[<i>Xanthomonas axonopodis</i> pv. <i>glycines</i> Zur is involved in pathogenicity in host and hypersensitive responses in nonhosts].

The zinc uptake regulator (Zur) has highly conserved sequences in the plant pathogen <i>Xanthomonas</i>, while its functions are diverse in different strains or races. To elucidate the functions of Zur in <i>Xanthomonas axonopodis</i> pv. <i>glycines</i> (<i>Xag</i>), we constructed a <i>zur</i>-deleted mutant (Δ<i>zur</i>) by homologous recombination. Compared with the wild type, Δ<i>zur</i> demonstrated reduced pathogenicity in the host soybean and reduced ability to trigger hypersensitive responses (HR) in nonhosts such as tobacco, tomato, chili pepper, and eggplant. Additionally, the deletion of <i>zur</i> significantly enhanced <i>Xag</i>'s sensitivity to Zn²⁺, Fe³⁺, and Cu²⁺, induced an imbalance in intracellular zinc homeostasis, decreased extracellular polysaccharide (EPS) production, and down-regulated the expression of extracellular hydrolases (cellulase, endo-glucanase, amylase, and protease). Functional complementation restored the defective properties of Δ<i>zur</i> to the wild-type levels. The qRT-PCR results showed that <i>zur</i> expression was remarkably induced by Zn²⁺. Moreover, the deletion of <i>zur</i> evidently reduced the expression levels of <i>hrp</i> representative genes (<i>hrpB1</i>, <i>hrpD6</i>, <i>hrpE</i>, <i>hrcV</i>, and <i>hrcC</i>), extracellular hydrolase encoding genes (<i>engXCA</i>, <i>egl2</i>, <i>pro1</i>, <i>pro2</i>, <i>pro8</i>, <i>pro11</i>, and <i>alpha1</i>), and EPS synthesis genes (<i>gumB</i>, <i>gumD</i>, <i>gumK</i>, <i>gumM</i>, <i>gumG</i>, and <i>gumH</i>) relative to the wild type. In summary, the results suggested that Zur may be involved in pathogenicity in the host soybean and in triggering HR in nonhosts of <i>Xag</i> by regulating the synthesis of virulence factors and the expression of <i>hrp</i> genes. This laid a foundation for further analysis of the mechanism of Zur in <i>Xanthomonas</i>-plant interaction.