Establishment of the rhizobia-legume symbiosis is usually accompanied by hydrogen peroxide (H2O2) production by the legume host at the site of infection, a process detrimental to rhizobia. In Azorhizobium caulinodans ORS571, deletion of chp1, a gene encoding c-di-GMP phosphodiesterase, led to increased resistance against H2O2 and to elevated nodulation efficiency on its legume host Sesbania rostrata. Three domains were identified in the Chp1: a PAS domain, a degenerate GGDEF domain, and an EAL domain. An in vitro enzymatic activity assay showed that the degenerate GGDEF domain of Chp1 did not have diguanylate cyclase activity. The phosphodiesterase activity of Chp1 was attributed to its EAL domain which could hydrolyse c-di-GMP into pGpG. The PAS domain functioned as a regulatory domain by sensing oxygen. Deletion of Chp1 resulted in increased intracellular c-di-GMP level, decreased motility, increased aggregation, and increased EPS (extracellular polysaccharide) production. H2O2-sensitivity assay showed that increased EPS production could provide ORS571 with resistance against H2O2. Thus, the elevated nodulation efficiency of the ∆chp1 mutant could be correlated with a protective role of EPS in the nodulation process. These data suggest that c-di-GMP may modulate the A. caulinodans-S. rostrata nodulation process by regulating the production of EPS which could protect rhizobia against H2O2.

中文翻译：

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满江红假单胞菌c-di-GMP磷酸二酯酶Chp1参与了寄主植物的活力，EPS产生和结瘤。

根瘤菌-豆科植物共生的建立通常伴随着豆科植物宿主在感染部位产生过氧化氢（H2O2），这是对根瘤菌有害的过程。在菜豆假单胞菌ORS571中，chp1（一种编码c-di-GMP磷酸二酯酶的基因）的缺失导致对H2O2的抗性增强，并导致其豆科植物寄主Sesbania rostrata的结瘤效率提高。在Chp1中确定了三个域：PAS域，简并的GGDEF域和EAL域。体外酶活性测定表明，Chp1的简并GGDEF域不具有双鸟苷酸环化酶活性。Chp1的磷酸二酯酶活性归因于其EAL结构域，它可以将c-di-GMP水解为pGpG。PAS结构域通过感测氧而起调节结构域的作用。Chp1的删除导致增加的细胞内c-di-GMP水平，降低的运动性，增加的聚集和增加的EPS（细胞外多糖）的生产。H2O2敏感性测定表明，增加的EPS产量可以为ORS571提供对H2O2的抗性。因此，Δchp1突变体的结瘤效率提高可能与EPS在结瘤过程中的保护作用有关。这些数据表明c-di-GMP可能会调节A. caulinodans-S。通过调节EPS的生成来形成根瘤菌结节过程，可以保护根瘤菌免受H2O2侵害。Δchp1突变体的结瘤效率提高可能与EPS在结瘤过程中的保护作用有关。这些数据表明c-di-GMP可能会调节A. caulinodans-S。通过调节EPS的生成来形成根瘤菌结节过程，可以保护根瘤菌免受H2O2侵害。Δchp1突变体的结瘤效率提高可能与EPS在结瘤过程中的保护作用有关。这些数据表明c-di-GMP可能会调节A. caulinodans-S。通过调节EPS的生成来形成根瘤菌结节过程，可以保护根瘤菌免受H2O2侵害。