MdCoL regulating columnar growth in apple trees positively mediates drought stress responses

Recent research has identified MdCoL, which encodes a putative 2-oxoglutarate (2OG)- and Fe(II)-dependent oxygenase, as a key regulator of the columnar growth habit in apple. However, its role in drought stress responses remains largely unexplored. In this study, shoot cultures of columnar apple showed increased drought tolerance compared to standard apple varieties when subjected to PEG-induced osmotic stress. MdCoL expression was significantly upregulated during PEG treatment. Ectopic expression of MdCoL in apple calli and transgenic poplar plants conferred increased drought tolerance. Physiological and biochemical assessments revealed that MdCoL-overexpressing lines maintained higher photosynthetic activity and more efficient reactive oxygen species (ROS) scavenging under drought conditions than wild-type (WT) counterparts. Moreover, MdCoL enhanced photosynthetic electron transport efficiency in poplar through interaction with PagFd1. Transgenic lines also developed epidermal surfaces with increased glandular trichome density. Leaf ultrastructure analysis showed pronounced leaf blades and palisade tissue thickening in transgenic plants relative to WT. Furthermore, MdCoL contributed to improved water retention and regulated stomatal behavior by modulating abscisic acid (ABA) levels. These findings underscore the role of MdCoL in increasing drought tolerance and highlight its potential utility in improving stress tolerance in woody plants.