<i>Xanthomonas oryzae</i> pv. <i>oryzae</i> Type III Effector Tal9b Targets a Broadly Conserved Disease Susceptibility Locus to Promote Pathogenesis in Rice

<i>Xanthomonas oryzae</i> pv. <i>oryzae</i> (<i>Xoo</i>), the causal agent of bacterial blight of rice, translocates multiple transcription activator-like effectors (TALEs) into rice cells. The TALEs localize to the host cell nucleus, where they bind to the DNA in a sequence-specific manner and enhance gene expression to promote disease susceptibility. <i>Xoo</i> strain PXO99^A encodes 19 TALEs, but the host targets of all these TALEs have not been defined. A meta-analysis of rice transcriptome profiles revealed a gene annotated as flavonol synthase/flavanone-3 hydroxylase (henceforth <i>OsS5H/FNS-03g</i>) to be highly induced upon <i>Xoo</i> infection. Further analyses revealed that this gene is induced by PXO99^A using Tal9b, a broadly conserved TALE of <i>Xoo</i>. Disruption of <i>tal9b</i> rendered PXO99^A less virulent. OsS5H/FNS-03g functionally complemented its <i>Arabidopsis</i> homologue AtDMR6, a well-studied disease susceptibility locus. Biochemical analyses suggested that OsS5H/FNS-03g is a bifunctional protein with salicylic acid 5' hydroxylase (S5H) and flavone synthase-I (FNS-I) activities. Further, an exogenous application of apigenin, the flavone that is enzymatically produced by OsS5H/FNS-03g, on rice leaves promoted virulence of PXO99^A <i>tal9b^-</i>. Overall, our study suggests that OsS5H/FNS-03g is a bifunctional enzyme, and its product, apigenin, is potentially involved in promoting <i>Xoo</i> virulence. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.