strigolactones: role in plant development

Welcome to

Seminar-1

• Ravi Kumar

• PGS14AGR6523

STRIGOLACTONES: Role In Plant Development

Contents

1. Introduction

2. What are strigolactones

3. Strigolactone biosynthesis

4. Strigolactone with root parasitic plant

5. Strigolactone with AM fungi.

6. Strigolactones in suboptimal conditions.

7. Strigolactones in root development.

8. Additional functions of Strigolactones.

9. Future issues

10. Conclusion

Hormones are chemicals produced in one part of an organism and

transported to another part where they exert a response.

Only a small amount of hormone is required to alter cell metabolism.

Cells respond to a hormone when they express a specific receptor for that

hormone

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Introduction

The hormone binds to the receptor protein, resulting in the activation of a

signal transduction mechanism that ultimately leads to cell type-specific

responses.

Hormones may act differently on different cell types.

They play critical roles during all developmental stages in plants, from

early embryogenesis to senescence.

plant hormone families

Auxins

Giberellins

Cytokinins

Ethylene

Abscisic Acid

Brassinosteroids

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Hormone Where produced/found Major Functions

Auxin Shoot apical meristems and

young leaves

Stimulates stem elongation

Cytokinins Roots Regulate cell division

Gibberellins Meristems of buds and roots Stimulates stem elongation,

reproduction

Brassinosteroids All tissue Promote cell expansion

Abscisic acid

(ABA)

All tissue Inhibits growth

Ethylene All tissue Promotes ripening of fruits

Other signaling molecules (hormones)

Strigolactone. (recently identified)

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What are strigolactones?

Strigolactones (SLs) constitute a new class of plant hormones which are

active as germination stimulants for seeds of parasitic weeds of Striga,

Orobanche, and Pelipanchi spp, in hyphal branching of arbuscular

mycorrhizal (AM) fungi and as inhibitors of shoot branching.

Strigolactones are signaling compounds made by plants.

They have two main functions:

1.Endogenous hormones to control plant development.

2.Components of root exudates to promote symbiotic interactions between

plants and soil microbes.

Some plants that are parasitic on other plants have established a third

function,

3.which is to stimulate germination of their seeds when in close proximity to

the roots of a suitable host plant.

It is this third function that led to the original discovery and naming of

strigolactones.

DISCOVERY OF STRIGOLACTONES

The first naturally occurring germination stimulant for Strigolactones was

isolated as early as 1966 from root exudates of cotton (Gossypium hirsutum

L.), which, is not a host for Striga or Orobanche (Cook et al., 1966).

Strigolactones were also discovered in root exudates due to their ability to

stimulate germination of seeds of the parasitic plant Striga, the ‘witch

weed’in sorghum[Sorghum bicolor (L.) Moench], maize (Zea mays L.), and

common millet (Panicum miliaceum L.)

Plants of the witch weed Striga hermonthica parasitizing maize plants

in Africa.

www.biomedcentral.com

Molecular Formula

C17H14O5

Strigolactones (SLs) are a group of terpenoid lactones in which a

tricyclic lactone (ABC ring) and a methyl butenolide (D ring) are

connected by an enol ether bridge

Strigolactone Biosynthesis

Lopez-Raez et al., (2008)

Strigolactone biosynthesis and signaling pathway

Strigolactones are derived from the carotenoids through two subsequent

enzymatic cleavage steps performed by the carotenoid cleaving dioxygenases

CCD7 and CCD8 described in Arabidopsis (MAX), Rice (D/HTD), pea (RMS)

and petunia (DAD).

For further conversion into strigolactones, a cytochrome P450 enzyme (cyp450)

has been reported to be required in Arabidopsis (MAX1) while two other

proteins, identified in rice (D27) and pea (RMS2) whose functions have not yet

been clarified, must also be involved in strigolactone biosynthesis.

In the tomato mutant Sl-ORT1, in which the mutation is still unidentified,

strigolactone biosynthesis and CCD7 expression are both down-regulated.

The perception of strigolactones and/or the downstream signaling pathway

are mediated by an F-box leucine rich protein (MAX2, D3, RMS4), and/-

hydrolase found in rice (D14) and a protein of unknown function in pea

(RMS3).

Biosynthesis of Strigolactones

Genes involved in the synthesis or signaling pathway of strigolactone in different species

STRIGOLACTONES GERMINATION STIMULANTS

FOR

ROOT PARASITIC PLANTS

Lopez-Raez, et al., 2008

Root parasitic plant species and their life cycle

Plant species of the genera Striga, Orobanche and Phelipanche are root parasites that affect many important food crops.

These three genera, belonging to the Orobanchaceae family.

Striga species have an absolute host requirement to develop and complete their life cycle.

Striga spp. occur in tropical and subtropical regions, with special incidence in Sub-Saharan Africa infecting mainly cereal crops.

The most damaging is Striga hermonthica, followed by Striga asiatica and Striga gesnerioides.

The broomrapes, of the genera Orobanche and Phelipanche, cause similar damage to food production as the Striga spp.

The life cycles of Striga, Orobanche and Phelipanche spp. are many similarities.

All of them produce minute seeds that remain viable in the soil for up to fourteen

years.

Once germination has been triggered, the radicle produces from the testa,

elongates towards the host root and develops a haustorium, an organ that can

attach to and penetrate the roots of the host plant.

In Striga spp., the haustorium establishes a xylem–xylem connection with the

host from where it can with draw water and nutrients.

Phelipanche and Orobanche spp. form connections with both phloem and xylem.

Once attached to the host and having established a connection with the vascular

system, the parasites can take up nutrients, water, and possibly also hormones

from the host.

Root Parasitic Plants

Life cycle of a root parasitic plant, Orobanche minor. (a) Seed germination is elicited by host-

derived stimulants, including strigolactones. (b) Seedling attaches to host root with haustoria. (c–

d ) Parasite tubercles grow underground for several weeks or months before emergence of the

flowering shoots. (e) The parasite produces a large number of seeds, which remain viable for

many years in soil.

STRIGOLACTONES AS BRANCHING FACTORS

FOR

ARBUSCULAR MYCORRHIZAL FUNGI

Zwanenburg, et al.,2013

Branching Factors for AM Fungi

Why plants exude SLs that enable them to be located by their enemies.

because these compounds may have other roles that outweigh the

potential risks of parasitism.

Such a beneficial role of SLs was unveiled through the discovery that these

compounds function as branching factors for symbiotic AM fungi, from

which the plants benefit.

AM fungi are soil borne microorganisms that form symbiotic

associations with the majority of land plants.

The symbiotic interaction between plants and the AM fungi is initiated by

mutual exchange of signaling molecules.

Under appropriate temperature conditions, the spores of AM fungi germinate,

and their hyphae elongate to a limited extent.

If a host root is not present, the hyphae stop growing and become inactive.

In the presence of host roots, AM hyphae differentiate into specific

morphological structures characterized by extensive branching.

These fungi penetrate and colonize plant roots, where they develop

highly branched structures called arbuscules, which are the sites of

nutrient exchange.

The fungi supply their hosts with water and nutrients, primarily

phosphate and nitrogen that are obtained through the hyphae that

reside outside in the soil, and in turn the AM fungi receive

photosynthates(carbohydrates) from their host plants.

AM symbiosis also give up the plant more tolerant to biotic and

abiotic stresses.

Arbuscular mycorrhizae in clover root, showing arbuscules (blue arrows) within

the root cortex, and hyphae radiating from the root surface.

Auxin is transported from the apical meristem while cytokinin and strigolatone are

transported from the roots (shown by black arrows). Red bars show repression while

blue arrows show activation. Lateral buds have the potential to grow into side shoots

when the hormone balance permits, and can be achieved experimentally by removing

the apical meristem.

The strigolactone-deficient mutant of Arabidopsis shows exuberant branching. The

wild-type plant (left) has few secondary shoots compared to the mutant (right).

Strigolactones help plants to cover with suboptimal

conditions

Brewer, et al., 2013

when the plant encounters certain difficulties in the environment, such as

suboptimal nutrient availability, strigolactone levels rise in order to

optimize and adapt the plant’s growth strategy to fit the conditions. (Umehara et al., 2008; Kohlen et al., 2011).

The most extensively studied, strigolactone-related, suboptimal plant

growth condition is phosphate deprivation.

Strigolactone levels increase in red clover grown under low phosphate

conditions (Yoneyama et al., 2007).

In red clover plant role of SL elevation in response to Pi deficiency. (Left) When Pi is sufficient, plants can

absorb Pi through their roots. SL levels are low in roots, and outgrowth of axillary buds is not inhibited.

Black arrows indicate the flow of Pi, which is distributed from the roots to the leaves, outgrowing axillary

buds, and apical meristem. (Right) Under low-Pi conditions, SL levels in roots are highly elevated, inhibiting

bud outgrowth in shoots (1). Senescence of old leaves is activated (2), nutrients such as Pi are translocated

and concentrated within the apical meristem and young tissues (3), and supplied by AMF in soil (4). Root-

parasitic plants might respond to SLs to detect potential hosts (5). Black arrows indicate the flow of Pi; gray

arrows indicate the flow and action of SLs.

STRIGOLACTONES IN ROOT

DEVELOPMENT

Brewer, et al., 2013

Under optimal growth conditions, strigolactones repress lateral root

formation(Kapulnik et al., 2011a; Ruyter-Spira et al., 2011) and promote root

hair elongation(Kapulnik et al., 2011a).

In the case of lateral root formation, auxin is a key regulator and the distribution

of auxin determines lateral root positioning, initiation, and elongation (reviewed

by De Smet (2012).

Strigolactones may affect lateral root formation via changes in auxin efflux in

the root.

Similarly, cytokinins can negatively affect lateral root formation, which is

suggested to be due to interference with auxin transport in lateral root parts

(Bishopp et al., 2011). Hence, in the case of lateral roots, strigolactones and

cytokinins may be considered to act similarly, in the sense that both may

influence auxin distribution.

The Proposed Roles of Strigolactones in Adult Plant Growth and Development.

(A) Under normal conditions, a basal level of strigolactone production in a wild-type plant reduces

lateral shoots and roots, but enhances plant height, secondary growth, senescence, and root hairs.

(B) Much of this influence of strigolactones can be seen in mutants that are unable to make or respond

to strigolactones. They display more lateral branches and lateral roots, and less secondary growth and

arbuscular mycorrhizal (AM) fungi symbiosis (in compatible species).

(C) Reduced phosphate triggers increased strigolactone production. This leads to greater branch

repression and, initially, to enhanced lateral roots and root hairs, and enhances AM fungi symbiosis.

Other phenotypes like plant height, secondary growth, senescence, especially with regard to low-phosphate-

induced strigolactone production.

Additional Functions of Strigolactones

Seto, et al., 2012

SLs control a wide range of developmental features. In vascular plants, SLs positively regulate secondary growth (A), leaf senescence (B),

root hair elongation (C) and primary root growth (D), while they negatively regulate axillary bud growth (E) and adventitious root formation (F).

SLs negatively regulate chloronemata branching (G) and colony extension (H) in the moss Physcomitrella patens. Red arrows indicate positive

effects. T-bars in blue depict negative effects.

Effect of GR24, a synthetic analogue of strigolactones, on gene

expression of solopathogenic strain of Sporisorium reilianum

S. K. Sabbagh., (2011)

• Sporisorium reilianum f.sp zeae a soilborne basidiomycetous fungus belonging to Ustilaginaceae, is the causal agent of maize head smut.

• showed that strigolactones rapidly trigger O2 consumption of the fungi.

• effects of GR24 on some genes involved in cell respiration and pathogenicity of U. maydis, a fungus phyllogeniticaly related to Sporisorium reilianum.

• At 1 h post supplementation of GR24, all genes involved in cell respiration were induced.

• Cell respiration increased a bit at 5 h, and was null at 8 h post supplementation of GR24.

• No induction of genes involved in cell respiration was observed in those times, but an induction of actin and putative 12 kDa heat shock protein was observed.

• The quantitative analysis of cells induced at 8 h post supplementation of GR24 did not show any increase in candidate genes transcripts.

Strigolactones are positive regulators of light-harvesting

genes in tomato

Mayzlish-Gati, et al., 2010

Illustration of the light reaction-associated biological pathways in which differentially regulated

genes putatively participate.Differentially regulated genes were identified from hybridization

data of roots exposed to GR24 and IAA treatments [IAA (10_8 M), IAA(10_8 M)+GR24 (13.5

lM), and GR24 (27 lM)] versus non-treated controls. Blue or red squares represent individual

genes. The colourwithin the squares represents fold change in gene expression in treatments

versus controls; values of fold change are as indicated in thecolour scale. Chl signifies

chlorophyll.

FUTURE ISSUES

1. SLs, which are exuded by plant roots, serve as branching factors for

beneficial AM symbionts but at the same time also trigger the germination

of root parasites. So far there is no knowledge on whether there is a

possibility to manipulate SL biosynthesis, metabolism, and exudation in a

way that will avoid or inhibit the germination of root parasitic weeds

without effecting AM symbiosis.

2. The diverse functions of SLs, hint that this group of compounds may also

have other functions in plant metabolism and development. Are SLs

involved in the regulation of growth and development of other plant

organs? Are SLs involved in the regulation of the reproductive organs? Are

SLs involved in the regulation of leaf initiation and development?

CONCLUSION

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