© 2015 American Society of Plant Biologists

STRIGOLACTONES

www.plantcell.org/cgi/doi/10.1105/tpc.109.tt0411

© 2015 American Society of Plant Biologists

Strigolactones contribute to a

devastating form of plant parasitism

Striga are parasitic

plants that are the

single largest biotic

cause of reduced crop

yields throughout Africa

(> $10 billion per year in

yield losses)

StrigaHost

Root

parasites

Striga germination is

induced by

strigolactones produced

by host plant roots Striga hermonthica

Image source: USDA APHIS PPQ Archive

© 2015 American Society of Plant Biologists

Strigolactones (SLs) regulate

seemingly unrelated events

Striga

Host

Root

AM fungi

SLs inhibit shoot

branching

SLs promote associations

with arbuscular mycorrhizal

(AM) fungi

SLs promote germination of

parasitic Striga plants

© 2015 American Society of Plant Biologists

Strigolactones inhibit shoot

branching

Image courtesy RIKEN

Wild type SL-deficient

In mutant plants unable to

make SLs, many more

shoot branches grow out

© 2015 American Society of Plant Biologists

Strigolactones promote beneficial

symbiotic interactions

Image courtesy of Mark Brundrett

Symbiotic AM fungi

SLs promotes the symbiotic association with AM

fungi. This symbiosis occurs in 80% of land plants

and helps them assimilate nutrients from the soil

Arbuscular is derived

from Latin for tree

(arbor). Mycorrhiza

means “fungus root”

© 2015 American Society of Plant Biologists

Strigolatones promote germination

of parasitic Striga seeds

Striga-infested maize field

Their common name is witchweed

because the plants appear to be cursed.

Typically Striga infestation causes

reductions in crop yields of 50 – 100%

Image source USDA APHIS PPQ Archive

© 2015 American Society of Plant Biologists

Image courtesy RIKEN

What is the

connection between:

•Shoot branching

•Root parasitism, and

•Root symbiosis?

© 2015 American Society of Plant Biologists

Three seemingly independent topics

converged on SLs

Evolution of plant

parasitism

Origins of plant /

mycorrhizal symbiosis

> 460 mya

Search for

branching factor...

1960s – 1970s

Purification and

characterization of

strigol from roots

1900s - Role of auxin in

shoot branching described

1990s – 2000s

Branching mutants

described in petunia, pea,

Arabidopsis and rice

2008:

Strigolactones

inhibit shoot

branching

1800s - Recognition of

AM fungus / plant

symbiosis

1800s - Host plant factor

required for parasitic seed

germination

2005:

Strigolactones

promote hyphal

branching

© 2015 American Society of Plant Biologists

Strigolactones have been co-opted

for various functions

O O

O OO

OH

SL’s ancestral

function may have

been communication

between individuals of

the same species (as

pheromones or

quormones)

AM fungi

Root parasite

O

O OO

H

O

SLs now contribute to communication within

an individual (as hormones), and between

individuals of different species (and

kingdoms) (as allelochemicals)

See for example Tsuchiya, Y. and McCourt, P. (2012). Strigolactones as small molecule communicators. Mol. BioSyst. 8: 464-469; Delaux, P.-M., et al., (2012). Origin of strigolactones in the green lineage. New

Phytol. 195: 857-871; Proust, H., et al., (2011). Strigolactones regulate protonema branching and act as a quorum sensing-like signal in the moss Physcomitrella patens. Development. 138: 1531-1539.

© 2015 American Society of Plant Biologists

Lecture Outline

• Synthesis

• Perception and signaling

• Strigolactones in whole-

plant processes:

– Shoot branching

– Other developmental effects

– Moss colony growth

– Symbiosis

– Germination

• Towards the elimination of

Striga parasitism

Image source USDA APHIS PPQ Archive

© 2015 American Society of Plant Biologists

Synthesis and transport

In a search for stimulators of Striga

germination, strigolactones were purified

from cotton roots in 1966 and the

chemical structure determined in 1972

Cook, C.E., Whichard, L.P., Turner, B., Wall, M.E., and Egley, G.H. (1966). Germination of witchweed (Striga lutea Lour.): Isolation and properties of a potent stimulant.

Science 154: 1189-1190; Reprinted with permission from Cook, C.E., Whichard, L.P., Wall, M., Egley, G.H., Coggon, P., Luhan, P.A., and McPhail, A.T. (1972).

Germination stimulants. II. Structure of strigol, a potent seed germination stimulant for witchweed (Striga lutea). J. Am. Chem. Soc. 94: 6198-6199.

© 2015 American Society of Plant Biologists

Strigolactones (SLs) are a small

family of compounds

Reprinted from Humphrey, A.J., and Beale, M.H. (2006). Strigol: Biogenesis and physiological activity. Phytochemistry 67: 636-640 with permission from Elsevier; See also Boyer, F.D., et al.. and

Rameau, C. (2012). Structure-activity relationship studies of strigolactone-related molecules for branching inhibition in garden pea: molecule design for shoot branching. Plant Physiol. 159: 1524-1544.

5-Deoxystrigol

SYNTHESIS

There are many

naturally occurring

SLs, derived from

5-deoxystrigol

© 2015 American Society of Plant Biologists

The stimulator of Striga germination

derives from the carotenoid pathway

fluridone

MEP pathway

GGPP

phytoene

b-carotene

5-deoxystrigol

Carotenoid-

deficient

mutants do

not make

germination

stimulator

WT WT

mutant mutant

Through the use of maize mutants and

enzyme inhibitors, carotenoids were

demonstrated to be the precursors of SLs

Matusova, R., Rani, K., Verstappen, F.W.A., Franssen, M.C.R., Beale, M.H., and Bouwmeester, H.J. (2005). The strigolactone germination

stimulants of the plant-parasitic Striga and Orobanche spp. are derived from the carotenoid pathway. Plant Physiol. 139: 920-934.

© 2015 American Society of Plant Biologists

Genes involved in SL biosynthesis

were identified by genetic methods

Reprinted from Booker, J., et al. (2004). MAX3/CCD7 is a carotenoid cleavage dioxygenase required for the synthesis of a novel plant signaling molecule. Curr. Biol. 14: 1232-1238

with permission from Elsevier; Morris, S.E., et al. (2001). Mutational analysis of branching in pea. Evidence that Rms1 and Rms5 regulate the same novel signal. Plant Physiol. 126:

1205-1213; Ishikawa, S., et al. (2005). Suppression of tiller bud activity in tillering dwarf mutants of rice. Plant Cell Physiol. 46: 79-86 by permission of the Japanese Society of Plant

Physiologists. Simons, J.L., et al. (2007). Analysis of the DECREASED APICAL DOMINANCE genes of petunia in the control of axillary branching. Plant Physiol. 143: 697-706.

WT max3 WT rms5

Strigolactone-deficient

mutants in Arabidopsis,

pea, rice and petunia show

similar short, branchy

phenotypes

The MORE AXILLARY GROWTH3 (MAX3),

RAMOSUS5 (RMS5) , DWARF17 (D17) and

DECREASED APICAL DOMINANCE3 (DAD3)

genes all encode a carotenoid cleavage

dioxygenase (CCD7)

WT dad3

© 2015 American Society of Plant Biologists

SL biosynthesis pathway

carlactone

MAX3, D17, RMS5, DAD3:

MAX4, D10 ,RMS1, DAD1:

MAX1

STRIGOLACTONES

CCD7

CCD8

(P450)MAX; Arabidopsis

D; rice

RMS; pea

DAD; petunia

These

reactions occur

in the plastid

D27 (β-carotene-9-isomerase)

Umehara, M., Hanada, A., Yoshida, S., Akiyama, K., Arite, T., Takeda-Kamiya, N., Magome, H., Kamiya, Y., Shirasu, K., Yoneyama, K., Kyozuka, J., and Yamaguchi,

S. (2008). Inhibition of shoot branching by new terpenoid plant hormones. Nature 455: 195-200.; Seto Y, Kameoka H, Yamaguchi S, Kyozuka J. (2012) Recent advances

in strigolactone research: chemical and biological aspects. (in press). Alder, A., Jamil, M., Marzorati, M., Bruno, M., Vermathen, M., Bigler, P., Ghisla, S.,

Bouwmeester, H., Beyer, P., and Al-Babili, S. (2012). The path from β-carotene to carlactone, a strigolactone-like plant hormone. Science. 335: 1348-1351.

© 2015 American Society of Plant Biologists

D27 encodes an Fe-binding enzyme

necessary for SL synthesis

Lin, H., Wang, R., Qian, Q., Yan, M., Meng, X., Fu, Z., Yan, C., Jiang, B., Su, Z., Li, J. and Wang, Y. (2009). DWARF27, an iron-

containing protein required for the biosynthesis of strigolactones, regulates rice tiller bud outgrowth. Plant Cell. 21: 1512-1525.

Wild-type

(Shiokari) d27

Strigolactones are detected in

exudates of wild-type but not d27 roots

Standard

Wild-type exudate

d27 exudate

The rice D27 is a β-

carotene-9-isomerase also

found in other plants

© 2015 American Society of Plant Biologists

Rice SL biosynthesis mutants are

rescued by exogenous SL

Umehara, M., Hanada, A., Yoshida, S., Akiyama, K., Arite, T., Takeda-Kamiya, N., Magome, H., Kamiya, Y., Shirasu, K., Yoneyama, K.,

Kyozuka, J., and Yamaguchi, S. (2008). Inhibition of shoot branching by new terpenoid plant hormones. Nature 455: 195-200;

Control

2nd leaf tiller1st leaf tiller

1 c

m1 c

m

WT d10 d17

GR24(1 µM)

d10 and d17 are

rescued by

exogenous SL

(GR24 is a

synthetic SL)WT d10 d17

D17

D10

© 2015 American Society of Plant Biologists

WT max1 max3 max4

Control

GR24 (5 µM)

Arabidopsis SL biosynthesis mutants

are rescued by SL

Umehara, M., Hanada, A., Yoshida, S., Akiyama, K., Arite, T., Takeda-Kamiya, N., Magome, H., Kamiya, Y., Shirasu, K., Yoneyama, K.,

Kyozuka, J., and Yamaguchi, S. (2008). Inhibition of shoot branching by new terpenoid plant hormones. Nature 455: 195-200; Seto Y,

Kameoka H, Yamaguchi S, Kyozuka J. (2012) Recent advances in strigolactone research: chemical and biological aspects. (in press).

MAX1

MAX4

MAX3

© 2015 American Society of Plant Biologists

MAX1 encodes a P450 enzyme

involved in shoot branching

Reprinted from Booker, J., Sieberer, T., Wright, W., Williamson, L., Willett, B., Stirnberg, P., Turnbull, C., Srinivasan, M., Goddard, P., and Leyser, O. (2005). MAX1 encodes a cytochrome P450 family member that acts downstream of MAX3/4 to

produce a carotenoid-derived branch-inhibiting hormone. Developmental Cell 8: 443-449 with permission from Elsevier.

max1WT

MAX1 is

expressed

throughout the

plant, primarily

in association

with vascular

tissuesMAX1

© 2015 American Society of Plant Biologists

SL synthesis in root or shoot is

sufficient to control shoot branching

Booker, J., Sieberer, T., Wright, W., Williamson, L., Willett, B., Stirnberg, P., Turnbull, C., Srinivasan, M., Goddard, P., and Leyser, O. (2005). MAX1 encodes a

cytochrome P450 family member that acts downstream of MAX3/4 to produce a carotenoid-derived branch-inhibiting hormone. Developmental Cell 8: 443-449.

max3

WT

max3 Scion

Root

WT

max3

WT

Grafts

Reciprocal grafts, in which wild-

type tissue is either the root or

scion, have normal phenotypes;

this says that the branch-

controlling signal can be made in

either tissue, and can move from

root to shoot

© 2015 American Society of Plant Biologists

Booker, J., Sieberer, T., Wright, W., Williamson, L., Willett, B., Stirnberg, P., Turnbull, C., Srinivasan, M., Goddard, P., and Leyser, O. (2005). MAX1 encodes a cytochrome P450 family member that acts downstream

of MAX3/4 to produce a carotenoid-derived branch-inhibiting hormone. Developmental Cell 8: 443-449; Alder, A., Jamil, M., Marzorati, M., Bruno, M., Vermathen, M., Bigler, P., Ghisla, S., Bouwmeester, H., Beyer,

P., and Al-Babili, S. (2012). The path from β-carotene to carlactone, a strigolactone-like plant hormone. Science. 335: 1348-1351.

An intermediate between MAX4 and

MAX1 action can move in the plant

max4

max1

max1

max4

MAX1

STRIGOLACTONES

MAX4

In this experiment mobile, graft-

transmissible intermediate in SL

synthesis is produced in max1

roots, and converted into SL in

max4 shoots

Carlactone is a good

candidate as the SL mobile

signalling molecule

© 2015 American Society of Plant Biologists

Kretzschmar, T., Kohlen, W., Sasse, J., Borghi, L., Schlegel, M., Bachelier, J.B., Reinhardt, D., Bours, R., Bouwmeester, H.J. and

Martinoia, E. (2012). A petunia ABC protein controls strigolactone-dependent symbiotic signalling and branching. Nature. 483: 341-344.

WTdad1

(=max4)

PDR1 has been identified as a

strigolactone exporter

pdr1

SLs are present

in pdr1 root

extracts, but not

prd1 exudates,

supporting its

role as a

transporter

pdr1 mutants are

less colonized by

AM fungi, and

stimulate less

parasitic seed

germination

Loss-of-function pdr1 mutant show:

•increased shoot branching

•decreased exudation of SLs

•decreased symbiotic interactions

© 2015 American Society of Plant Biologists

The distribution of PDR1 is

consistent with its roles in transport

Reprinted from Sasse, J., Simon, S., Gübeli, C., Liu, G.-W., Cheng, X., Friml, J., Bouwmeester, H., Martinoia, E. and Borghi, L. (2015). Asymmetric localizations of the ABC transporter PaPDR1 trace paths of directional

strigolactone transport. Curr. Biol. 25: 647-655 and Ruyter-Spira, C., Al-Babili, S., van der Krol, S. and Bouwmeester, H. (2013). The biology of strigolactones. Trends Plant Sci. 18: 72-83 with permission from Elsevier.

SLs are transported

shootward and

outward from the root

© 2015 American Society of Plant Biologists

Synthesis and transport - summary

Alder, A., Jamil, M., Marzorati, M., Bruno, M., Vermathen, M., Bigler, P., Ghisla, S., Bouwmeester, H., Beyer, P., and Al-

Babili, S. (2012). The path from β-carotene to carlactone, a strigolactone-like plant hormone. Science. 335: 1348-1351.

SLs are derived from

carotenoids;

Early steps occur in

the plastids of root

and shoot

carlactone

MAX3, RMS5, D17, DAD3

MAX4, RMS1, D10, DAD1

MAX1

STRIGOLACTONES

(CCD7)

(CCD8)

(P450)MAX; Arabidopsis

RMS; pea

D; rice

DAD; petunia

D27 (β-carotene-9-isomerase)

Carlactone is an SL

intermediate (and may

be a mobile signal)

© 2015 American Society of Plant Biologists

Perception and signaling

Reprinted by permission from Macmillan Publishers Ltd: Zhou, F., et al and Wan, J. (2013). D14-SCFD3-dependent degradation of D53 regulates strigolactone signalling. Nature 504: 406-410.

1. D14 is an α/β-

fold hydrolase that

binds SLs

2. D3 is an F-box protein that promotes

interaction with the proteasome

3. The interaction between SLs, D14

and D3 leads to degradation of target

proteins including D53 (SMAX)

© 2015 American Society of Plant Biologists

1. D14 is an α/β-fold hydrolase that

binds SLs ¥

Loss-of-function

d14 mutants are

SL insensitive

and show

increased shoot

branching; the

orthologous

gene in petunia

is dad2

Arite, T., Umehara, M., Ishikawa, S., Hanada, A., Maekawa, M., Yamaguchi, S. and Kyozuka, J. (2009). d14, a strigolactone-insensitive mutant of rice, shows an accelerated outgrowth of tillers. Plant Cell Physiol. 50:

1416-1424; Hamiaux, C., Drummond, R.S., Janssen, B.J., Ledger, S.E., Cooney, J.M., Newcomb, R.D. and Snowden, K.C. (2012). DAD2 is an α/β hydrolase likely to be involved in the perception of the plant branching

hormone, strigolactone. Curr Biol. 22: 2032–2036

Genetic studies Biochemical

studies

D14 and DAD2

are α/β-fold

hydrolases

similar to the

GID1 protein

involved in

gibberellin

perception

© 2015 American Society of Plant Biologists

The α/β-fold hydrolase D14/DAD2

cleaves SL

Nakamura, H., Xue, Y.L., Miyakawa, T., Hou, F., Qin, H.M., Fukui, K., Shi, X., Ito, E., Ito, S., Park, S.H., Miyauchi, Y., Asano, A., Totsuka, N., Ueda, T., Tanokura, M., and Asami, T. (2013). Molecular mechanism of

strigolactone perception by DWARF14. Nat Commun. 4: 2613. Seto, Y., and Yamaguchi, S. (2014). Strigolactone biosynthesis and perception. Curr. Opin. Plant Biol. 21: 1-6 with permission from Elsevier.

Hydrolysis

© 2015 American Society of Plant Biologists

SL receptors D14/DAD2 are related

to the KAI2 karrikin receptors

Conn, C.E., Bythell-Douglas, R., Neumann, D., Yoshida, S., Whittington, B., Westwood, J.H., Shirasu, K., Bond, C.S., Dyer, K.A., and Nelson, D.C. (2015). Convergent evolution of

strigolactone perception enabled host detection in parasitic plants. Science 349: 540-543. Reprinted with permission from AAAS.

Karrikins are small

molecules present in

smoke that are

structurally similar to

SLs and can induce

seed germination

D14 probably arose from a duplication of KAI2

before the evolution of seed plants

© 2015 American Society of Plant Biologists

Parasitic Striga make highly-

sensitive SL receptors

From Toh, S., Holbrook-Smith, D., Stogios, P.J., Onopriyenko, O., Lumba, S., Tsuchiya, Y., Savchenko, A. and McCourt, P. (2015). Structure-

function analysis identifies highly sensitive strigolactone receptors in Striga. Science. 350: 203-207. Reprinted with permission from AAAS.

The SL receptor gene family

is amplified in StrigaShHTL7 encodes a highly

sensitive receptor protein

that confers germination

sensitivity to SLs in vivo

© 2015 American Society of Plant Biologists

2. D3, an F-box protein, promotes

interaction with the proteasome

GR24

(1 µM)

Control

d3WT

MAX2, D3 and

RMS4 encode F-box

proteins related to

those involved in

auxin and jasmonate

signaling

Auxin receptor

Jasmonate co-receptor

Umehara, M., Hanada, A., Yoshida, S., Akiyama, K., Arite, T., Takeda-Kamiya, N., Magome, H., Kamiya, Y., Shirasu, K., Yoneyama, K., Kyozuka, J., and Yamaguchi, S. (2008). Inhibition of shoot branching by new

terpenoid plant hormones. Nature 455: 195-200. Johnson, X., Brcich, T., Dun, E.A., Goussot, M., Haurogne, K., Beveridge, C.A., and Rameau, C. (2006). Branching genes are conserved across species. Genes

controlling a novel signal in pea are coregulated by other long-distance signals. Plant Physiol. 142: 1014-1026.

© 2015 American Society of Plant Biologists

In the presence of SLs, DAD2/D14

interacts with D3/MAX2

Hamiaux, C., Drummond, R.S., Janssen, B.J., Ledger, S.E., Cooney, J.M., Newcomb, R.D. and Snowden, K.C. (2012). DAD2 is an α/β hydrolase likely to be involved in the perception

of the plant branching hormone, strigolactone. Curr Biol. 22: 2032–2036 ;Nelson, D.C., Scaffidi, A., Dun, E.A., Waters, M.T., Flematti, G.R., Dixon, K.W., Beveridge, C.A.,

Ghisalberti, E.L. and Smith, S.M. (2011). F-box protein MAX2 has dual roles in karrikin and strigolactone signaling in Arabidopsis thaliana. Proc Natl Acad Sci USA. 108: 8897-8902.

SL concentration-

dependent interaction

between DAD2 (D14)

and MAX2 (D3)

Pro

tein

inte

raction

Model of SL signal transduction

Note the similarity of this model with those for auxin,

jasmonate, salicylate and gibberellin signalling

A current model is that SLs promote the interaction

between DAD2/D14 receptor and MAX2/D3,

leading to degradation of a signaling repressor

(D3)

© 2015 American Society of Plant Biologists

Karrikin signals are transduced in a

similar manner as SLs

Nelson, D.C., Scaffidi, A., Dun, E.A., Waters, M.T., Flematti, G.R., Dixon, K.W., Beveridge, C.A., Ghisalberti, E.L. and Smith, S.M. (2011). F-box protein MAX2 has dual roles in karrikin and strigolactone

signaling in Arabidopsis thaliana. Proc Natl Acad Sci USA. 108: 8897-8902.; Waters, M.T., Nelson, D.C., Scaffidi, A., Flematti, G.R., Sun, Y.K., Dixon, K.W. and Smith, S.M. (2012). Specialisation within

the DWARF14 protein family confers distinct responses to karrikins and strigolactones in Arabidopsis. Development. 139: 1285-1295.

By analogy to SLs, karrikin-

binding to KAI2 could promote its

interaction with MAX2 and the

degradation of repressor proteinsMAX2 is

needed for SL

and karrikin

signaling

© 2015 American Society of Plant Biologists

3. Genetic approaches identified

D53/SMXLs in SL signalling

Stanga, J.P., Smith, S.M., Briggs, W.R. and Nelson, D.C. (2013). SUPPRESSOR OF MORE AXILLARY GROWTH2 1 controls seed germination and seedling development in Arabidopsis. Plant Physiol. 163: 318-330.

Jiang, L., and Li, J. (2013). DWARF 53 acts as a repressor of strigolactone signalling in rice. Nature 504: 401-405. Reprinted by permission from Macmillan Publishers Ltd: Zhou, F. et al.,, and Wan, J. (2013). D14-

SCFD3-dependent degradation of D53 regulates strigolactone signalling. Nature 504: 406-410.

SMAX = Suppressor of MAX2

Loss-of-function smax1

mutants suppress SL

mutant phenotypes

Gain-of-function dominant d53 mutants show

a SL-resistant, branchy phenotype

© 2015 American Society of Plant Biologists

Zhou, F., Lin, Q., Zhu, L., Ren, Y., Zhou, K., Shabek, N., Wu, F., Mao, H., Dong, W., Gan, L., Ma, W., Gao, H., Chen, J., Yang, C., Wang, D., Tan, J., Zhang, X., Guo, X., Wang, J., Jiang, L., Liu, X., Chen, W., Chu,

J., Yan, C., Ueno, K., Ito, S., Asami, T., Cheng, Z., Wang, J., Lei, C., Zhai, H., Wu, C., Wang, H., Zheng, N., and Wan, J. (2013). Reprinted by permission from Macmillan Publishers Ltd: D14-SCFD3-dependent

degradation of D53 regulates strigolactone signalling. Nature 504: 406-410.

The D53 protein is a SL signaling repressor. D53 degradation by the proteasome depends on

interaction with the D14/DAD2 receptor bound to SL.

D14-SL and D3/MAX2 intermediate interaction between the D53 repressor and the SCF complex.

In Arabidopsis, D14 degradation is induced by SL via a MAX2-dependent proteasome

mechanism.

Model: Strigolactone promotes D14-

SCFD3-mediated degradation of the

repressor D53

© 2015 American Society of Plant Biologists

Reprinted by permission from Macmillan Publishers Ltd: Jiang, L., Liu, X., Xiong, G., Liu, H., Chen, F., Wang, L., Meng, X., Liu, G., Yu, H., Yuan, Y., Yi, W., Zhao, L., Ma, H., He, Y., Wu, Z., Melcher, K., Qian, Q.,

Xu, H.E., Wang, Y., and Li, J. (2013). DWARF 53 acts as a repressor of strigolactone signalling in rice. Nature 504: 401-405

• D14/DAD2 is an a/b hydrolyze

that binds SL, allowing

interactions with the F-box

protein D3/MAX2

• D3/MAX2 brings the SCF

complex and E2 ubiquitin ligase

to D53 for polyubiquitination, and

further degradation by the

proteasome

• D53 degradation leads to

transcriptional activation of SL-

target genes

Signaling summary

© 2015 American Society of Plant Biologists

Strigolactones and whole-plant

processes

Seto, Y., Kameoka, H., Yamaguchi, S., and Kyozuka, J. (2012). Recent advances in strigolactone research: chemical and biological aspects. Plant Cell Physiol. 53: 1843-1853 by permission of Oxford University Press.

Strigolactones

have diverse

roles in the

development of

vascular plants

and moss

This include

positive and

negative effects

© 2015 American Society of Plant Biologists

Strigolactones regulate shoot /root

branching and nutrient responses

Reprinted from Brewer, P.B., Koltai, H., and Beveridge, C.A. (2013). Diverse roles of strigolactones in plant development. Mol Plant. 6: 18-28 with permission of Elsevier.

SL mutants

produce more

shoot and

root branches

In nutrient

deficiency,

elevated SLs

repress shoot

branching and

promote root

hair elongation

© 2015 American Society of Plant Biologists

Strigolactones dampen polar auxin

transport (PAT)

Reprinted from Bennett, T., Sieberer, T., Willett, B., Booker, J., Luschnig, C., and Leyser, O. (2006). The Arabidopsis MAX pathway controls shoot branching by regulating auxin transport. Curr.

Biol. 16: 553-563 with permission from Elsevier; Crawford, S., Shinohara, N., Sieberer, T., Williamson, L., George, G., Hepworth, J., Müller, D., Domagalska, M.A., and Leyser, O. (2010).

Strigolactones enhance competition between shoot branches by dampening auxin transport. Development 137: 2905-2913 reproduced with permission.

SL-deficient plants show

increased polar auxin transport

Transported

auxin

Auxin

+SL

PIN

In shoots, strigolactones

promote internalization of PIN

auxin efflux transporters,

decreasing polar auxin transport

© 2015 American Society of Plant Biologists

How do

strigolactones

inhibit bud

outgrowth?

Goulet, C. and Klee, H.J. (2010). Climbing the branches of the strigolactones pathway one discovery at a time. Plant Physiol. 154: 493-496.

© 2015 American Society of Plant Biologists

Axillary bud outgrowth is hormonally

and environmentally responsive

McSteen, P. (2009). Hormonal regulation of branching in grasses. Plant Physiol. 149: 46-55; Brewer, P.B., Dun, E.A., Ferguson, B.J., Rameau, C., and Beveridge,

C.A. (2009). Strigolactone acts downstream of auxin to regulate bud outgrowth in pea and Arabidopsis. Plant Physiol. 150: 482-493.

(Axillla means armpit)

bud• Auxin and SLs

inhibit outgrowth

• Cytokinins (CKs)

promote it

© 2015 American Society of Plant Biologists

SL effect on branching can occur via

effects on polar auxin transport

Shinohara, N., Taylor, C., and Leyser, O. (2013) Strigolactone can promote or inhibit shoot branching by triggering rapid depletion of the auxin efflux protein PIN1 from the plasma membrane. PLoS Biol 11: e1001474.

© 2015 American Society of Plant Biologists

SL effects on shoot branching can

also be auxin-independent

Brewer, P.B., Dun, E.A., Gui, R., Mason, M.G. and Beveridge, C.A. (2015). Strigolactone inhibition of branching independent of polar auxin transport. Plant Physiol. 168: 1820-1829.

Transcriptional targets of

SLs include BRC1, a

transcription factor that

represses bud outgrowth

© 2015 American Society of Plant Biologists

SLs in whole plant responses -

summary

Reprinted from Smith, Steven M. and Waters, Mark T. (2012). Strigolactones: Destruction-

dependent perception? Curr. Biol. 22: R924-R927, with permission from Elsevier.

SLs have pleiotropic

effects on plant

development, mediated

in part by auxin and

other hormones

© 2015 American Society of Plant Biologists

Strigolactones promote branching in

arbuscular mycorrhizal fungi

Reprinted by permission from Macmillan Publishers Ltd: Akiyama, K., Matsuzaki, K.-i., and Hayashi, H. (2005). Plant

sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi. Nature 435: 824-827 copyright 2005.

SL

SL

SL

SL

Time Zero Time 24 hours

SLs promote

hyphal

branching

© 2015 American Society of Plant Biologists

When AM fungi perceive SLs, they

initiate symbiosis with the host plant

Reprinted by permission from Macmillan Publishers Ltd: Parniske, M. (2008) Arbuscular mycorrhiza: the

mother of plant root endosymbiosis. Nat. Rev. Microbiol. 6: 763 – 775 copyright 2008.

© 2015 American Society of Plant Biologists

Both partners benefit from the

symbiosisThe fungus gets sugars

produced by photosynthesis

The plant gets

nitrogen and

phosphorus

from the soil

by way of the

symbiotic

fungal

association

The arbuscule

provides a large

surface area for

nutrient

exchange

© 2015 American Society of Plant Biologists

0 500 1000 1500 2000 25005-deoxystrigol (pg/plant/5 days)

Low Mg

Low Ca

Low K

Low P

Low N

control

SL levels are elevated in sorghum

root exudates under low P and N

Courtesy of K. Yoneyama and adapted from Yoneyama, K., Xie, X., Kusumoto, D., Sekimoto, H., Sugimoto, Y., Takeuchi, Y., and Yoneyama, K. (2007). Nitrogen deficiency as well as phosphorus

deficiency in sorghum promotes the production and exudation of 5-deoxystrigol, the host recognition signal for arbuscular mycorrhizal fungi and root parasites. Planta 227: 125-132.

© 2015 American Society of Plant Biologists

–P+P

AM fungi

In nutrient-poor soils:

•SL synthesis increases

•Shoot branching decreases and

root branching increases

•AM symbiosis increases

These responses

enhance plant

survival under low

nutrient conditions

© 2015 American Society of Plant Biologists

Strigolactones promote germination

in parasitic and other plants

Umehara, M., Hanada, A., Yoshida, S., Akiyama, K., Arite, T., Takeda-Kamiya, N., Magome, H., Kamiya, Y., Shirasu, K., Yoneyama, K.,

Kyozuka, J., and Yamaguchi, S. (2008). Inhibition of shoot branching by new terpenoid plant hormones. Nature 455: 195-200 Dörr, I. (1997). How

Striga parasitizes its host: a TEM and SEM study. Annals of Botany. 79: 463-472, by permission of Oxford University Press;

O O

O OO

OH

Strigolactones were first

characterized as

inducers of Striga

germination (1960s)

© 2015 American Society of Plant Biologists

Striga species (witchweeds) are

serious agricultural pests

•Major cereal crops are infested: corn, sorghum, millet and rice

•70 million hectares are infested

•Food productions for 300 million people are affected

•Financial loss is estimated to be approximately 10 billion USD

•No effective control measure has been developed

Adapted from Ejeta, G. and Gressel, J. (eds) (2007) Integrating new technologies for striga control: towards

ending the witch-hunt. World Scientific Publishing, Singapore; Image sources: USDA APHIS PPQ Archive,

Florida Division of Plant Industry Archive, Dept Agriculture and Consumer Services.

Witchweed infestation

HeavyModerateLight

Striga asiatica

Striga hermonthica

© 2015 American Society of Plant Biologists

How can we move towards Striga-

resistant crops?

Agricultural practices:

•Field treatments

•Allelopathic approaches

Genetic approaches:

•Modify SL structure:

encrypt the signal

•Suppress branching in SL-

deficient plants

Image courtesy of International Institute of Tropical Agriculture (IITA)

© 2015 American Society of Plant Biologists

Orobanche (broomrape)-infested

carrot field

Photo credit Shmuel Golan courtesy of Yaakov Goldwasser

© 2015 American Society of Plant Biologists

Striga

asiatica

Image courtesy of Prof. Julie Scholes & Mamadou Cissoko

Rice field

infested

with Striga

© 2015 American Society of Plant Biologists

Parasitic plants infect

60% of farmlands in

sub-Saharan Africa

Image source USDA APHIS PPQ Archive

© 2015 American Society of Plant Biologists

Agricultural practices can reduce

crop losses from Striga

Photo courtesy Ken Hammond (USDA)

• Before planting, apply germination

stimulants to promote “suicidal

germination” (no host = no survival)

• Apply fertilizers to reduce SL

production by crop plants.

• These methods are prohibitively

expensive in many parts of the

developing world.

© 2015 American Society of Plant Biologists

Intercropping with beneficial plants

can reduce Striga infestation

Maize  intercropped with  Desmodium uncinatum

Desmodium is a nitrogen-fixing

legume that enriches the soil, and

also produces allelopathic chemicals

that interfere with Striga parasitism

Hassanali, A., Herren, H., Khan, Z.R., Pickett, J.A. and Woodcock, C.M. (2008). Integrated pest management: the push–pull approach for controlling insect pests and

weeds of cereals, and its potential for other agricultural systems including animal husbandry. Phil. Trans. R. Soc. B 363: 611-621 copyright 2008 The Royal Society.

Desmodium uncinatum

© 2015 American Society of Plant Biologists

Do all SLs promote Striga

germination and affect branching?

5-Deoxystrigol Orobanchol 2’-epi-orobanchol

Sorgomol SolanacolOrobachyl acetate

Strigyl acetateStrigolSorgolactone

Fabacyl acetate7-oxoorobanchyl

acetateOrobanchyl acetate

A BC

D

There are many

different naturally

occurring SLs –

how does their

structure affect

their function?

© 2015 American Society of Plant Biologists

Saturated (3,6'-dihydro-) GR24 isomers

do not induce Striga germination

O

O OO

O O

O OO

O O

O OO

O O

O OO

O

Isomer I Isomer II Isomer III Isomer IV

GR24

Enol etherO O

O OO

Isomers of saturated GR24

© 2015 American Society of Plant Biologists

…but an isomer of saturated GR24

promotes interactions with AM fungi

Adapted from Akiyama, K., Ogasawara, H., Ito, S. and Hayashi, H. (2010) Structural requirements of strigolactones for hyphal branching in AM fungi. Plant Cell Physiol., 51: 1104-1117 see also Boyer, F.D., de Saint

Germain, A., Pillot, J.P., Pouvreau, J.B., Chen, V.X., Ramos, S., Stevenin, A., Simier, P., Delavault, P., Beau, J.M. and Rameau, C. (2012). Structure-activity relationship studies of strigolactone-related molecules for

branching inhibition in garden pea: molecule design for shoot branching. Plant Physiol. 159: 1524-1544.

Can we make synthetic SLs with beneficial but not

detrimental effects? YES

Can we engineer plants to make these? MAYBE

AM

fungi Striga

+++ –O

O OO

O

Saturated

GR24

O O

O OO

+++ +++GR24

© 2015 American Society of Plant Biologists

Can we engineer Striga resistance?

Rice d10 SL-deficient mutant...

(1) Parasitism

(2) Symbiosis

Root parasite

(3) Shoot branching

O

O OO

H

O

O O

O OO

(1) No germination of Striga

seeds around the root

(2) Reduced AM fungi symbiosis

（but not fully inhibited）(3) Too many shoot branches

Are these effects separable?

AM fungi

© 2015 American Society of Plant Biologists

Can we engineer Striga resistance?

Rice d10 SL-deficient mutant...

(1) Parasitism

(2) Symbiosis

Root parasite

(3) Shoot branching

O

O OO

H

O

O O

O OO

Modify downstream component

（e.g. d10 suppressor mutants）

Can we normalize shoot branching

in SL-deficient mutants?

These experiments are in progress

with a goal to producing Striga-

resistant plants

(1) No germination of Striga

seeds around the root

(2) Reduced AM fungi symbiosis

（but not fully inhibited）(3) Too many shoot branches

AM fungi

© 2015 American Society of Plant Biologists

Summary (1) Perception and

signaling

de Saint Germain, A., Bonhomme, S., Boyer, F.D., and Rameau, C. (2013). Novel insights into strigolactone distribution and signalling. Curr Opin Plant Biol. 16: 583-589.

© 2015 American Society of Plant Biologists

Conclusions and future directions

AM fungi

O

O OO

H

O

O O

O OO

Strigolactones are synthesized in roots of

nutrient-limited plants

SLs enhance AM fungi symbiosis

SLs repress shoot branching and

adventitious root formation

SLs stimulate primary root growth, root hair

elongation, secondary growth, and leaf

senescence

© 2015 American Society of Plant Biologists

AM fungi

O

O OO

H

O

O O

O OO

Conclusions and future directions

How do SLs integrate with other hormones

and signals to control shoot, root branching

and leaf senescence?

Can we modify the SL biosynthesis pathway

to alter the types and activities of SLs

produced?

What are the evolutionary origins and

ancestral functions of SLs?