Genome-Wide Analysis of the RNase T2 Family and Identification of Interacting Proteins of Four ClS-RNase Genes in ‘XiangShui’ Lemon

S-RNase plays vital roles in the process of self-incompatibility (SI) in Rutaceae plants. Data have shown that the rejection phenomenon during self-pollination is due to the degradation of pollen tube RNA by S-RNase. The cytoskeleton microfilaments of pollen tubes are destroyed, and other components cannot extend downwards from the stigma and, ultimately, cannot reach the ovary to complete fertilisation. In this study, four S-RNase gene sequences were identified from the ‘XiangShui’ lemon genome and ubiquitome. Sequence analysis revealed that the conserved RNase T2 domains within S-RNases in ‘XiangShui’ lemon are the same as those within other species. Expression pattern analysis revealed that S(3)-RNase and S(4)-RNase are specifically expressed in the pistils, and spatiotemporal expression analysis showed that the S(3)-RNase expression levels in the stigmas, styles and ovaries were significantly higher after self-pollination than after cross-pollination. Subcellular localisation analysis showed that the S(1)-RNase, S(2)-RNase, S(3)-RNase and S(4)-RNase were found to be expressed in the nucleus according to laser confocal microscopy. In addition, yeast two-hybrid (Y2H) assays showed that S(3)-RNase interacted with F-box, Bifunctional fucokinase/fucose pyrophosphorylase (FKGP), aspartic proteinase A1, RRP46, pectinesterase/pectinesterase inhibitor 51 (PME51), phospholipid:diacylglycerol acyltransferase 1 (PDAT1), gibberellin receptor GID1B, GDT1-like protein 4, putative invertase inhibitor, tRNA ligase, PAP15, PAE8, TIM14-2, PGIP1 and p24beta2. Moreover, S(3)-RNase interacted with TOPP4. Therefore, S(3)-RNase may play an important role in the SI of ‘XiangShui’ lemon.