Zur参与大豆斑疹病菌在寄主上的致病性和在非寄主上的过敏性反应

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

摘要:

锌吸收调控蛋白(zinc uptake regulator,Zur)在植物病原黄单胞菌中的序列高度保守，但其在不同菌株或小种中的功能却差异显著。为阐明Zur在大豆斑疹病菌(Xanthomonas axonopodis pv.glycines,Xag)中的功能，本研究利用同源重组策略获得了zur基因的缺失突变株(∆zur)。该突变株在寄主大豆上的致病性和在非寄主(烟草、番茄、辣椒和茄子)上激发超敏反应(hypersensitive responses,HR)能力相较于野生型均显著减弱。此外，与野生型菌株相比，突变株∆zur的胞内锌稳态失衡，细胞外多糖(extracellular polysaccharide,EPS)产量和胞外水解酶(纤维素酶、内切葡聚糖酶、淀粉酶和蛋白酶)表达均大幅降低，而且其对Zn²⁺、Fe³⁺和Cu²⁺的敏感性显著增强。功能互补可将突变株Δzur的上述缺陷性表型恢复至野生型水平。荧光定量PCR结果表明，zur基因受Zn²⁺诱导表达；且zur基因突变大幅降低hrp基因簇代表性基因(hrpB1、hrpD6、hrpE、hrcV和hrcC)、胞外水解酶编码基因(engXCA、egl2、pro1、pro2、pro8、pro11和alpha1)和EPS合成基因(gumB、gumD、gumK、gumM、gumG和gumH)的表达水平。这些结果表明，Zur可能通过调控毒性因子合成和hrp基因表达，从而参与Xag在寄主大豆上的致病性和在非寄主植物上激发HR能力。本研究为进一步解析Zur在黄单胞菌-植物互作中的作用机制奠定了基础。

Abstract:

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.