Donald James

Xanthomonas oryzae pv. oryzae ( Xoo), 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. Xoo 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 OsS5H/FNS-03g) to be highly induced upon Xoo infection. Further analyses revealed that this gene is induced by PXO99^A using Tal9b, a broadly conserved TALE of Xoo. Disruption of tal9b rendered PXO99^A less virulent. OsS5H/FNS-03g functionally complemented its Arabidopsis 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 tal9b^–. Overall, our study suggests that OsS5H/FNS-03g is a bifunctional enzyme, and its product, apigenin, is potentially involved in promoting Xoo virulence. [ ] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

SUMMARY: Xanthomonas oryzae pv. oryzae (Xoo), 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. Xoo strain PXO99 ^A encodes nineteen 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 OsS5H/FNS-03g ) to be highly induced upon Xoo infection. Further analyses revealed that this gene is induced by PXO99 ^A using TAL9b, a broadly conserved TALE of Xoo. Disruption of tal9b rendered PXO99 ^A less virulent. OsS5H/FNS-03g functionally complemented its Arabidopsis 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 on rice leaves, the flavone that is enzymatically produced by OsS5H/FNS-03g, promoted virulence of PXO99 ^A tal9b -. Overall, our study suggests that OsS5H/FNS-03g is a bifunctional enzyme and its product apigenin is potentially involved in promoting Xoo virulence.

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