Microbes in plant endosphere: Pathogenesis and disease antagonism

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Microbes in Plant Endosphere: Pathogenesis and Disease Antagonism

IntroductionEndophytes are omnipresent colonizers persist in the inner plant tissues where

they do not normally cause any considerable morphological changes and disease symp-toms. The term endophytes generally used for particular group of host and microbes (‘Indo’-Living in ‘Phyco’- Plants). Mainly fungi and various bacteria comprises the important group of endophytes. In general, an endophyte can be defined as a microor-ganism, typically bacterial or fungal, that lives asymptomatically within a plant, caus-ing no signs of harm to the host (Bacon and White, 2000). Majority of the endophytes colonize different compartments of the plant apoplast, including the intercellular spac-es of the cell walls and xylem vessels. Some of them are able to colonize reproductive organs of plants, e.g. flowers, fruits and seeds.

Chapter

5.MICROBES IN

PLANT ENDOSPHERE: PATHOGENESIS AND DIS-

EASE ANTAGONISMRoshan Kumar, Shailendra Kumar, Vivek, Avijit Tarafdar and Koushik Biswas

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Plant Pathogen Interactions : Recent Trends

Figure1. Schematic reproduction of endophytic interactionsbetween plant and mi-crobes

Source: http://www.springerimages.com/Images/LifeSciences/110.1007_s00442-009-1351-8-0

In figure 1, the interaction between microbes and succesional plant is discussed. Where the ecosysytem above ground and belowground interactions between an earlier (left) and later (right) successional plant, their herbivores, symbionts (endophytes and mycorrhizal fungi), their predators and top predators is storm. Closed lines indicate direct interactions and interrupted interactions are indirect; thick lines indicate strong interactions and thin lines weak. Long arrows indicate strong effects on the rate of suc-cession, and short arrows weak; the direction implies driving (arrow pointing to the right), or slowing down (arrow to the left) of succession. Coarse interrupted lines relate to the early succession species; fine interrupted lines relate to the successor plant; a coarse/fine interrupted line relates to both plant species. In the case of the aboveground and belowground herbivores, we only produced an example that had stronger effects on the early than on the later successional plant species. The decomposer subsystem responds relatively slowly when compared to the herbivores and symbionts and will generally promote succession. This simplification of reality may help to explore bound-ary conditions where net positive effects turn into net negative effects.

Endophytes are generally two categories.

1. Endophytic bacteria

2. Endophytic Fungi

1. Endophytic bacteria Virtually all plants are inhabited by diverse bacteria known as endophytes. En-

dophytic bacteria are referred to as those which can be detected at a particular moment within the tissues of apparently healthy plant hosts (Schulz and Boyle, 2006). Most of the endophytes colonize different compartments of the plant apoplast, include the inter-cellular spaces of the cell walls and xylem vessels. Some of them are able to colonize reproductive organs of plants, e.g. flowers, fruits and seeds. Inside a plant, endopjytic bacteria do not normally cause any substantial morphological changes like root-nodule symbionts do(Fig2). They also do not cause any disease symptoms, in contrast to phy-topathogens. Several endophytic bacteria have a number of plant-beneficial traits in vi-tro; few of those exhibit them in planta and only a small number of endophytes proved to be very effective plant-growth promoting and/or biocontrol agents under agricultural conditions (Scherwinski et al., 2008).

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The phyllosphere is the microbial habitat extending from the surface of the cu-ticle covering of leaf epidermis to the outer edge of the boundary layer surrounding the leaf and inward into the leaf tissues.

Source: http://www.els.net/WileyCDA/ElsArticle/refId-a0000400.html

1.1 Diversity of bacteria found as Endophytes

Since the first reliable reports about the isolation of endophytic bacteria from surface - sterilized plants (Mundt and Hinkle, 1976) more than 200 bacterial genera from 16 phyla have been reported as endophytes. These contain both culturable and unculturable bacteria belonging to Acidobacteria, Actinobacteria, Aquificae, Bac-teroidetes, Cholorobi, Chloroflexi, Cyanobacteria, Deinococcus-Thermus, Firmicutes, Fusobacteria, Gemmatimonadetes, Nitrospira, Planctomycetes, Proteobacteria, Spiro-chaetes and Verrucomicrobiae (Manter et al., 2010; Sessitsch et al., 2012). However, the principal and studied endophytes belong to three major phyla (Actinobacteria, Pro-teobacteria and Firmicutes) and included members of the genus, Azoarcus (Krause et

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al., 2006), Bacillus (Deng et al., 2011), Enterobacter (Taghavi et al., 2010), Bur-kholderia (Weilharter et al., 2011), Herbaspirillum (Pedrosa et al. 2011), Pseudomonas (Taghavi et al., 2009), and Streptomyces (Suzuki et al., 2005). Species of these genera are ubiquitous in the soil/rhizosphere which represents the main source of en-dophytic colonizers (Hallmann and Berg, 2006). Other possible sources of endophytes include the phyllosphere, the anthosphere and seeds (Compant et al., 2010). Naturally occurring endophytes can be visualized by FISH (fluorescence in situ hybridization) combined with confocal laser scanning microscopy using specific probes (Amann et al. 1990; Loy et al. 2007).

Source: http://dx.doi.org/10.1590/S1517-83822010000400014

1.2. Ways of colonization of plant by endophytic bacteriaIn number of ways endophytic bacteria can get access to a plant’s interior. In

this section the different route of endophyte colonization is discussed briefly.

1.3. Endophytic colonization in rhizoplaneColonization of the plant’s interior by bacteria initiate with their establishment

in the rhizosphere. The early events of this process such as recognition and chemotaxis have been extensively reviewed by Lugtenberg et al. (2001) and Lugtenberg and Kamilova (2009). After in colonization rhizosphere, bacteria attach, to the rhizoplane, i. e. the root surface. A number of mutational studies showed that attachment of bacte-rial cells to the root is a crucial step for subsequent endophytic establishment. Several

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bacterial surface components can be involved in this process. For Azoarcus sp. BH72, an endophytic diazotroph of rice, type IV pili encoded by pilAB are required for attach-ment to the root surfaces (Dorr et al., 1998). A mutant impaired in the expression of pilAB fails to successfully colonize roots and shoots of rice plants (Reinhold-Hurek et al., 2006). The attachment of another diazotrophic endophyte, Herbaspirillum sero-pedicae to root surfaces of maize depends on LPS (liposaccharide) (Balsanelli et al., 2010). A mutant strain with changed monosaccharide composition in the core domain of LPS showed a hundred-fold lower root adhesion and endophytic spreading com-pared to the wild type. Similarly (exopolysaccharide) is necessary for rhizoplane and endosphere colonization of rice plants by Gluconacetobacter diazotrophicus (Meneses et al., 2011). Since none of these mutant strains completely lost their ability for adhe-sion, it can be expected that other bacterial surface components are also involved in this process.

1.4. Bacterial Penetration in Plant root

The preferable sites of bacterial attachment and consequent entry are the apical root zone with the thin-walled surface root layer such as the cell elongation and the root hair zone (zone of active penetration), and the basal root zone with small cracks caused by the emergence of lateral roots (zone of passive penetration) (figure3). At these sites bacteria are often arranged in microcolonies comprising several hundreds of cells (Zachow et al., 2010). For active penetration, endophytic bacteria have to be well-equipped with cellulolytic enzymes which hydrolyze the plan’s exodermal cell.

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Fig.3: Bcaterial Penetration through cortex or basal root and colonization in root cell and intra cellular pore

Source: Zachow et al., 2010

1.5. The various plant colonization routes by endophytic bacteria.

By entering a plant through natural cracks at the region where the lateral roots appear, bacteria remain “invisible” for the plant’s immune system. This mode of entry (often combined with active penetration) has been suggested for Azoarcus sp. BH72 (Reinhold-Hurek and Hurek, 1998) and Burkholderia vietnamiensis (Govindarajan et al., 2007) in rice, B. phytofirmans PsJN in grape (Compant et al., 2005), B. subtilis Lu144 (Ji et al., 2008).

1.6. Colonization in Plant cortexAfter passed the exodermal barrier bacterial cells, can remain at the site of entry

eg. ( Paenibacillus polymyxa in Arabidopsis) (Timmusk et al., 2005) or move deeper inside and occupy the intercellular space of the cortex (James et al., 1994; Roncato-Maccari et al., 2003; Compant et al., 2005; Gasser et al., 2011). It is unusual for endophytic bacteria to penetrate plant cells and cause formation of specific morpho-logical structures like root-nodule bacteria do. on the other hand, Huang et al. (2011)

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showed that Bacillus subtilis GXJM08 colonizes the root of the leguminous plant Robinia pseudoacacia L. in a mode similar to that used by rhizobia.

1.7. Xylem colonizationOnly a some bacteria can penetrate the endodermal barrier and invade the

xylem vessels (James et al., 2002; Roncato-Maccari et al., 2003; Compant et al., 2005; Gasser et al., 2011). This generally happens through unsuberized endodermal cells in the apical root zone and/or in the basal root zone, where the emerging lateral roots interrupt the continuity of the Casparian band in the wall of endodermal cells. The long-distance transport of water, ions and low-molecular weight organic compounds, such as sugars, organic and amino acids, takes place in the xylem (Sattelmacher, 2001).

1.8. Reproductive organs colonizationIt is likely that the concentration of available nutrients in xylem is decreasing

along the plant axis. This can explain the facts that the diversity and population den-sity of endophytic bacteria decreases with the distance from the root and that only a small number of bacteria reaches the upper parts of shoots, the leaf apoplast and repro-ductive organs, such as flowers, fruits and seeds (Compant et al., 2010). The presence of endophytic bacteria in reproductive organs of plants was confirmed by cultivation (Fürnkranz et al., 2011).

1.9. Some other ways of plant colonizationWhile the rhizosphere is supposed to be the main source of endophytic colo-

nizers, other sites of entry cannot be ignored. Some bacteria are able to enter a plant through stomata as has been shown for Gluconobacter diazotrophicus on sugarcane (James et al., 2001) and for Streptomyces galbus on rhododendron (Suzuki et al., 2005). In the later case, production of non-specific wax-degrading enzymes might have fa-cilitated the leaf surface colonization and the later endophytic establishment of this microbe (Suzuki et al., 2005). Bacterial entry can also occur in a plant through flowers, fruits and seeds.

1.10. Beneficial endophytic bacteria and their effects on a plant After establishment in a plant, endophytes can positively influence plant growth and its resistance to different stresses. plant growth-promoting effects mediated by en-dophytic bacteria are grouped as direct PGP (plant growth promotion) and biocontrol of phytopathogens.

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Source : Malfanova et al., 2011

Illustration of the chief mechanisms of PGP and BC mediated by en-dophytic plant-beneficial bacteria. Indicated in bold are the mechanisms used by endophytes as shown by experimental studies. Other mechanisms are putatively involved based on genomic data.

PGP has been shown for many endophytic bacteria (Zachow et al., 2010; Gas-ser et al., 2011). Direct PGP mediated by endophytes is by and large based on provid-ing essential nutrients to plants and production and/or regulation of phytohormones.

Many IAA-producing endophytes possess ACC (1-aminocyclopropane-1- carboxylate)-deaminase activity is reported to be involved in lowering the level of plant ethylene (Long et al., 2008). high levels of ethylene caused by some stresses are known to hinder root elongation and lateral root coming out (Ivanchenko et al., 2008). According to the model proposed by Glick (2005) bacterial IAA activates ACC-synthase of plants resulting in the production of ACC, the ethylene precursor. Some bacteria can use ACC as a nutrient source and thereby decrease the synthesis of ethylene in plants. ACC-deaminase activity was described for plant growth-promoting endophytic strains of Burkholderia (Sun et al., 2009; Gasser et al.2011).

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Source: http://www.izmb.uni-bonn.de/uelker/projectlandsberg.html

1.11. Endophytic bacteria Biocontrol of phytopathogens

Colonization of plants by biocontrol endophytes induces several cell-wall modifications, such as deposition of callose, pectin, cellulose and phenolic com-pounds leading to the formation of a structural obstacle at the site of potential attack by phytopathogens (Benhamou et al.,1998; Benhamou etal.,2000).

1.12. Genomic and postgenomic view of plant-endophyte interactions

A number of genomes of endophytic bacteria has been sequenced in recent years. every beneficial traits such as (N fixation, IAA, ACC deaminase, etc.) are re-flected in their genomes. Moreover, genomic analysis also revealed the existence of a high number of genes involved in iron uptake and metabolism. For example, the ge-nome of Enterobacter sp. 638 has nine ABC transporters for siderophore complexes in contrast to four in E.coli K12 (Taghavi et al.,2010).

Further genome analysis revealed genes which might be important for the endophytic lifestyle. For example, the genome of diazotrophic K. pneumoniae (Kp) 342 contains genes for superoxide dismutases, putative catalases, peroxidases and reductases which are involved in the protection of bacterial cells against plant ROS (reactive oxygen species) (Fouts et al., 2008).

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Analysis of metagenomic of the most abundant endophytic bacteria of rice verified traits which are shared among endophytes and hence potentially important for their interactions with plants (Sessitsch et al., 2012). These include (i) a whole set of specialized secretion systems, except the type III secretion system which was not highly conserved among rice endophytes, (ii) cellulolytic and pectinolytic enzymes, (iii) flagellins, (iv) enzymes involved in ROS degradation, (v) receptors and transport-ers for iron uptake, (vi) QS (quorum sensing) systems, (vii) metabolic pathways for degradation of plant compounds, and (viii) numerous plant-growth promoting and bio-control traits (ACC- deaminase activity, BNF, production of antimicrobial compounds, phytohormones and volatiles).

1. Endophytic FungiFungal endophytes are eukaryotic microorganisms colonizing healthy tissues of

living plants without causing disease symptoms (Wilson 1995). They have frequently been reported associated with roots, where they appear to protect plants exposed to various abiotic (Marquez et al. 2007) and biotic stresses Omacini et al. 2001; Rai et al. 2004) and to promote plant growth (Deshmukh & Kogel 2007).

It has been observed that endophytic fungi may steps forward drought resis-tance in a few species of agronomically important forage and turf grasses, namely pe-rennial ryegrass (Lolium perenne L.) and tall fescue (Lolium arundinaceum Darbyshire ex. Schreb.). Under drought, endophyte infection can enhance host growth and survival relative to endophyte free (EF) grasses (Read and Camp 1986; Elmi and West 1995). In contrast, increasing evidence shows that the response of endophyte-infected (EI) grasses to drought depends largely on the availability of other resources, in particular nutrients (McCormick et al. 2001).

Fungal-plant mutualistic interactions in the rhizosphere show a diversity of in-teractions by a group of fungal taxa and over 90% of plant species (Bonfante and Genre, 2010). Interactions with trees account for the bulk of the variety of interactions with fungi, utilizing ectomycorrhizal mecha nisms in the root hair in which the fungal my-celium are extracellular. Most vascular plants have evolved endosymbiotic interactions with arbuscular mycorrhiza, a common fungus in soil. Such relationship seems to have occurred early in the development of land plants and represents the most widespread type of symbiosis in nature. Members of the fungi phylum Glomeromycota are part of the soil matrix, and their hyphae can infect the root hair and form arbuscule structures (endomycorrhizobial) in the plant root cells to exchange nutrients. Without the roots of plants, these microorganisms would be unable to complete their Iifecycle and they would die. These fungal advantage the host plant by providing additional phosphorus, but can also provide additional micronutrients and water due to the increase Gsurfaee area· of the filamentous mycorrhiza hyphae widely distributed through the soil, where-as the plant can provide the fungi with carbon, often the sole source of carbon, for the

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arbuscular mycorrhiza. Disruption of the rhizosphere, or the soil surrounding the plant root system, can disrupt the mycorrhiza hyphal network and impede the symbiosis-based development. Mainly, tillage, fungicides, and application of phosphorus fertiliz-ers are modern practices that go against the potential benefits of arbuscular mycorrhiza by disrupting fungal growth and minimizing infection of the host plant. In comparison, this symbiosis could be managed and exploited by farming systems where inputs are minimal, such as organic farms, to improve plant growth and crop yields.

The potential of endophytic fungi as sources of drugs was stirred by the sur-prising discovery that certain endophytic fungi produced the anticancer drug taxol ( Tan RX and Zou WX 2001; Strobel G and Daisy B 2003; Strobel G, Daisy B, Castillo U, et al 2004). Taxol is a diterpenoid, which was first isolated from Taxus brevifolia. It has a matchless mode of action preventing the depolymerization of tu-bulin during cell division. The most common source of taxol is the bark of Yew trees. Unfortunately, however, these trees are not only rare but also slow growing, and large amounts of bark yield only small amounts of the drug, which has contributed to its high price.

The discovery that endophytic fungi produce taxol has raised the possibility that this key drug may be produced via industrial fermentation.

2.1. Community interactions based on endophytic fungiMultitrophic interactions, herbivory and intraand interspecific competition can

influence the performance of plants either directly or indirectly.

E+ grasses out-competed E- grasses in a four year study, resulting in an over-all decrease in plant-species richness [Clay K, Holah J,1999]. In addition, a study of interspecific competition between Lp and the annual grass Digitaria sanguinalis showed that E+ Lp plants had reduced competitive ability compared with E- Lp plants (Rich-mond DS, Grewal PS, Cardina J, 2003).

Tall fescue (Festuca arundinacea Schreb.) is the most commonly used culti-vated grass in the United States to feed beef cattle. Tall fescue is a cool-season peren-nial grass that many cattle producers ‘can’t live with, but can’t live without’ because of its hardiness and good forage yields, but adverse effects on cattle well-being and yields. The history of this forage and its effects on animal performance have been extensively reviewed (Hemken et al., 1984; Bacon et al., 1986).

Numerous studies have since demonstrated the adverse effects that endophyte-infected tall fescue can have on beef cattle performance (Paterson et al., 1995; Ball, 1997). The nutritional quality of endophyte-infected tall fescue is comparable to other similar forages and is not an influential factor in most studies. Fescue toxicosis costs the U.S. beef industry an estimated $500 million to $1 billion annually in lost revenue because of reduced reproductive and growth rates in cattle herds. More than 100 yr of

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research suggests that most, if not all, plants in natural ecosystems are symbiotic with mycorrhizal fungi and/or fungal endophytes (Petrini, 1986). These fungal symbionts can have profound effects on plant ecology, fitness, and evolution (Brundrett, 2006), shaping plant communities (Clay & Holah, 1999) and manifesting strong effects on the community structure and diversity of associated organisms (e.g. bacteria, nematodes and insects; Omacini et al., 2001).

2.2. Major groups of endophytic fungiIn general, two major groups of endophytic fungi have been recognized previ-

ously, reflecting differences in evolutionary relatedness, taxonomy, plant hosts, and ecological functions (Table 1): the clavicipitaceous endophytes (C-endophytes), which infect some grasses; and the nonclavicipitaceous endophytes (NC-endophytes), which can be recovered from asymptomatic tissues of nonvascular plants, ferns and allies, conifers, and angiosperms. C-endophytes (hereafter, Class 1 endophytes) represent a small number of phylogenetically related clavicipitaceous species that are fastidious in culture and limited to some cool- and warm-season grasses (Bischoff & White, 2005). Typically these endophytes occur within plant shoots, where they form systemic inter-cellular infections. Clay & Schardl (2002) recognized three types of clavicipitaceous endophytes, ranging from symptomatic and pathogenic species (Type I) to mixed- in-teraction and asymptomatic endophytes (Types II and III, respectively). spread of Class 1 endophytes is primarily vertical, with maternal plants passing fungi on to offspring via seed infections (Saikkonen et al., 2002). Colonized plants usually harbor one domi-nant fungal isolate/genotype (Wille et al., 1999). Class 1 endophytes repeatedly in-crease plant biomass, confer drought tolerance, and produce chemicals that are toxic to animals and decrease herbivory (Clay, 1988). However, the benefits conferred by these fungi appear to depend on the host species, host genotype and environmental conditions (Saikkonen et al., 1999; Faeth & Sullivan, 2003; Faeth et al., 2006). NC-endophytes can be differentiated into three functional classes based on host colonization patterns, mechanism of transmission between host generations, in planta biodiversity levels, and ecological function (Table 1). Although all three classes have broad host ranges, Class 2 endophytes may grow in both above- and below-ground tissues. By contrast, Class 3 and 4 endophytes are restricted to above-ground tissues and roots, respectively.

Table 1 Symbiotic criteria used to characterize fungal endophytic classes

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Criteria Clavicipitaceous NonclavicipitaceousClass 1 Class 2 Class3 Class 4

Host range Narrow Broad Broad BroadTissue(s)

colonized

Shoot and rhi-zome

Shoot, root and rhizome

Shoot Root

colonization Extensive Extensive Limited Extensivebiodiversity Low Low High UnknownTransmission Vertical and hori-

zontalVertical and hori-zontal

Horizon-tal

Horizontal

Fitness benefits NHA NHA and HA NHA NHA

2.3. Roles in plant ecophysiology

Class 1 endophytes possibly will enhance the ecophysiology of host plants and allow plants to counter abiotic stresses such as drought (Arechavaleta et al., 1989) and metal infectivity, In some cases, endophytes arouse longer root hairs and enhance exudation of ‘phenolic-like compounds’ into the rhizosphere, resulting in more ef-ficient absorption of soil phosphorus and enhanced aluminum tolerance via chela-tion (Malinowski & Belesky, 2000). Class 2 endophytes are distinct from the other NC-endophytes because in general they colonize roots, stems and leaves; are capable of forming extensive infections within plants; are transmit via seed coats and/or rhi-zomes; have low presence in the rhizosphere; confer habitat-adapted fitness benefits in addition to nonhabitat-adapted benefits; and typically have high infection frequen-cies (90–100%) in plants growing in high-stress habitats.

2.4. Endophyte-conferred fitness benefits and ecological adaptations of plantsIt seems that most, if not all, of the Class 2 endophytes examined to date in-

crease host shoot and/or root biomass, possibly as a result of the induction of plant hormones by the host or biosynthesis of plant hormones by the fungi (Tudzynski & Sharon, 2002). Many Class 2 endophytes protect hosts to some extent against fungal pathogens (Danielsen & Jensen, 1999; Narisawa et al., 2002; Campanile et al., 2007), reflecting the production of secondary metabolites (Schulz et al., 1999), fungal para-sitism (Samuels et al., 2000), or induction of systemic resistance (Vu et al., 2006). It is also probable that symbiotically conferred disease protection may be a result of a lack of ability of pathogens to compete with endophtyes for resources or niche space. Only a few have been examined for their direct interactions with host defenses.

2.5. Class 2 endophytes

Endophytic isolates of Fusarium oxysporum and a Cryptosporiopsis sp. con-

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ferred disease resistance against virulent pathogens in barley (Hordeum vulgare) and larch (Larix decidua), respectively (Schulz et al., 1999), and resistance was corre-lated to increased concentrations of phenolic metabolites. A mutant of Colletotrichum (isolate Path-1; Freeman & Rodriguez, 1993) expressing a mutualistic lifestyle as-ymptomatically colonized roots, stems and leaves of watermelon (Citrullus lanatus) without inducing host defense systems (Redman et al., 1999). though, when Path-1-colonized plants were challenged with a virulent pathogen, host defense systems were activated very quickly to levels not achieved by nonsymbiotic plants that suc-cumbed to disease. One characteristic that appears unique to Class 2 endophytes is the ability of individual isolates to asymptomatically colonize and confer habitat-adapted fitness benefits on genetically distant host species representing monocots and eudi-cots (Rodriguez et al., 2008). This phenomenon was discovered by comparing fitness benefits conferred by Class 2 endophytes in plants growing in geothermal soils (Cur-vularia protuberata), coastal beaches (Fusarium culmorum) and agricultural fields (Colletotrichum spp.). A series of laboratory studies indicated that C. protuberata conferred heat but not salt or disease tolerance, F. c u l mo r u m conferred salt but not heat or disease tolerance, and Colletotrichum spp. conferred disease resistance but not heat or salt tolerance (Redman et al., 2001, 2002).

2.6. Class 3 endophytes are distinguished on the basis of their occurrence primar-ily or exclusively in above-ground tissues; horizontal transmission; the formation of highly localized infections; the potential to confer benefits or costs on hosts that are not necessarily habitat-specific; and extremely high in planta biodiversity (Table 1). Class 3 endophytes include the hyperdiverse endophytic fungi associated with leaves of tropical trees (Lodge et al., 1996; Fröhlich & Hyde, 1999; Arnold et al., 2000; Gamboa & Bayman, 2001). plants infected by multiple Class 3 endophytes typically show no observable change in growth rate, biomass accumulation, root:shoot ratio, or other easily quantifiable characteristics following inoculation under in vivo conditions (Arnold, 2002).

3. Conclusions

Plants benefit broadly by harbouring endophytic microbes. They promote plant growth and give enhanced resistance to a range of pathogens. However, the way the interactions among endophytes influence the plant productivity has not been explained. Present study experimentally showed that endophytes isolated from rice (Oryza sativa) used as the test plant produced two types of interactions; biofilms (bac-teria attached to mycelia) and mixed cultures with no such attachments. Acidity, as

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measured by pH in cultures with biofilms was higher than that of fungi alone, bacteria alone or the mixed cultures. Production of indole-acetic acid like substances (IAAS) of biofilms was higher than that of mixed cultures, fungi or bacteria. Bacteria and fungi produced higher quantities of IAAS than mixed cultures. In mixed cultures, the potential of IAAS production of resident microbes was reduced considerably. There was a negative relationship between IAAS and pH of the bio films, indicating that IAAS was the main contributor to the acidity. However, such a relationship was not observed in mixed cultures. Microbial acid production is important for suppressing plant pathogens.

Fungal endophytes comprise a diverse group of species that vary in symbiotic and ecological functions. Here, we used host range, colonization and transmission patterns, tissue specificity, and symbiotically conferred fitness benefits to differenti-ate endophytes into four functional classes. While the impact of each class on plants may differ significantly, the vast majority of endophytes have yet to be adequately characterized. Regardless, it is clear that these fungi can have profound impacts on the survival and fitness of plants in all terrestrial ecosystems, and therefore likely play a significant role in plant biogeography, evolution and community structure. many re-searchers perform Koch’s postulates to assess host range, colonization and transmis-sion patterns, and symbiotic/ecological function, our viewpoint on endophytes will modify. Until we understand more about the importance of endophytes in plant biol-ogy, our understanding of plant community dynamics and ecosystem function will be incomplete. It is important to point out that individual plants comprise communities of microorganisms including fungi, bacteria, viruses and sometimes algae.

It can be concluded that natural condition of plants seems to be in a close inter-action with endophytes. Endophytes seem promising to increase crop yields, remove contaminants, hamper pathogens, and produce fixed nitrogen or novel substances.

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Microbes in plant endosphere: Pathogenesis and disease antagonism

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