Patterns of seed-to-seedling transmission of Xanthomonas citri pv. malvacearum, the causal agent of cotton bacterial blight

Cotton bacterial blight (CBB), caused by Xanthomonas citri pv. malvacearum (Xcm), was a major disease of cotton in the United States in the early part of the 20th century. The recent reemergence of CBB revealed many gaps in our understanding of this important disease. In this study, we employed a wild type (WT) field isolate of Xcm from Georgia USA to generate a non-pathogenic, hrcV mutant lacking a functional type III secretion system (T3SS-). We tagged the WT and T3SS- strains with an auto-bioluminescent Tn7 reporter and compared colonization patterns of susceptible and resistant cotton seedlings using macroscopic image analysis and bacterial load enumeration. Wildtype and T3SS- Xcm strains colonized cotton cotyledons of resistant and susceptible cotton cultivars. However, Xcm populations were significantly higher in susceptible seedlings inoculated with the WT strain. Additionally, WT and T3SS- Xcm strains systemically colonized true leaves, although at different rates. Finally, we observed that seed-to-seedling transmission of Xcm may involve systemic spread through the vascular tissue of cotton plants. These findings yield novel insights into potential Xcm reservoirs for CBB outbreaks.