PbrMYB169 positively regulates lignification of stone cells in pear fruit

Stone cells negatively affect fruit quality because of their firm and lignified cell walls, so are targets for reduction in pear breeding programmes. However, there is only limited knowledge of the molecular mechanisms underlying the formation of stone cells. Here, we show that PbrMYB169, an R2R3 MYB transcription factor, of Pyrus bretschneideri positively regulates lignification of stone cells in pear fruit. PbrMYB169 was shown to be co-expressed with lignin biosynthesis genes during pear fruit development, and this co-expression pattern was coincident with stone cell formation in the fruit of Pyrus bretschneideri 'Dangshansuli'. The PbrMYB169 expression level was also positively correlated with stone cell content in 36 pear cultivars tested. PbrMYB169 protein significantly activated the promoter of lignin genes C3H1, CCR1, CCOMT2, CAD, 4CL1, 4CL2, HCT2, and LAC18 via binding to AC elements ACC(T/A)ACC in these promoters. Furthermore, overexpression of PbrMYB169 in transgenic Arabidopsis plants enhanced the expression of lignin genes, and increased lignin deposition and cell wall thickness of vessel elements, but did not change the ratio of syringyl and guaiacyl lignin monomers. In conclusion, PbrMYB169 appears to be a transcriptional activator of lignin biosynthesis and regulates secondary wall formation in fruit stone cells. This study advances the understanding of the regulation of lignin biosynthesis and provides valuable molecular genetic information for reducing stone cell content in pear fruit.