PtoHsfB1 regulates growth and salt response by affecting ABA biosynthesis in Populus tomentosa

Poplar is an important commercial timber species and a model organism for forest molecular biology. Here, we identified PtoHsfB1, a subgroup B heat shock transcription factor from Populus tomentosa that is homologous to AtHsfB1 and PtrHsfB1, and contains the conserved repression motif '-LFGV-'. To elucidate its function, transgenic poplars overexpressing PtoHsfB1 were generated. Overexpression significantly enhanced growth rate and biomass accumulation. Histological analyses revealed increased cambial cell layers and enlarged phloem caps, indicating a positive role in cambial activity and phloem development. In roots, PtoHsfB1 promoted growth by suppressing abscisic acid (ABA) biosynthesis. Conversely, PtoHsfB1 overexpression reduced salt stress tolerance, as evidenced by increased oxidative damage under salt stress conditions. Collectively, these results show that PtoHsfB1 plays a dual regulatory role by promoting growth but negatively regulating salt stress responses, highlighting its potential application in improving poplar phloem yield and soil conservation.