Characterization and expression analysis of PbEXP genes in the epidermis of pear (Pyrus bretschneideri Rehd.)

In the present study, "Dangshansuli" (wild type, WT) and its russet mutant "Xiusu" (mutant type, MT) were used to characterize the PbEXPs in "Dangshansuli" as well as the role of IAA in mediating the expression of PbEXPss. Forty-one expansins were identified in WT and divided into EXPA, EXPB, EXLA and EXLB families. In the same EXP family, they shared similar exon-intron structure and phylogenetic characters. Eight of 41 expansin genes, screened from the transcriptome of WT and its russet mutant "Xiusu", exhibited differential expression patterns in the epidermis of both WT and MT with the development of pear fruit. In addition, the correlation coefficients between IAA content and expression of expansins of PbEXPB3b and PbEXP4a in MT were 0.763 and 0.762, respectively. After treatments with IAA, NPA and AVG, the expression levels of PbEXPB3b and PbEXP4a changed significantly 15, 30, or 45 days after full bloom (DAFB). The longitudinal and vertical diameters of the epidermis of WT and MT at different stages were measured. We found that high IAA levels enhanced the expression of PbEXPB3b and PbEXP4a in epidermis of MT at the russet turning stage of 75-100 DAFB, which promoted cell expansion and microcrack occurrence and led to russet on the pear fruits.