PagBTB74 Negatively Regulates Secondary Wall Biosynthesis During Wood Formation in Populus alba × Populus glandulosa

Secondary cell wall (SCW) synthesis is a critical process of wood formation. Extensive studies have identified NAC and MYB transcription factors as key regulators of SCW thickening. Currently, no studies reveal the involvement of BTB/POZ proteins during wood formation. In this study, we investigated the role of a BTB/POZ protein during wood formation in Populus alba × Populus glandulosa. PagBTB74 overexpression (PagBTB74-OE) and dominant repression (PagBTB74-SRDX) transgenic plants exhibited narrowed xylem and thinner SCWs of xylem cells. Conversely, RNAi silencing (PagBTB74-RNAi) transgenic plants displayed opposite phenotypes with widened xylem and thickened walls. Integrated chromatin immunoprecipitation (ChIP), RNA-seq and yeast one-hybridisation (Y1H) analyses identified PagVDN6-C1, PagCesA7 and PagXTH4 as direct targets of PagBTB74. RNA-seq and effector-reporter assays showed that these three genes were downregulated by PagBTB74, consisting with the result that PagBTB74 had transcriptional repression activities. Our study represents the first report of a BTB transcription factor as a negative regulator of SCW biosynthesis during wood formation, and this regulation may occur through direct repression of the key genes of SCW biosynthesis, including PagVND6-C1, PagCesA7 and PagXTH4.