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. Agrobacterium-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 Agrobacterium-induced immune responses trigger callus death, hindering transformation. To improve efficiency, we engineered an Agrobacterium strain carrying the type III secretion system (T3SS) and effector gene AvrPto. 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-AvrPto-harboring Agrobacterium showed delayed browning and necrosis versus those infected with the empty vector (NV). The transformation rate with Agrobacterium (T3SS-AvrPto-EGFP) reached 1.6%, while Agrobacterium (NV-EGFP) failed entirely. These findings demonstrate that T3SS and AvrPto enhance mango transformation efficiency, offering a promising strategy for breeding multi-resistant varieties.