Progress towards elucidating the mechanisms of self-incompatibility in the grasses: further insights from studies in Lolium.

Abstract

Self-incompatibility (SI) in flowering plants ensures the maintenance of genetic diversity by ensuring outbreeding. Different genetic and mechanistic systems of SI among flowering plants suggest either multiple origins of SI or considerable evolutionary diversification. In the grasses, SI is based on two loci, S and Z, which are both polyallelic: an incompatible reaction occurs only if both S and Z alleles are matched in individual pollen with alleles of the pistil on which they alight. Such incompatibility is referred to as gametophytic SI (GSI). The mechanics of grass GSI is poorly understood relative to the well-characterized S-RNase-based single-locus GSI systems (Solanaceae, Rosaceae, Plantaginaceae), or the Papaver recognition system that triggers a calcium-dependent signalling network culminating in programmed cell death. There is every reason to suggest that the grass SI system represents yet another mechanism of SI. S and Z loci have been mapped using isozymes to linkage groups C1 and C2 of the Triticeae consensus maps in Secale, Phalaris and Lolium. Recently, in Lolium perenne, in order to finely map and identify S and Z, more closely spaced markers have been developed based on cDNA and repeat DNA sequences, in part from genomic regions syntenic between the grasses. Several genes tightly linked to the S and Z loci were identified, but so far no convincing candidate has emerged. From subtracted Lolium immature stigma cDNA libraries derived from S and Z genotyped individuals enriched for SI potential component genes, kinase enzyme domains, a calmodulin-dependent kinase and a peptide with several calcium (Ca(2+)) binding domains were identified. Preliminary findings suggest that Ca(2+) signalling and phosphorylation may be involved in Lolium GSI. This is supported by the inhibition of Lolium SI by Ca(2+) channel blockers lanthanum (La(3+)) and verapamil, and by findings of increased phosphorylation activity during an SI response.