Department of Microbiology and Immunology, University of Otago, Dunedin, New Zealand.
Mol Plant Microbe Interact. 2013 Mar;26(3):319-29. doi: 10.1094/MPMI-09-12-0227-R.
Rhizobial surface polysaccharides are required for nodule formation on the roots of at least some legumes but the mechanisms by which they act are yet to be determined. As a first step to investigate the function of exopolysaccharide (EPS) in the formation of determinate nodules, we isolated Mesorhizobium loti mutants affected in various steps of EPS biosynthesis and characterized their symbiotic phenotypes on two Lotus spp. The wild-type M. loti R7A produced both high molecular weight EPS and lower molecular weight (LMW) polysaccharide fractions whereas most mutant strains produced only LMW fractions. Mutants affected in predicted early biosynthetic steps (e.g., exoB) formed nitrogen-fixing nodules on Lotus corniculatus and L. japonicus 'Gifu', whereas mutants affected in mid or late biosynthetic steps (e.g., exoU) induced uninfected nodule primordia and, occasionally, a few infected nodules following a lengthy delay. These mutants were disrupted at the stage of infection thread (IT) development. Symbiotically defective EPS and Nod factor mutants functionally complemented each other in co-inoculation experiments. The majority of full-length IT observed harbored only the EPS mutant strain and did not show bacterial release, whereas the nitrogen-fixing nodules contained both mutants. Examination of the symbiotic proficiency of the exoU mutant on various L. japonicus ecotypes revealed that both host and environmental factors were linked to the requirement for EPS. These results reveal a complex function for M. loti EPS in determinate nodule formation and suggest that EPS plays a signaling role at the stages of both IT initiation and bacterial release.
根瘤菌表面多糖对于至少一些豆科植物根部的根瘤形成是必需的，但它们的作用机制仍有待确定。作为研究胞外多糖（EPS）在确定型根瘤形成中的功能的第一步，我们分离了在 EPS 生物合成的各个步骤中受到影响的 Mesorhizobium loti 突变体，并对其在两种 Lotus spp.上的共生表型进行了表征。野生型 M. loti R7A 产生高分子量 EPS 和低分子量（LMW）多糖分数，而大多数突变株仅产生 LMW 分数。受预测早期生物合成步骤（例如，exoB）影响的突变体在 Lotus corniculatus 和 L. japonicus 'Gifu'上形成固氮根瘤，而受中或晚期生物合成步骤（例如，exoU）影响的突变体诱导未感染的根瘤原基，并在长时间延迟后偶尔诱导一些感染的根瘤。这些突变体在感染线程（IT）发育阶段被破坏。共生缺陷 EPS 和 Nod 因子突变体在共接种实验中相互功能互补。观察到的大多数全长 IT 仅携带 EPS 突变株，并且没有显示细菌释放，而固氮根瘤包含两个突变株。在各种 L. japonicus 生态型上对 exoU 突变体的共生效率进行检查表明，宿主和环境因素都与 EPS 的需求有关。这些结果揭示了 M. loti EPS 在确定型根瘤形成中的复杂功能，并表明 EPS 在 IT 起始和细菌释放阶段都发挥信号作用。
