Graduate School of Life Sciences, Tohoku University.
Faculty of Life Sciences, Kyoto Sangyo University.
Microbes Environ. 2020;35(1). doi: 10.1264/jsme2.ME19141.
Bradyrhizobium elkanii, a rhizobium with a relatively wide host range, possesses a functional type III secretion system (T3SS) that is involved in symbiotic incompatibility against Rj4-genotype soybean (Glycine max) and some accessions of mung bean (Vigna radiata). To expand our knowledge on the T3SS-mediated partner selection mechanism in the symbiotic legume-rhizobia association, we inoculated three Lotus experimental accessions with wild-type and T3SS-mutant strains of B. elkanii USDA61. Different responses were induced by T3SS in a host genotype-dependent manner. Lotus japonicus Gifu inhibited infection; L. burttii allowed infection, but inhibited nodule maturation at the post-infection stage; and L. burttii and L. japonicus MG-20 both displayed a nodule early senescence-like response. By conducting inoculation tests with mutants of previously reported and newly identified effector protein genes of B. elkanii USDA61, we identified NopF as the effector protein triggering the inhibition of infection, and NopM as the effector protein triggering the nodule early senescence-like response. Consistent with these results, the B. elkanii USDA61 gene for NopF introduced into the Lotus symbiont Mesorhizobium japonicum induced infection inhibition in L. japonicus Gifu, but did not induce any response in L. burttii or L. japonicus MG-20. These results suggest that Lotus accessions possess at least three checkpoints to eliminate unfavorable symbionts, including the post-infection stage, by recognizing different T3SS effector proteins at each checkpoint.
慢生根瘤菌（Bradyrhizobium elkanii）是一种宿主范围较广的根瘤菌，具有功能性的 III 型分泌系统（T3SS），该系统参与了与 Rj4 基因型大豆（Glycine max）和一些绿豆（Vigna radiata）品种的共生不相容性。为了扩展我们对共生豆科植物-根瘤菌关联中 T3SS 介导的伙伴选择机制的认识，我们用野生型和 T3SS 突变型菌株 USDA61 接种了三个实验型 Lotus 品系。T3SS 以宿主基因型依赖的方式诱导了不同的反应。日本百脉根（Lotus japonicus）Gifu 抑制了感染；L. burttii 允许感染，但在感染后期抑制了根瘤的成熟；而 L. burttii 和 L. japonicus MG-20 都表现出类似早期衰老的根瘤反应。通过对先前报道的和新鉴定的 B. elkanii USDA61 效应蛋白基因的突变体进行接种试验，我们确定 NopF 是触发感染抑制的效应蛋白，而 NopM 是触发类似早期衰老的根瘤反应的效应蛋白。这些结果与以下结果一致：将 B. elkanii USDA61 的 nopF 基因引入 Lotus 共生菌 Mesorhizobium japonicum 中，会在 L. japonicus Gifu 中诱导感染抑制，但不会在 L. burttii 或 L. japonicus MG-20 中引起任何反应。这些结果表明，Lotus 品系至少有三个检查点来消除不利的共生体，包括在感染后期，通过在每个检查点识别不同的 T3SS 效应蛋白。
