[Xanthomonas axonopodis pv. glycines Zur is involved in pathogenicity in host and hypersensitive responses in...

The zinc uptake regulator (Zur) has highly conserved sequences in the plant pathogen Xanthomonas, while its functions are diverse in different strains or races. To elucidate the functions of Zur in Xanthomonas axonopodis pv. glycines (Xag), we constructed a zur-deleted mutant (Δzur) by homologous recombination. Compared with the wild type, Δzur 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 zur significantly enhanced Xag'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 Δzur to the wild-type levels. The qRT-PCR results showed that zur expression was remarkably induced by Zn²⁺. Moreover, the deletion of zur evidently reduced the expression levels of hrp representative genes (hrpB1, hrpD6, hrpE, hrcV, and hrcC), extracellular hydrolase encoding genes (engXCA, egl2, pro1, pro2, pro8, pro11, and alpha1), and EPS synthesis genes (gumB, gumD, gumK, gumM, gumG, and gumH) 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 Xag by regulating the synthesis of virulence factors and the expression of hrp genes. This laid a foundation for further analysis of the mechanism of Zur in Xanthomonas-plant interaction.