Xanthomonas phaseoli pv. Manihotis effector XopAG impairs cassava immunity by hijacking MeHSP26 to suppress flavonoid biosynthesis.

Cassava bacterial blight (CBB), caused by Xanthomonas phaseoli pv. manihotis (Xpm), is a major disease constraining cassava production. Although overexpression of the Xpm effector XopAG enhances Arabidopsis susceptibility to Pseudomonas syringae pv. tomato DC3000, its virulence mechanism in cassava remains unclear. The study showed that deletion of XopAG did not affect morphology or extracellular enzyme production of Xpm but significantly attenuated its virulence on cassava. Quantitative proteomic analysis revealed that 267 differentially accumulated proteins (DAPs) at 6 h post-inoculation (hpi) and 429 DAPs at 6 d post-inoculation (dpi) were identified in cassava leaves inoculated with the ΔxopAG mutant compared with Xpm. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that DAPs were significantly enriched in phenylpropanoid and flavonoid biosynthesis pathways. In the absence of XopAG, flavonoid biosynthesis-related proteins and genes (DFR, ANR, ANS, CYP75A1, FLS) were up-regulated, accompanied by increased total flavonoid content in cassava leaves during early infection. Furthermore, cassava small heat shock protein MeHSP26 was identified as a host target of XopAG, and 2 proteins interact in both the cytoplasm and nucleus in N. benthamiana. Importantly, a degradation assay reveals that XopAG promotes MeHSP26 degradation in N. benthamiana. Silencing MeHSP26 compromised cassava resistance to Xpm, following reduced flavonoid content and suppression of flavonoid gene expression. Together, these results demonstrate that XopAG promotes MeHSP26 degradation to suppress flavonoid biosynthesis, thereby attenuating cassava immunity during early Xpm inoculation. This work provides insights into cassava-Xpm interactions by revealing that effector manipulation of metabolic pathways facilitates inoculation.