Available online at www.sciencedirect.com

Strigolactones: a new hormone with a pastYuichiro Tsuchiya and Peter McCourt

The recent discovery of an endogenous hormonal activity for

strigolactones in shoot branching was surprising since these

molecules were thought to mostly play roles as signaling

molecules between organisms. Even in the context of plant

hormones, strigolactones appear to be different in that their

role in plant development is quite restricted. This most probably

reflects early days and new hormonal functions will most

probably be found for these compounds in the future. In this

respect, the exogenous role of strigolactones in parasitic plant

seed germination may hint to functions of this compound in

seed development. However, showing new roles for

strigolactones in the seed or any other aspect of plant

development for that matter will require developing assays in

model genetic systems such as Arabidopsis and rice where we

can take full advantage of the experimental tools that are

available.

Address

Department of Cell & Systems Biology, University of Toronto, 25

Willcocks Street, Toronto ON M5S 3B2, Canada

Corresponding author: McCourt, Peter (peter.mccourt@utoronto.ca)

Current Opinion in Plant Biology 2009, 12:556–561

This review comes from a themed issue on

Cell signalling and gene regulation

Edited by Jan U. Lohmann and Jennifer L. Nemhauser

Available online 31st August 2009

1369-5266/$ – see front matter

# 2009 Elsevier Ltd. All rights reserved.

DOI 10.1016/j.pbi.2009.07.018

IntroductionUsually plant biologists are interested in five plant hor-

mones – auxins, cytokinins, ethylene, gibberellins and

abscisic acid – which is enough since this gang of five

seems to be involved in modulating everything. To make

matters worse, the list of hormones appears to be growing

with new molecules such as bassinosteroids and jasmo-

nates been added to the list. To keep this list manageable,

developmental biologists have come up with some simple

rules of what is a hormone. First, a compound of interest

must be made in small quantities and work at nanomolar

to micromolar ranges. Second, mutants defective in the

synthesis of an interesting molecule must have specific

developmental defects before it can be classified as a

hormone. These rules have meant most developmental

biologists did not have to worry about the myriad of

secondary products produced by plants. With this said,

Current Opinion in Plant Biology 2009, 12:556–561

the recent discovery that a collection of organically

related compounds, generically called strigolactones,

are also hormones was still surprising. Strigolactones have

long been of interest because they act as germination

stimulants for seeds of parasitic weeds of the genera Strigaand Orobanche and this parasitism is the scourge of agri-

culture in the developing world [1,2��,3]. In Africa alone

Striga species have infested up to two-thirds of the arable

land and are thought to cause 10 of billions of dollars in

lost crop yields. Striga species parasitize mostly tropical

monocotyledonous cereal crops whereas Orobanchespecies are limited to dicotyledonous crops found in

semi-temperate regions.

Although separated geographically and by host range, the

lifecycles of both Striga and Orobanche ssp. are similar.

Their seeds remain dormant in the soil until they sense

the growing roots of a potential host (Figure 1). Following

this event, the seeds germinate and grow toward a host

until their roots attach. After the successful establishment

of root vascular connections, the parasitic plant begins to

withdraw both nutrients and water resulting in dramatic

decreases in crop growth. Parasitic plant seeds are rela-

tively small and contain little nutrient reserve. Hence, the

germinating seeds must find a host root within a few days

of germination or it will die. Parasitic seeds will only

germinate when they are within 20 mm of a host root

(Figure 1). This small activation zone between a parasitic

seed and its host root suggested the host was producing a

germination stimulant. Subsequent analysis of root exu-

dates that could induce parasitic seed germination led to

the identification strigolactones [4��]. The basic structural

unit of a strigolactone is a tricyclic lactone that is con-

nected to a butyrolactone by an enol ester bridge

(Figure 2). On the basis of this core structure and on

the reduced efficiency at which various maize carotenoid

biosynthetic mutants could stimulate Striga germination

had led to the suggestion that strigolactones are derived

from a carotenoid-based pathway that has shared steps

with the hormone abscisic acid (ABA) [5].

However, it is clear that host plants do not make strigo-

lactones for the benefit of the parasite. This was further

supported by the fact that the first identified strigolactone

was purified from cotton plants, which are not normally

infected by Striga species [6]. The ability of non-host

plants to germinate parasitic plant seeds suggested stri-

golactones may exist ubiquitously in higher plants and

may have role(s) that are independent of host–parasite

interactions. Evidence for new roles of strigolactones

came from an unusual source when it was discovered

that the strigolactone, 5-deoxy-strigol, was required as a

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Strigolactones: a new hormone with a past Tsuchiya and McCourt 557

Figure 1

Strigolactones influence the germination of parasitic seeds and branching patterns of mycorryzial fungi. On the left a host plant exudes strigolactone

into the soil, which activates dormant parasitic plant seed to germinate. If the seed radicle reaches the host root it will establish a parasitic interaction.

On the right a host plant exudes strigolactone into the soil, which is sensed by the hyphe of an arbuscular mycorrhizal fungus. In responding to

strigolactone the hyphal growth is directed toward the plant root. Once an interaction is established the two organisms mutually exchange sugar from

the plant and nitrogen from the fungi.

branching factor to help arbuscular mycorrhizal fungi to

interact with plant roots (Figure 1) [7]. Mycorrhizal fungi

cannot develop in the absence of their plant partner

because the plant supplies carbon in the form of glucose

to the fungi. In return, the fungus improves the transfer of

nutrients to the plant root that is used for the synthesis of

amino acids [8]. This mutualistic relationship does

explain why some plants not involved in plant parasitic

interaction may exude strigolactones. However, this sym-

biotic communication could not be the whole story.

Although eighty percent of land plants form a mycorrhizal

interaction, many plants that produce strigolactones, do

not require any fungal partners for good growth [9,10�].

Branching out is hard to doThe absence of a natural plant parasite or a mycorrhizal

interaction for some plants suggested strigolactones have

other functions. But what were these role(s)? An insight

came with the finding that a collection of shoot branching

defective mutants in a variety of species were phenoty-

pically rescued by strigolactone addition [11��,12��]. This

story began with a collection of loss-of-function mutants

of pea, petunia, rice, and Arabidopsis that were originally

identified by their enhanced shoot branching [13].

Because of its sophisticated molecular genetics Arabidop-

sis researchers were able to show that two of these genes,

MAX3 and MAX4, encoded carotene-deoxygenases

(CCDs) that were most probably involved in carotene

degradation (Figure 2) [14,15]. Subsequent studies

showed that some of the branching defective mutants

in the other species were also defective in these CCD

genes [15]. Although the products of the MAX3 and

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MAX4 CCDs in planta are sketchy, some knowledge of

their potential cleavage products has been predicted from

expression of these genes in E. coli [16]. This finding

paired with the belief that strigolactones were carotenoid

derivatives was bought together when analytical analysis

demonstrated that various branching mutants in pea and

rice showed reduced levels of strigolactones [11��,12��].Finally, a synthetic strigolactone analogue, GR24, was

able to phenotypically rescue a variety of branching

mutants in different species including Arabidopsis clearly

showed strigolactones were necessary for normal plant

shoot development.

The carotenoid-based origins of strigolactones and ABA

have interesting biosynthesis and developmental implica-

tions since ABA is a germination inhibitor whereas stri-

golactones appear to stimulate germination at least in

parasitic plants. Actually of the nine CCDs that exist in

the Arabidopsis genome five are involved in ABA syn-

thesis. This raises questions about the interactions be-

tween these two biosynthetic pathways and the role of

ABA itself on strigolactone production. We know that null

mutations in CCDs involved in ABA synthesis in tomato

or maize do reduce the ability of these plants to germinate

parasitic seeds and this has been correlated with a

reduction in strigolactone levels [17]. This could mean

a CCD involved in ABA synthesis is also involved in

strigolactone biosynthesis. Alternatively, the reduced stri-

golactone levels may indirectly reflect a regulatory role of

ABA on the synthesis of this hormone. These possibilities

could be resolved by measuring strigolactone levels in

ABA biosynthesis mutants versus signaling mutants.

Current Opinion in Plant Biology 2009, 12:556–561

558 Cell signalling and gene regulation

Figure 2

A partially predicted pathway of strigolactone biosynthesis based on branching biosynthetic mutants from a variety of plant species. The products of

CC7 and CCD8 enzymes are based on heterologous enzymatic analysis in E. coli. The gene names on the right are based on mutants identified from

various plant species. The D27 mutant has only been identified in rice and the P450/MAX1 mutant has only been identified in Arabidopsis. At the

bottom are structures of a small number of natural and synthetic strigolactones. Strigol stimulates Striga germination whereas Orobanchol stimulates

Orobanche species seed germination. Strigol and 5-deoxystrigol have been identified in monocotyledonous plants and Orobanchol is usually found in

dicotyledonous species.

A third gene, MAX1, which encodes a Cytochrome P450,

also appears to be involved in strigolactone synthesis at

least in Arabidopsis [18]. At this stage it is difficult to

know where MAX1 fits into the biosynthetic pathway but

it is assumed this cytochrome acts downstream of the two

CCDs [11��,12��]. Interestingly, the equivalent loss-of-

function max1 mutations have not been identified in any

other plant species, which may mean the ‘MAX pathway’

in Arabidopsis, is not completely conserved across plant

species. Perhaps this just reflects genetic redundancy

issues between species or maybe there is a more provo-

cative reason that reflects selective differences between

non-mycorrhizal species such as Arabidopsis with species

that have evolved a mycorrhizal symbiosis. Finally, a new

gene involved in strigolactone biosynthesis has recently

been identified in rice [19�]. Originally identified as the

recessive dwarf27 (d27) mutation that conferred an

increased branching phenotype, this mutant was later

shown to have low strigolactone levels and its mutant

phenotypes could be rescued by strigolactone addition.

Current Opinion in Plant Biology 2009, 12:556–561

Molecular characterization of the D27 gene shows it

encodes a novel iron containing protein [19�]. The role

of this gene in strigolactone synthesis is unclear and this

finding does demonstrate how much we still have to learn

about the strigolactone biosynthesis.

Although it is now obvious that strigolactones are

involved in shoot branching, how does this molecule

contribute to this developmental response? This will of

course require the identification of its receptor and the

genes that it regulates. However, we do know that part of

the answer involves how strigolactones interact with other

hormones and in particular auxin and cytokinin. Auxin is

actively transported downward from the top of a plant and

this action inhibits axillary bud outgrowth and hence

branching [20�]. Conversely, cytokinin is transported

upwards from the root to activate bud outgrowth. In

strigolactone deficient mutants there is a higher transport

capacity in the stem owing to upregulation of auxin

transporters [21]. These observations have led to a clever

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Strigolactones: a new hormone with a past Tsuchiya and McCourt 559

model that is akin to traffic congestion. In wild-type

plants, strigolactones or a downstream product, acts like

a flashing orange light slowing the flow of auxin down the

main artery of the stem. As a consequence, auxin made in

axillary buds, which would be analogous to cars trying to

enter onto the main road cannot go in and this backlog

leads to increased auxin levels in the bud, which inhibits

outgrowth. However, recent reports also suggest strigo-

lactones act downstream of auxins and are dependent on

auxin for their synthesis [22�]. Undoubtedly these nuan-

ces will be resolved but these conflicting results still show

that strigolactones, like all plant hormones, have complex

relationships with other hormones. With this said, it is

surprising that strigolactone deficient mutants have phe-

notypes that are limited to shoot branching and a slightly

reduced stature. This is in contrast to most hormone

mutants, which often show a dizzying array of pheno-

types. Perhaps this apparent developmental specificity

will fall away when more detailed phenotypic analysis is

done on these branching mutants.

More mutants, more phenotypingThe lack of more strigolactone-dependent phenotypes on

the basis of genetic analysis does appear to set this

hormone apart from others traditional plant hormones.

This has led to suggestions that strigolactones may

represent more specialized signaling molecules [11��].Some hints that this may not be the case comes from

mutations that appear to influence strigolactone signaling.

Loss-of-function mutations in the MAX2 gene have the

same branching defects of strigolactone deficient

mutants. However, unlike the biosynthetic mutants,

the increased branching in a max2 plant is not rescued

by exogenous strigolactone [23]. Molecular identification

of the MAX2 gene showed it encoded an F-box protein

indicating protein turnover is most probably an integral

part of strigolactone signal transduction [23]. Interest-

ingly, F-box proteins have turned out to be key regulators

of a number of hormone signaling pathways that target

specific transcription factors [24]. What is interesting

about max2 mutants is that they have added phenotypes

beyond increased branching such as altered light respon-

siveness and senescence [25,26]. A simple explanation is

the biosynthetic mutants are leaky, which may be true

since CCD mutants still showed strigolactone activity in

parasitic plant seed germination assays [11��,12��]. This

remaining activity may reflect the genetic redundancy of

the CCD gene family or may mean strigolactones have

alternative biosynthetic pathways [27]. It is also possible

that unlike strigolactone biosynthesis mutants the MAX2

F-box is used in strigolactone-independent processes.

This would be unusual since the F-box proteins involved

in other hormone signaling pathways appear to be specific

to that particular hormone [24].

To clearly define the roles of strigolactones in plant growth

and development will first require more strigolactone

www.sciencedirect.com

mutants and second, better developmental and molecular

phenotyping. In these respects rice may actually lead the

way. Defects in shoot branching can be easily identified at

early stages of rice growth and this has led to the identi-

fication of many varieties with altered branching patterns.

For example, the DWARF14 (D14) mutant of rice looks like

a strigolactone deficient mutant, however, it is not rescued

by strigolactone addition [28��]. Positional cloning of the

D14 locus shows it encodes a protein related to a hydrolase

superfamily that has previously included proteins with

roles in hormone metabolism and signaling. Intriguingly,

the rice GA receptor GID1 is a member of this family. This

molecular relatedness, coupled with the role of F-box

proteins in strigolactone signaling, could mean many of

the core components of strigolactone signaling may be

similar to those found in GA signaling.

With respect to finding new phenotypes, perhaps seed

development is first place to start. Because strigolactones

are important regulators in parasitic plant germination is it

possible that this hormone has a general developmental

role in this process? Early work on lettuce and wild oats

seed germination did suggest strigolactones have a

positive influence on germination [29,30]. Thus, the

response of parasitic plants to strigolactones may reflect

an evolved response that sensitizes germination responses

of these plants to strigolactones. A widespread role for

strigolactones in seed germination, however, is not sup-

ported by the observation that strigolactone mutants do

not show obvious seed germination defects. Although the

genetics argues against a general role for strigolactones in

germination it is also possible that the model plants in

which strigolactone mutants have been identified are

themselves biased. For example, the commonly used

Arabidopsis laboratory accessions such as Col and Ler

have relatively weak seed dormancy under normal growth

and storage conditions. Therefore, these ecotypic back-

grounds may not be well suited for addressing relation-

ships between strigolactones and seed development [31].

Possibly strigolactone studies involving Arabidopsis

should be carried out using accessions such as Cvi, which

has deep seed dormancy compared with most laboratory

accessions. Weather strigolactones are specialized or have

broader roles in plant development will have to be seen

but certainly they have joined the ‘plant hormone club’.

However, although it is generally thought that plant

hormones are integrators of environmental inputs with

plant development, strigolactones have the added role of

also shaping the environmental signals themselves

through the organisms with which they interact. This

means the evolution of strigolactones may be different

than other endogenous plant hormones. Unlike ABA, for

example, which has a conserved structure across plant

species, a wide array of natural strigolactones has been

isolated. What this means about the evolutionary history

and current roles of this new hormone class remains to be

discovered.

Current Opinion in Plant Biology 2009, 12:556–561

560 Cell signalling and gene regulation

References and recommended readingPapers of particular interest, published within the period of review,have been highlighted as:

� of special interest

�� of outstanding interest

1. Joel DM, Hershenhom Y, Ejeta G, Rich JP: Biology andmanagement of weedy root parasites. Hort Rev 2007,33:267-349.

2.��

Lopez-Raez JA, Matusova R, Cardoso C, Jamil M, Charnikhova T,Kohlen W, Ruyter-Spira C, Verstappen F, Bouwmeester H:Strigolactones: ecological significance and use as a target forparasitic plant control. Pest Manag Sci 2009, 65:471-477.

This is an excellent up to date overview of parasitic plants in the context ofstrigolactones. It integrates much of the literature from both an ecologicaland molecular perspective. It also talks about how this work may beimplemented to control Striga infestations in the future.

3. Scholes JD, Press MC: Striga infestation of cereal crops—anunsolved problem in resource limited agriculture. Curr OpinPlant Biol 2008, 11:180-186.

4.��

Zwanenburg B, Mwakaboko AS, Reizelman A, Anilkumar G,Sethumadhavan D: Structure and function of natural andsynthetic signalling molecules in parasitic weed germination.Pest Manag Sci 2009, 65:478-491.

As a biologist, this review tells you more than you will ever need to knowabout the chemistry of strigolactones. However, this review demon-strates how understanding the chemistry of a molecule and its chemicalanalogues can be used at both a basic level to predicting the nature of thestrigolactone receptor. This review also demonstrates how powerfulchemical biology will be in the field of strigolactones and parasitic weeds.

5. Matusova R, Rani K, Verstappen FW, Franssen MC, Beale MH,Bouwmeester HJ: The strigolactone germination stimulants ofthe plant-parasitic Striga and Orobanche spp. are derivedfrom the carotenoid pathway. Plant Physiol 2005, 139:920-934.

6. Cook CE, Whichard LP, Turner B, Wall ME, Egley GH:Germination of witchweed (Striga lutea Lour.): isolation andproperties of a potent stimulant. Science 1966, 154:1189-1190.

7. Akiyama K, Matsuzaki K, Hayashi H: Plant sesquiterpenesinduce hyphal branching in arbuscular mycorrhizal fungi.Nature 2005, 435:824-827.

8. Govindarajulu M, Pfeffer PE, Jin H, Abubaker J, Douds DD,Allen JW, Bucking H, Lammers PJ, Shachar-Hill Y: Nitrogentransfer in the arbuscular mycorrhizal symbiosis. Nature 2005,435:819-823.

9. Goldwasser Y, Yoder JI: Differential induction of Orobancheseed germination by Arabidopsis thaliana. Plant Sci 2001,160:951-959.

10.�

Goldwasser Y, Yoneyama K, Xie X, Yoneyama K: Production ofstrigolactones in Arabidopsis thaliana responsible forOrobanche aegyptiaca seed germination. Plant Growth Reg2008, 55:21-28.

This study is the first to clearly show that Arabidopsis does makestrigolactone like molecules. It also shows that because of its small sizeand lack of characterized internal standards it is not yet possible to doquantitative analysis. For these reasons although max mutants in Arabi-dopsis have been instrumental in understanding the strigolactonebranching connection all analytical measurements have to be done inlarger species like pea [11��] and rice [12��]. This shows the advantage ofhaving related mutants in multiple species so as to take advantage ofeach ones attributes.

11.��

Gomez-Roldan V, Fermas S, Brewer PB, Puech-Pages V, Dun EA,Pillot JP, Letisse F, Matusova R, Danoun S, Portais JC et al.:Strigolactone inhibition of shoot branching. Nature 2008,455:189-194.

This study along with reference [12��] shows that a number of branchingmutants in were actually deficient in strigolatone. In this case, the authorsfocussed on pea. What is interesting is that although the structure ofstrigolactones has been known for forty years and it was predicted thesemutants were deficient in a carotenoid-based hormone it was onlyrecently that these two leads were put together. By contrast, theapproach to use metabolomics to find the signal does not appear tobe sensitive enough to find new growth regulators.

Current Opinion in Plant Biology 2009, 12:556–561

12.��

Umehara M, Hanada A, Yoshida S, Akiyama K, Arite T, Takeda-Kamiya N, Magome H, Kamiya Y, Shirasu K, Yoneyama K et al.:Inhibition of shoot branching by new terpenoid planthormones. Nature 2008, 455:195-200.

This study, like the previous one [11��], clearly showed that a number ofbranching mutants were actually deficient in strigolatone. These authorsfocussed on rice. Again this study linked the carotenoid pathway infor-mation to strigolactone structure by using mutants. It again shows howgenetics linked to good biochemistry can be so powerful in dissectinghormone synthesis and action.

13. McSteen P, Leyser O: Shoot branching. Annu Rev Plant Biol2005, 56:353-374.

14. Booker J, Auldridge M, Wills S, McCarty D, Klee H, Leyser O:MAX3/CCD7 is a carotenoid cleavage dioxygenase requiredfor the synthesis of a novel plant signaling molecule. Curr Biol2004, 14:1232-1238.

15. Sorefan K, Booker J, Haurogne K, Goussot M, Bainbridge K,Foo E, Chatfield S, Ward S, Beveridge C, Rameau C et al.: MAX4and RMS1 are orthologous dioxygenase-like genes thatregulate shoot branching in Arabidopsis and pea. Genes Dev2003, 17:1469-1474.

16. Alder A, Holdermann I, Beyer P, Al-Babili S: Carotenoidoxygenases involved in plant branching catalyse a highlyspecific conserved apocarotenoid cleavage reaction. BiochemJ 2008, 416:289-296.

17. Booker J, Sieberer T, Wright W, Williamson L, Willett B,Stirnberg P, Turnbull C, Srinivasan M, Goddard P, Leyser O:MAX1 encodes a cytochrome P450 family member that actsdownstream of MAX3/4 to produce a carotenoid-derivedbranch-inhibiting hormone. Dev Cell 2005, 8:443-449.

18. Lopez-Raez JA, Charnikhova T, Gomez-Roldan V, Matusova R,Kohlen W, De Vos R, Verstappen F, Puech-Pages V,Becard G, Mulder P et al.: Tomato strigolactones arederived from carotenoids and their biosynthesis ispromoted by phosphate starvation. New Phytol 2008,178:863-874.

19.�

Lin H, Wang R, Qian Q, Yan M, Meng X, Fu Z, Yan C, Jiang B, Su Z,Li J et al.: DWARF27, an iron-containing protein required for thebiosynthesis of strigolactones, regulates rice tiller budoutgrowth. Plant Cell 2009. [Epub ahead of print].

The discovery that the dwarf mutant d27 is defective in strigolactonesynthesis shows the advantage of using rice as a model system forstudying this hormone. This appears to be because defects in ricebranching can be identified quite early in development. With the wealthof dwarfed mutants that exist in rice it is possible rice will out competeother model systems such as Arabidopsis with regard to understandingstrigolactone synthesis and signaling at a molecular level.

20.�

Leyser O: The control of shoot branching: an example of plantinformation processing. Plant Cell Environ 2009 doi: 10.1111/1365-3040.2009.01930.x.

This is a very up to date review of axillary shoot branching both at adevelopmental and signaling perspective. The review also touches on therole of the environment in branching patterns with a very interestingsection on the light signaling and how it may play into various hormonepathways.

21. Bennett T, Sieberer T, Willett B, Booker J, Luschnig C, Leyser O:The Arabidopsis MAX pathway controls shoot branching byregulating auxin transport. Curr Biol 2006, 16:553-563.

22.�

Brewer PB, Dun EA, Ferguson BJ, Rameau C, Beveridge CA:Strigolactone acts downstream of auxin to regulate budoutgrowth in pea and Arabidopsis. Plant Physiol 2009,150:482-493.

This study shows the complexity of strigolactone signaling in the axillarybud and how the interplay of hormones will be essential to understandingthis developmental response. This study also marks the new era ofstudying branching in plants in the context of knowing that the branchingsignal is strigolactones.

23. Stirnberg P, van De Sande K, Leyser HM: MAX1 and MAX2control shoot lateral branching in Arabidopsis. Development2002, 129:1131-1141.

24. Santer A, Estelle M: Recent advances and emerging trends inplant hormone signalling. Nature 2009, 459:1071-1078.

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25. Shen H, Luong P, Huq E: The F-box protein MAX2 functions as apositive regulator of photomorphogenesis in Arabidopsis.Plant Physiol 2007, 145:1471-1483.

26. Woo HR, Chung KM, Park JH, Oh SA, Ahn T, Hong SH, Jang SK,Nam HG: ORE9, an F-box protein that regulates leafsenescence in Arabidopsis. Plant Cell 2001, 13:1779-1790.

27. Simons JL, Napoli CA, Janssen BJ, Plummer KM, Snowden KC:Analysis of the decreased apical dominance genes of petuniain the control of axillary branching. Plant Physiol 2007,143:697-706.

28.��

Arite T, Umehara M, Ishikawa S, Hanada A, Maekawa M,Yamaguchi S, Kyozuka J: d14, A strigolactone insensitivemutant of rice shows an accelerated outgrowth of tillers.Plant Cell Physiol 2009. [Epub ahead of print].

www.sciencedirect.com

This study again shows the power of the rice dwarf system to identify newmutants in strigolactone biosynthesis and signaling. Possibly like GAsignaling studies, rice will have advantages of other systems such asArabidopsis with respect to dissecting the molecular basis of this newhormone.

29. Bradow J, Connick WJ, Pepperman AB: Comparison of the seedgermination effects of synthetic analogs of strigol, gibberellicacid, cytokinins, and other plant growth regulators.J Plant Growth Regul 1988, 7:227-239.

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31. Koornneef M, Bentsink L, Hilhorst H: Seed dormancy andgermination. Curr Opin Plant Biol 2002, 5:33-36.

Current Opinion in Plant Biology 2009, 12:556–561