BpbZIP61 negatively regulates drought resistance in birch by reducing ascorbic acid content

Drought is one of the major limiting factors affecting forest growth and survival. Basic leucine zipper (bZIP) transcription factors (TFs) play essential roles in plant responses to drought stress. Here, we identified that BpbZIP61, a drought-induced bZIP TF in birch (Betula platyphylla), functions as a potential negative regulator of drought tolerance. Overexpression of BpbZIP61 resulted in increased leaf water loss, elevated levels of reactive oxygen species (H₂O₂) and malondialdehyde (MDA), and decreased activities of Superoxide Dismutase (SOD) and Peroxidase (POD) activities. The overexpressing lines exhibited a significant reduction in ascorbic acid (AsA) content, a finding supported by transcriptomic evidence of downregulated expression in the ascorbate metabolism. We further demonstrated that BpbZIP61 functions as a transcriptional repressor by directly binding to ABRE cis-element in the promoter of BpGGLO6, a key gene in the AsA biosynthesis pathway, thereby suppressing its expression. The findings suggest that BpbZIP61 is a potential negative regulator for drought tolerance in birch, which operates by inhibiting AsA biosynthesis and regulating antioxidant enzyme activities, offering crucial insights for molecular breeding of drought-resistant trees.