Molecular basis of Self Incompatibility In Crop Plants GOWTHAMI. R 13-611-102
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