Understanding the Role of Mitogen-activated Protein Kinases During Pollen-pistil Interaction

Abstract

Pollination involves a complex interplay of molecular communication and signaling between pollen grains and stigmatic papillary cells which decides the fate of the pollen that lands on the stigma. In Brassicaceae, a number of pollen and stigma proteins have been identified that are required for successful pollination. In spite of this, the molecular signaling mechanism behind, how these proteins coordinate pollination responses has remained unknown. By using various molecular, biochemical and cell biological approaches, I have identified a functionally redundant MAPK cascade that phosphorylates an important compatibility factor, to mediate successful pollination responses in Arabidopsis. Molecular genetic analysis revealed that two Arabidopsis mitogen-activated protein kinases regulate successful pollination in stigma. Double mutant of these two MAPKs exhibited substantial defects in pollination responses in naturally pollinated stigmas. Significant reduction in pollen adherence, germination and seed set was observed in these plants. Interestingly, contrary to our expectations of larger gene families associated with increased functional redundancy, the MAPK kinases (MKK) family with the least number of genes, displayed extreme functional redundancy during pollination responses. Five MKKs from three different clades were required to regulate the activity of downstream MAPKs in the stigmas to mediate compatible pollination. Loss of function of these five MKKs in the quintuple mutant phenocopied the same pollination phenotype observed in the mpk double mutant. Through utilizing biochemical and cell biological approaches, I was also able to show phosphorylation of a downstream substrate by the activated MAPKs. This phosphorylation was required for successful pollination to occur as loss of the phosphorylation motif resulted in inability of the protein to function in the compatibility pathway. bTaken together, through this study, I have been able to identify a signaling pathway in the stigma that uses MAPK phosphorylation cascade to regulate a downstream compatibility factor for successful pollination to occur. Given the agronomic importance of various crop plants that belong to Brassicaceae (canola, rape seeds, kale, broccoli, etc.), understanding the signaling mechanism during compatible pollination responses could lead to identifying strategies to improve crop yield.