The proteome reveals the involvement of serine/threonine kinase in the recognition of self- incompatibility in almond.

The self-incompatibility recognition mechanism determines whether the gametophyte is successfully fertilized between pollen tube SCF (SKP1-CUL1-F-box-RBX1) protein and pistil S-RNase protein during fertilization is unclear. In this study, the pistils of two almond cultivars 'Wanfeng' and 'Nonpareil' were used as the experimental materials after self- and nonself/cross-pollination, and pistils from the stamen-removed flowers were used as controls. We used fluorescence microscopy to observe the development of pollen tubes after pollination and 4D-LFQ to detect the protein expression profiles of 'Wanfeng' and 'Nonpareil' pistils and in controls. The results showed that it took 24-36 h for the development of the pollen tube to 1/3 of the pistil, and a total of 7684 differentially accumulated proteins (DAPs) were identified in the pistil after pollinating for 36 h, of which 7022 were quantifiable. Bioinformatics analysis based on the function of DAPs, identified RNA polymerases (4 DAPs), autophagy (3 DAPs), oxidative phosphorylation (3 DAPs), and homologous recombination (2 DAPs) pathways associated with the self-incompatibility process. These results were confirmed by parallel reaction monitoring (PRM), protein interaction and bioinformatics analysis. Taken together, these results provide the involvement of serine/threonine kinase protein in the reaction of pollen tube recognition the nonself- and the self-S-RNase protein. SIGNIFICANCE: Gametophytic self-incompatibility (GSI) is controlled by the highly polymorphic S locus or S haplotype, with two linked self-incompatibility genes, one encoding the S-RNase protein of the pistil S-determinant and the other encoding the F-box/SLF/SFB (S haplotype-specific F-box protein) protein of the pollen S-determinant. The recognition mechanism between pollen tube SCF protein and pistil S-RNase protein is divided into nonself- and self-recognition hypothesis mechanisms. At present, two hypothetical mechanisms cannot explain the recognition between pollen and pistil well, so the mechanism of gametophytic self-incompatibility recognition is still not fully revealed. In this experiment, we investigated the molecular mechanism of pollen-pistil recognition in self-incompatibility using self- and nonself-pollinated pistils of almond cultivars 'Wanfeng' and 'Nonpareil'. Based on our results, we proposed a potential involvement of the MARK2 (serine/threonine kinase) protein in the reaction of pollen tube recognition of the nonself- and the self-S-RNase protein. It provides a new way to reveal how almond pollen tubes recognize the self and nonself S-RNase enzyme protein.

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