MiR319a-Targeted PtoTCP20 Regulates Secondary Growth via interactions with PtoWOX4 and PtoWND6 in Populus tomentosa.

Secondary growth is a key characteristic of trees, which requires the coordination of multiple regulatory mechanisms including transcriptional regulators and microRNAs (miRNAs). However, the roles of microRNAs in the regulation of secondary growth need to be explored in depth. Here, the role of miR319a and its target,PtoTCP20, in the secondary growth ofPopulus tomentosastem was investigated using genetic and molecular analyses. The expression level of miR319a gradually decreased from primary to secondary growth inP. tomentosa, while that ofPtoTCP20gradually increased. MiR319a overexpression in seedlings resulted in delayed secondary growth and decreased xylem production, while miR319a knockdown andPtoTCP20overexpression promoted secondary growth and increased xylem production. Further analysis showed that PtoTCP20 interacted with PtoWOX4a and activatedPtoWND6transcriptionin vitroandin vivo. Our data show that PtoTCP20 controls vascular cambium proliferation by binding to PtoWOX4a, and promotes secondary xylem differentiation by activatingPtoWND6transcription, thereby regulating secondary growth inP. tomentosa. Our findings provide insight into the molecular mechanisms underlying secondary growth in trees.