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Plants have two major classes of self-incompatibilities (SI) systems: gametophytic and sporophytic. In the gametophytic incompatibility system, the pollen is rejected according to the gametophyte genotype. A pollen grain carrying an S1 allele will be rejected by an S1S2 style whereas pollen carrying an S3 allele will germinate. In the sporophytic system, pollen rejection occurs according to the genotype of the pollen parent. Pollen grain from an S1S3 plant, carrying either an S1 or S3 allele, will be rejected by an S1S2 papillar cell.
In a review article on, “Molecular Mechanisms of Self-Recognition in Brassica Self-Incompatibility” in October, 2000 issue of Trends in Plant Science (5(10):432-8.), Steven J. Rothstein and Norbert Brugière, currently at the Pioneer Hi-Bred International (Johnston, Iowa) and Yuhai Cui at Agriculture and Agri-Food (London, Canada) have discussed the present status of our knowledge of molecular mechanisms of sporophytic self-incompatibility in Brassica
The SLG (s-locus glycoprotein) and the SRK (S-locus receptor kinase), two highly polymorphic proteins, have been identified and characterized. The sequence variation of SRK apparently determines the S specificity. The two genes have base sequence similarity and are tightly linked at the S-locus. Their expression has only been detected in stigmas. In transgenics plants in which SLG no longer functions due to gene silencing, there is a breakdown of the SI phenotype, demonstrating that SLG might play a role in SI. However, recent studies showed that SLG is not necessary to determine SI specificity, as self-incompatible Brassica plants have been found to lack the SLG gene.
The authors describe how the rejection of the pollen occurs in the SI phenotype: the pollen coat protein (PCP) SCR (S locus cystein rich protein) from the pollen diffuses towards the papillar cell wall of the stigma. It is believed that binding of the SCR protein, the male determinant of the SI specificity, to the S-locus receptor kinase, the female determinant present on the papilla cell membrane, would trigger the phosphorylation of its intracellular kinase domain. This, in turn, would initiate a signal transduction cascade preventing pollen germination. The ARC1 (arm-repeat-containing) gene encodes the first downstream component of the pathway. Suppression of ARC1 expression by an antisense transgene partially breaks down the SI phenotype promoting seed set in otherwise self-incompatible Brassica plants.
Pollen adhesion on the stigma papillar cells is essential for the self-incompatibility process to occur. Two proteins, namely, SLG and SLR (S-locus related glycoproteins) are involved in this process. The role of SLR1′s in the process was studied using a transgenic approach. Brassica napus plants expressing an antisense construct of SLR1 show reduced levels of SLR1 transcripts and present a significant reduction of the pollen ability to adhere to the transgenic stigma. Several PCP proteins might play a role in the process of adhesion of the pollen to the stigma. Among them, a small defensin-like protein called PCP-A1 (PCP= pollen coat protein class A-1) interacts with SLG and SLR proteins. Interestingly, the pollen determinant of SI, SCR, is also a defensin-like protein. Recently SLR1-BP and SLR1-BP2, two other PCPs were shown to interact with SLR1 and SLG potentially playing a role in pollen adhesion.
Recent knowledge of the S specificity suggests that the small physical size of the S locus helped in the maintenance of this complex locus during evolution. A close relative of Brassica, Arabidopsis might have evolved to a self-incompatibility phenotype by the deletion of the self-recognition genes that characterize Brassica species.
Although a lot of ground remains to be covered to work out the molecular mechanisms of the SI phenomenon in its entirety, progress so far made in this field is impressive following the identification of some of the primary proteins involved in the process. SI in Brassica is an example of a rare two-gene system. In this system, the evolution of the two genes, SRK and SCR has proceeded simultaneously giving rise to as many as 80 different haplotypes showing characteristic self-recognition system between pollen and stigma.
The authors are of the opinion that the progress in this field will be quicker, given the variety of tools that are now available to study protein-protein interactions.
* S Haplotype = Occurrence of a large number of multiple alleles in the S locus of pollen determining compatibility response; the haplotype (specific allele) of the pollen will determine whether or not it will germinate on the stigma of the female parent.