Genome-wide identification of MYB family genes potentially related to SCW biogenesis in Korla pear fruit

The formation of fruits with a rough-skin is a physiological disorder in Korla pear (Pyrus sinki-angensis Yu) that is increasingly common in recent years. Compared with normal fruits, a main characteristic of such rough-skinned fruits is their greater stone cell content. These pear stone cells belong to the sclerenchyma formed by the secondary thickening of parenchyma cell walls, and MYB (myeloblastosis) family transcription factors (TFs) are known to have important functions during secondary cell wall (SCW) biosynthesis. Therefore, this work aimed to investigate MYB family genes in the pear genome and to identify SCW-related MYB family members in Korla pear fruits. A total of 193 PbMYBs, which were further divided into 11 clades, were identified in the pear genome. The gene phylogeny and a similarity and structure analysis showed that 33 PbMYBs were closely homologous to SCW-associated MYBs' in Arabidopsis. Gene expression analysis revealed that 14 of them were differentially expressed during the experimental time course (prime, late, and stationary stage of stone cell differentiation), being homologs to the SCW-related genes MYB20, MYB42, MYB52, MYB83, MYB85, and MYB103 in Arabidopsis. Expression pattern clusters indicated that for most of these MYBs expression levels peaked in the prime stage of stone cell differentiation, and then were gradually down-regulated in subsequent stages. Furthermore, RT-qPCR results showed that the pear homologs of AtMYB20,AtMYB42, AtMYB52, and AtMYB85 - downstream TFs of the SCW biosynthesis regulatory network - were significantly upregulated in either or both the peel and pulp of rough-skinned fruits. These results suggest these SCW-associated PbMYBs might play fundamental roles during the stone cell differentiation process in pear fruits.