adopt no-till farm-ing practices. Such climate change mitigation practices enhance soil qualityand retain carbon in the soil (Brookes and Barfoot 2008). Strategy of carbonsequestration by switching from annual crops to perennials or by introducingmore organic amendments (and doing this more effectively) runs counterto the current trends of going from grassland to cropland and the grow-ing demand for biomass/bioenergy. Perennial crops for renewable materialand biomass could sequester additional soil carbon. Developing perennialgrasses, legumes for hot/dry and low rainfall conditions, with greater toler-ance to overgrazing and uncontrolled grazing, is also gaining momentum asa component of mitigation and adaptation strategies.

Water-use efficiency needs to be achieved at both regional and globallevels. Crops differ widely in their WUE; some crops have low (need>2000 m3/ton), moderate (need >750 m3/ton), and high efficiency (need<500 m3/ton). As indicated earlier, farming system–specific varieties that fitin a farming system and are freely accessible to farmers are needed. Hardycrop species, such as sorghum and millets, need to be reintroduced for


commercial farming in water-scarce countries in Asia, Latin America, andAfrica. In Australia, a collection of greater lotus (Lotus uliginosus) breed-ing lines is being adapted to low latitudes and possibly with improvedheat tolerance, which could push greater lotus (with its ability to toler-ate waterlogging/acidity) further north and out of its current niche in NewSouth Wales. Similarly, birdsfoot trefoil (Lotus corniculatus) is being evalu-ated as a new perennial legume adapted to low-fertility acidic conditions.There is known genotypic diversity in birdsfoot trefoil to extend its usagethrough breeding into high altitude and low rainfall dryland grazing andcropping environments. Similarly, wild and related species of Medicagosativa (lucerne) are being evaluated because of their significantly higher tol-erance to grazing, ability to spread by rhizomes, and, in some cases, greatertolerance to salinity and waterlogging. These plants will potentially providea perennial legume for areas where lucerne will not persist because of over-grazing or uncontrolled grazing and/or salinity or drought. Other species ofperennial legumes of very early maturity that would be good at adapting tohotter/drier conditions are Yelbeni yellow serradella, Toreador hybrid medic(a hybrid between strand and disc medic), and Trigonella balansae, whichis about to be released. All of these species flower between 70 and 85 days.In addition, burgundy bean, a summer perennial legume, can flower and setseed in as little as 60 days under moisture stress, but under better condi-tions, it will not flower until 90 days. Such efforts need to be repeated withadditional species across the globe.

GENE DISCOVERY, GERMPLASM ENHANCEMENT, ANDGENOME-BASED APPROACHES TO IMPROVE BREEDING

EFFICIENCY

Development of new, more climate-resilient crop varieties, as always, willbe critical to agricultural adaptation. Because varietal development is a long-term endeavor, region-by-region prediction of future climatic conditionsand trait-by-trait cataloging of global crop genetic resources are extremelyimportant. Crop breeders need to pursue specific targets rather than movingtargets.

Use of genome-wide association studies, integrating genome sequenc-ing, single nucleotide polymorphism (SNP) analysis, epigenetic analysis, andtranscriptomics may help identify gene sources for developing plants adaptedto emerging climate change stresses. Such efforts may also include mas-sive genome sequencing, particularly in CWRs. Such sequencing data shouldallow selection of specific SNPs as candidates for adaptation to environmen-tal stresses. Identification of genes and alleles for improving nutrient-uptakeand nutrient-use efficiencies is also critical for reducing carbon footprint ofagriculture. Novel alleles are likely to be available in CWRs and landraces,


where the local environment has affected selection for tolerance to environ-mental extremities. New knowledge and resources should be freely availableto breeders. By screening populations with a large number of SNP markers,especially using SNP chips, signals of selection can be identified. Variation insuch markers will provide insights into functional rather than neutral geneticvariation. Next-generation sequencing should facilitate the study of geneexpression. Also, the epigenetic and transcriptome analyses will need tobe used to achieve a far more detailed gene mapping than currently pos-sible. Because such studies map heat-sensitive and nutrient utilization traitsin the genome, the data generated will directly affect gene models withinthe mapped regions. Specific information about SNPs, genes, genetic path-ways, and biomarkers can be used in genetic selection for better adaptationof commercial genotypes to efficient production under stress conditions.Conservation genetics needs to be integrated with modern techniques fora more representative estimation of genetic variation within and betweenpopulations and establishing distinction between neutral and non-neutralmarkers. Conservation genomics will increase our insight into how the envi-ronment and genes interact to affect phenotype and fitness (Angeloni et al.2012).

Globally coordinated breeding strategies and priorities are required toensure that products of current breeding programs remain relevant to currentand future climatic challenges. A variety of release systems must encouragethe maintenance of biodiversity (genetic diversity), including promoting freeexchange of seed among farmers and rapid supplies of fresh replacementseeds after environmental disasters, a case for the establishment of inter-national and regional seed banks. Climate change also warrants change ininternational policies for providing access to more genetic resource mate-rials, capitalizing on interdependence brought about by shifts in climatezones globally. It may also be possible to designate areas across the globewhere the current environment may mimic projected future environmentsfor some other regions. Such locations may be extensively used to screenavailable germplasm resources and also for raising crops continuously andnaturally, developing crop populations as germplasm conduits at the globallevel. Participatory plant breeding in diversity hotspots for effectively cap-italizing on centuries-old knowledge of indigenous people is an idea thatmerits serious consideration. There is a need to reanalyze the domesticationhistory of crops. Wild-to-domesticated complexes are excellent experimen-tal systems to investigate evolutionary issues. Both the progenitors and theirdescendants are known. This allows for the integration of evolutionary anddevelopmental genetics and for a closer look at those differences at themolecular level responsible for the phenotypic differences between wild anddomesticated types. This knowledge can assist with gene discovery and alsopossibly aid in finding for new domesticates.


Because the stakes are high, the Crop Science Society of America (CSSA)has developed a “Position Statement” on climate change, which identifies thechallenges that crop science can address to adapt cropping systems to climatechange in the short term (https://www.crops.org/files/science-policy/cssa-crop-adaptation-position-statement.pdf). Because of uncertainties and lim-ited predictability in the long term, a need exists for an infrastructure thatdrives innovation and implements crop adaptation strategies in a sustainablemanner. The Position Statement goes on to suggest that research investmentsand efforts are needed to further 1) understand the physiological, genetic,and molecular basis of adaptation to drought, heat, and biotic stresses likelyresulting from climate change; 2) translate new knowledge into new agricul-tural systems that integrate genetic and management technologies (i.e., bothbreeding and agronomy will contribute to adaptation); and 3) transfer knowl-edge effectively and make technologies and innovations widely available toincrease food production and stability. The CSSA statement also suggestedthat an effective, planned response to climate change should provide for sig-nificant investments in, and strengthening of, crop science research to yieldknowledge and information on adapting crops and cropping systems to thechanging environment.

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