The PtoERF1-PtomiR393a-PtoFBL4 module confers drought tolerance via ABA-auxin crosstalk in Populus.

Drought stress limits forest tree growth and adaptation, with xylem vessels critical for hydraulic transport and structural integrity. However, the molecular mechanisms of abscisic acid (ABA)-auxin interaction in regulating vessel morphogenesis under water-deficit conditions remain unclear. Here, we identified PtomiR393a, a drought-responsive microRNA in Populus tomentosa that mediates crosstalk between ABA and auxin signaling pathways under drought stress. Suppressing PtomiR393 enhanced drought tolerance and growth, whereas its overexpression had the opposite effect. Under drought conditions, suppression of PtomiR393 resulted in reduced vessel size (12.18-13.57%) and increased vessel density (27.22-30.14%), while its overexpression exhibited increased vessel size (15.42-16.01%) and reduced vessel density (19.80-20.62%). Functional assays showed that PtomiR393 specifically targets PtoFBL4, an F-box auxin receptor, modulating auxin signaling in response to drought stress. Expression analyses further revealed that PtomiR393 downregulates genes involved in vessel and fiber formation and secondary cell wall biosynthesis by repressing PtoFBL4-mediated auxin signaling. Furthermore, drought-induced ABA signaling activated PtoERF1 expression via PtoAREB13, thereby inhibiting PtomiR393a expression. The study revealed a PtoERF1-PtomiR393a-PtoFBL4 cascade that links ABA-auxin crosstalk and regulates vessel development under drought stress. These findings offer new insights into drought tolerance mechanisms in trees and suggest potential strategies to enhance forest tree resilience to water-deficit conditions.