Overexpression of the ABA-responsive transcription factor PtrMYBH alters shoot apical meristem, xylem, root, and leaf development in Populus

Abscisic acid (ABA) mediates stress responses and growth regulation in plants, but the roles of ABA-responsive transcription factors (TFs) in Populus development remain poorly characterized. Here, we identified an ABA-upregulated TF and investigated its function through overexpression in transgenic poplar. The PtrMYBH is upregulated during ABA treatment, and its overexpression in transgenic poplar leads to leaf malformations, including reduced size and curling, with severity correlating with PtrMYBH-overexpression levels in three independent transgenic lines. PtrMYBH regulates stomatal growth and development, resulting in decreased stomatal length and aperture, along with distinct leaf structural abnormalities. Additionally, PtrMYBH affects root development, with overexpressing lines showing an increase in adventitious root number but shorter lengths, alongside morphological changes in the root elongation zone. Furthermore, morphological changes are observed in the shoot apical meristem (SAM) and stem-differentiating xylem (SDX) of PtrMYBH-overexpressing poplar. RNA-seq analyses reveal PtrMYBH's influence on the expression of genes related to cellular proliferation in the SAM and developmental pathways in the SDX. Finally, in PtrMYBH-overexpressing lines, ABA treatment results in leaf tip damage, earlier leaf drop, and stunted growth, highlighting its critical role in the ABA response. These findings lay a foundation for further exploration of TFs like PtrMYBH to regulate growth in Populus species.