Genome-wide identification of the U-box E3 ubiquitin ligase family in kiwifruit and functional characterization of AdUbox79 under salinity stress

Salinity stress significantly limits the productivity of crops like kiwifruit ( Actinidia deliciosa cv. Hayward). U-box E3 ubiquitin ligases are pivotal in post-translational regulation during abiotic stress; however, the AdUbox gene family in kiwifruit remains largely uncharacterized. This study identified 88 AdUbox genes in the A. deliciosa genome, classified into eight phylogenetically distinct groups with diverse structural and physicochemical properties. Among them, AdUbox79 was selected for functional characterization. Its expression was significantly induced in roots 48 h after salinity and osmotic stress treatments, and the AdUbox79:eGFP fusion protein localized specifically to the plasma membrane. Heterologous overexpression of AdUbox79 in Nicotiana tabacum conferred sensitivity to salt stress, manifested by reduced leaf area, primary root length, fresh weight and plant growth. Transgenic lines exhibited lower relative water and chlorophyll contents, alongside increased leaf chlorosis and reactive oxygen species (ROS) accumulation. Physiological analysis revealed that AdUbox79 overexpression impaired stomatal dynamics, as shown by reduced stomatal closure capacity despite a lower stomatal density. Furthermore, the expression of key salinity-responsive genes in both ABA-dependent and ABA-independent pathways was significantly repressed in the transgenic lines. Collectively, these results demonstrate that in N. tabacum AdUbox79 acts as a negative regulator of the salinity stress response, providing new insights into ubiquitin-mediated regulatory networks in kiwifruit.