A Legume Genetic Framework Controls Infection of Nodules by Symbiotic and Endophytic Bacteria

Legumes have an intrinsic capacity to accommodate both symbiotic and endophytic bacteria within root nodules. For the symbionts, a complex genetic mechanism that allows mutual recognition and plant infection has emerged from genetic studies under axenic conditions. In contrast, little is known about the mechanisms controlling the endophytic infection. Here we investigate the contribution of both the host and the symbiotic microbe to endophyte infection and development of mixed colonised nodules in Lotus japonicus. We found that infection threads initiated by Mesorhizobium loti, the natural symbiont of Lotus, can selectively guide endophytic bacteria towards nodule primordia, where competent strains multiply and colonise the nodule together with the nitrogen-fixing symbiotic partner. Further co-inoculation studies with the competent coloniser, Rhizobium mesosinicum strain KAW12, show that endophytic nodule infection depends on functional and efficient M. loti-driven Nod factor signalling. KAW12 exopolysaccharide (EPS) enabled endophyte nodule infection whilst compatible M. loti EPS restricted it. Analysis of plant mutants that control different stages of the symbiotic infection showed that both symbiont and endophyte accommodation within nodules is under host genetic control. This demonstrates that when legume plants are exposed to complex communities they selectively regulate access and accommodation of bacteria occupying this specialized environmental niche, the root nodule.

豆科植物（Legumes）具有在根瘤（root nodules）内同时容纳共生细菌与内生细菌的固有能力。针对共生菌而言，无菌培养条件下的遗传学研究已揭示出一套介导相互识别与植物侵染的复杂遗传机制。与之形成鲜明对比的是，学界对调控内生细菌侵染的分子机制仍知之甚少。本研究以百脉根（Lotus japonicus）为实验体系，探究宿主与共生微生物对内生菌侵染以及混合定殖根瘤形成的贡献。我们发现，由百脉根的天然共生菌——洛氏根瘤菌（Mesorhizobium loti）所起始的侵染线（infection threads），可选择性地引导内生细菌向根瘤原基迁移，在该处具备侵染能力的菌株可与固氮共生伙伴一同增殖并定殖根瘤。进一步利用具备有效定殖能力的菌株——中生根瘤菌（Rhizobium mesosinicum）KAW12开展的共接种实验表明，内生细菌的根瘤侵染依赖于功能完整且活性高效的洛氏根瘤菌介导的结瘤因子（Nod factor）信号通路。KAW12的胞外多糖（exopolysaccharide, EPS）可促进内生细菌的根瘤侵染，而与之相容的洛氏根瘤菌胞外多糖则会抑制该过程。对调控共生侵染不同阶段的植物突变体进行分析后发现，根瘤内共生菌与内生细菌的定殖均受宿主遗传调控。这一结果证实，当豆科植物暴露于复杂菌群环境时，其可通过选择性调控来管控占据根瘤这一特殊生态位的细菌的入侵与定殖。