Phylogenetic and Expression Analyses of Cullin Family Members Unveil the Role of PbCUL1.C1 in Pollen Tube Growth Underlying Non-self S-RNase in Pear

Cullin ( CUL ) gene family organizes the largest class of RING E3 ligases which play crucial roles in the ubiquitin-proteasome system to affect plant physiological activities. However, little is known about the CUL genes in fruit trees. In this study, 57 CUL genes were isolated from the sequenced genomes of seven Rosaceae trees and were divided into three subfamilies according to phylogenetic analyses. Evolutionary analysis indicated that multiple duplication events play important role in the expansion of the CUL gene family in Rosaceae . The expression pattern shows that only four CUL genes were highly expressed in pollen and pollen tube in pear. Of these CUL genes, PbCUL1.C1 presented the highest levels of expression in pollen, but the knockdown of PbCUL1.C1 by antisense oligonucleotide (ASO) did not influence pollen tube growth. Interestingly, the pollen tube treated by both the non-self S-RNase and ASO of PbCUL1.C1 was longer than that treated by unique non-self S-RNase, suggesting that the PbCUL1.C1 plays an important role for pollen tube growth underlying non-self S-RNase in pear. These results will be useful for exploring the biological roles of CUL genes in self-incompatibility in Rosaceae trees.