Functional Characterization of BpYAB1 in Birch: Overexpression Leads to Leaf Curling, Developmental Defects, and Elevated Water Loss

Leaves are vital photosynthetic organs in plants, playing crucial roles in growth and development. The transcription factor YABBY (YAB) family is particularly important in leaf development, where it regulates abaxial polarity establishment. In this study, we analyzed BpYAB1 expression in the woody species Betula platyphylla . BpYAB1 was localized to the nucleus and showed the highest expression in apical buds, with gradually reduced levels from the 1st to the 4th leaves, stems, and roots. To further elucidate BpYAB1 function, we generated transgenic B. platyphylla lines. While BpYAB1 knockout birch lines showed no significant phenotypic differences compared to wild‐type (WT), BpYAB1 overexpression induced abaxial leaf curling, accompanied by structural alterations in the upper/lower epidermis, palisade/spongy tissues, vascular xylem, and ground tissue. Additionally, BpYAB1‐ overexpressing lines displayed developmental delays and increased water loss rates. Transcriptome analysis suggested that these phenotypic defects may be linked to altered expression of auxin‐responsive, growth‐related, and stress‐response genes. Collectively, our findings provide novel insights into the molecular mechanisms governing leaf development in B. platyphylla .