Immune Responses of Mango Callus Infected by Agrobacterium tumefaciens Inhibited Transformation

Mango is a vital fruit crop in tropical and subtropical regions, yet pests and diseases cause 30-70% production losses. Developing disease-resistant cultivars through transgenic methods could mitigate these issues. <i>Agrobacterium</i>-mediated callus transformation is a common genetic engineering approach, but successful transgenic mango plants from callus remain unreported due to severe browning and necrosis post-infection. We hypothesized that <i>Agrobacterium</i>-induced immune responses trigger callus death, hindering transformation. To improve efficiency, we engineered an <i>Agrobacterium</i> strain carrying the type III secretion system (T3SS) and effector gene <i>AvrPto</i>. Compared to controls, infected calluses exhibited elevated reactive oxygen species (ROS), along with up-regulated ROS-related, gallic acid biosynthesis, and defense genes. Calluses infected with T3SS-<i>AvrPto</i>-harboring <i>Agrobacterium</i> showed delayed browning and necrosis versus those infected with the empty vector (NV). The transformation rate with <i>Agrobacterium</i> (T3SS-<i>AvrPto</i>-EGFP) reached 1.6%, while <i>Agrobacterium</i> (NV-EGFP) failed entirely. These findings demonstrate that T3SS and <i>AvrPto</i> enhance mango transformation efficiency, offering a promising strategy for breeding multi-resistant varieties.