Molecular basis of Self Incompatibility In Crop Plants

Molecular basis of Self Incompatibility In Crop Plants

GOWTHAMI. R13-611-102

IntroductionMost flowering plants being bisexual have

elaborate genetic mechanisms of avoiding self fertilization and thereby preventing inbreeding depression

SI is one such natural out breeding system where self recognition and rejection is the rule.

It is based on allele specific- interaction between stigma receptors and pollen ligand that results in the arrest of pollen tube development

Researchers are unraveling the molecular mechanisms that are involved in self-incompatibility

The most common anti-selfing mechanism in flowering plants is known as self-incompatibility, the ability of a plant to reject its own pollen.

Distribution of self incompatibility

• To date Nearly 6000 species including 250 genera from 70 families representing 19 orders of both monocots and dicots have been reported to exhibit SI.

Self incompatibility

Heteromorphic(Primula)

Homomorphic

Gametophytic(Solanaceae)

Sporophytic(Brassicaceae)

S1S2 Anther S1S2 PistilIncompatibl

e

S1S3 PistilSemi-compatible

S3S4 PistilCompatible

Pollen

Anther

Stigma

Style

Ovary

Ovule

A. Gametophytic Self-incompatibility

S1S2 PistilIncompatibl

e

S1S3 PistilIncompatible

S3S4 PistilCompatible

S1S2 Anther

B. Sporophytic Self-incompatibility

Table:1Characteristics associated with the three main classes of homomorphic SI

Mofactorial GSI Bifactorial GSI Monofactorial SSI

Pollen phenotype Haploid Haploid Diploid

Controlled by Genotype genotype Genotype

No. of genes 1 2 1

Allelic state Multiallelic multiallelic multiallelic

Stigma type Wet Dry dry

Inhibition site Style Stigma Stigma

Pollen nuclei Binucleate trinucleate Trinucleate

Pollen germination slow Fast fast

Molecular basis of S- locus With classical genetics S-locus was assumed

to be a single gene but after 1987 molecular studies revealed S-locus to be much more complex

S-locus glycoprotein (SLG) gene which encodes a secreted glycosylated protein (Nasrallah et. al., 1987)

SRK gene which encodes a receptor protein at stigmatic surface (Steln et al,1991)

S-locus cysteine rich protein (SCR) gene (Schopfer et al ,1999)

expression of these genes are temporally (anthesis) and spatially (Stigma) regulated

Schematic Drawing Of S-Locus

Table:2 Schematic drawing of the S-locus and a list of the identified female and male determinant genes

Family Types of SI Male determinant

Female determinant

Brassicaceae SSI SP11/SCR SRK

Solanaceae, Rosaceae, Scrophulariaceae

GSI SLF/SFB S-RNase

Papaveraceae GSI unknown S-protein

(Takayama et al.,2005)

Models for S-haplotype-specific inhibition of pollen tube growth and competitive interaction

(A) Simple inhibitor model (B) Modified inhibitor model

(C) Competitive interaction based on the modified inhibitor modelKao et al., 2004

SI in Brassicaceae

SRK, the female determinant spans the plasma membrane of the stigma papilla cell.

SP-11 male determinant expression occurs in anther tapetum.

Upon pollination SP-11 binds SRK in an S- haplotype specific manner.

Binding induces auto- phosphorylation of SRK, triggering a signal cascade resulting in rejection of self pollen

SLG enhances the SI reaction in some S -haplotypes Positive effectors MLPK, ARC-1. Proteasomal degradation.

SRK (S-locus receptor kinase)

• The female determinant of SSI• Encodes allelic forms of a receptor serine/

threonine receptor domain• Expressed in the epidermal cells (papillae) of

the stigma• Transgenic gain of function mutation

experiments showed that SRK alone determine SI specificity and its ability is enhanced by SLG

(Takayama et al.,2005)

SLG (S-locus glycoprotein) gene

• The first S-linked gene identified in Brasssica

• Encodes a stigma specific glycoprotein localized to cell wall of papilla cells

• It shares as much as 98% nucleotide sequence identity with SRK

• Loss of function experiments showed that SLG is not essential for haploid specific pollen recognition even though presence of SLG enhance SRK response

(Takasaki et al.,2000)

Molecular model of the SI in Brassicaceae

GSI in the Solanaceae Tobacco (Nicotiana), Petunia, tomato

(Lycopersicon) Inheritance as a single locus (S) with multiple

alleles (S1, S2, S3….) Molecular studies revealed at least 2 tightly loci Stylar S gene product has ribonuclease activity

(S-RNase) SLF/ SFB is the male determinant and a member

of the F-Box family of proteins SLF/SFB is expected to be involved in ubiquitin

mediated protein degradation of non self S-RNases

Molecular model of the SI in Solanaceae

Receptor or Gatekeeper ModelCell

wallCell wall

Inhibitor ModelCell

wallCell wall

SI in Papaveraceae

Female determinant gene↓

Secrete stigma protein↓

Interaction with the assumed S- haplotype specific pollen receptor

↓Induction of Ca+2 influx

↓Triggering of downstream signaling cascades

↓Rapid growth inhibition and ultimately PCD of

incompatible pollen tubes

Molecular model of the SI in Papaveraceae

Molecular model of the SI in Papaveraceae

Molecular model of the SI response in the Papaveraceae

Franklin-Tong et al., 2008

Conclusions Plant Family Typ

e of SI

Female Determinant

Male Determinan

t

Mechanism

Solanaceae, Rosaceae,Scrophulariaceae

GSI S-RNase

SLF/SFB

S-RNase–mediated degradation of pollen tube RNA

Papaveraceae GSI S-gene PrpSCa2+–mediated signaling cascade in pollen

Brassicaceae SSI SRK SCR/SP11Receptor-kinase-mediated signaling in stigma

Summary In many species, the specificity of the SI

response is determined by the haplotypes of the S-locus, which contains at least two separate multiallelic genes, the female and male determinant genes

SI does not represent one system, but rather a collection of divergent mechanisms, suggesting that SI evolved independently in several lineages

In Brassicaceae the determinant genes encode a pollen ligand and its stigmatic receptor kinase and their interaction induces incompatible signaling within the stigma papilla cells

Cont… In the Solanaceae, Rosaceae, Scrophulariaceae,

the determinants are a ribonuclease and an F-Box protein, suggesting the involvement of RNA and protein degradation within the system

In the Papaveraceae, the only identified female determinant induces a Ca2+ dependent signaling network that ultimately results in the death of incompatible pollen