Genetics Behind Sexual Incompatibility in Plants: How Much We Know and What More to Uncover?

Self-incompatibility (SI) is an adaptive strategy in plants that increase genetic variability by promoting cross-pollination and reducing the chances of inbreeding depression. After pollination, the pistil can recognize genetically related (self) or different (non-self) pollen grains that fall on the stigma surface. Plants have adapted diverse biological mechanisms to reject self-pollens. The SI response of pollen is determined either by the genotype of the diploid parent plants that produce pollen (sporophytic self-incompatibility, SSI) or the genotype of the haploid microspores (gametophytic self-incompatibility, GSI). In either case, the incompatibility reaction is controlled by genes located in the multi-allelic S loci. In the event of SSI, the recognition of pollen by the stigma is rapid. Both hydration and germination of self-pollen are prevented at the stigma surface. On the other hand, in case of GSI, self- as well as non-self-pollens are initially allowed to germinate. However, only non-self-pollen can enter the ovary for fertilization; while growth of self-pollen is prevented in style. In SSI, a receptor kinase-based interaction between S LOCUS CYSTEINE RICH (SCR) and S LOCUS RECEPTOR KINASE (SRK) genes determines the incompatibility response. During GSI, the interaction between a pistil-specific S-RNase gene and the pollen determinant S locus F box ( SLF ) prevents the growth of self-pollen. In contrast, an additional GSI response is dependent on Ca 2+ -mediated receptor-ligand signaling. This review discusses on important genes related to GSI and SSI, their evolutionary divergence caused by duplication, and targeted editing of SI/SC-associated genes converting SI plants to SC and vice versa. It also illuminates the genetics of GSI in Poaceae and presents experimental challenges associated with it. Furthermore, emerging scopes for further research for crop improvement with special emphasis on genome editing are discussed.