Type III secretion effectors of Xanthomonas oryzae: molecular functions, evolutionary dynamics and implications for rice resistance breeding

Xanthomonas oryzae pv. oryzae ( Xoo ) and X. oryzae pv. oryzicola ( Xoc ) are major bacterial pathogens of rice that cause bacterial blight and bacterial leaf streak, respectively. Successful infection by these pathogens depends largely on a functional type III secretion system (T3SS), which delivers diverse type III secretion effectors (T3SEs) into rice cells to manipulate host transcription, immunity, metabolism, and cellular processes. T3SEs in X. oryzae comprise transcription activator-like effectors (TALEs) and a structurally and functionally diverse group of non-TALE effectors, commonly designated Xops. Canonical TALEs activate host susceptibility genes, including members of the OsSWEET family, or TALE-inducible executor resistance genes. In contrast, non-TALE effectors employ diverse biochemical strategies, including enzymatic activities and protein–protein interactions, to modulate host immunity, metabolism, and cellular processes. However, the strength of evidence supporting individual non-TALE mechanisms varies substantially among effectors and experimental systems. In this review, we provide an evidence-oriented synthesis of T3SS regulation and effector biology in X. oryzae , focusing on the relationship between molecular mechanisms, pathogen population variation, and resistance breeding applications. We distinguish mechanisms demonstrated in native Xoo/Xoc –rice interactions from those inferred from rice protoplast assays, heterologous systems, or related Xanthomonas pathovars. We further examine effector diversification and its implications for pathogen adaptation, while emphasizing the limitations of inferring host-driven selection from population variation alone. Finally, we evaluate effector-informed resistance strategies, including SWEET promoter editing, executor resistance gene deployment, and synthetic promoter engineering. Although these approaches provide valuable opportunities for improving rice resistance, their long-term effectiveness requires validation across diverse pathogen populations, genetic backgrounds, and field environments. Future priorities include strengthening mechanistic evidence and evaluating resistance durability alongside agronomic performance.