The formation of nitrogen-fixing nodules on legume hosts is a finely tuned process involving many components of both symbiotic partners. Production of the exopolysaccharide succinoglycan by the nitrogen-fixing bacterium Sinorhizobium meliloti 1021 is needed for an effective symbiosis with Medicago spp., and the succinyl modification to this polysaccharide is critical. However, it is not known when succinoglycan intervenes in the symbiotic process, and it is not known whether the plant lysin-motif receptor-like kinase MtLYK10 intervenes in recognition of succinoglycan, as might be inferred from work on the Lotus japonicus MtLYK10 ortholog, LjEPR3. We studied the symbiotic infection phenotypes of S. meliloti mutants deficient in succinoglycan production or producing modified succinoglycan, in wild-type Medicago truncatula plants and in Mtlyk10 mutant plants. On wild-type plants, S. meliloti strains producing no succinoglycan or only unsuccinylated succinoglycan still induced nodule primordia and epidermal infections, but further progression of the symbiotic process was blocked. These S. meliloti mutants induced a more severe infection phenotype on Mtlyk10 mutant plants. Nodulation by succinoglycan-defective strains was achieved by in trans rescue with a Nod factor-deficient S. meliloti mutant. While the Nod factor-deficient strain was always more abundant inside nodules, the succinoglycan-deficient strain was more efficient than the strain producing only unsuccinylated succinoglycan. Together, these data show that succinylated succinoglycan is essential for infection thread formation in M. truncatula, and that MtLYK10 plays an important, but different role in this symbiotic process. These data also suggest that succinoglycan is more important than Nod factors for bacterial survival inside nodules.

中文翻译：

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苜蓿中华根瘤菌琥珀酰化高分子量琥珀聚糖和蒺藜苜蓿 LysM 受体样激酶 MtLYK10 独立参与共生感染。

在豆科植物宿主上形成固氮结核是一个精细调整的过程，涉及两个共生伙伴的许多成分。与苜蓿属植物的有效共生需要由固氮细菌苜蓿中华根瘤菌 1021 生产胞外多糖琥珀聚糖，而对这种多糖的琥珀酰修饰是至关重要的。然而，尚不清楚琥珀聚糖何时干预共生过程，也不知道植物溶素基序受体样激酶 MtLYK10 是否干预对琥珀聚糖的识别，这可能从莲花 japonicus MtLYK10 直系同源物 LjEPR3 的研究中推断出来。 . 我们研究了缺乏琥珀聚糖产生或产生修饰的琥珀聚糖的苜蓿根瘤菌的共生感染表型，在野生型 Medicago truncatula 植物和 Mtlyk10 突变植物中。在野生型植物上，不产生琥珀聚糖或仅产生未琥珀酰化的琥珀聚糖的苜蓿根瘤菌菌株仍诱导结节原基和表皮感染，但共生过程的进一步进展受阻。这些苜蓿根瘤菌突变体在 Mtlyk10 突变植物上诱导了更严重的感染表型。琥珀聚糖缺陷菌株的结瘤是通过使用 Nod 因子缺陷型苜蓿根瘤菌突变体进行反式救援来实现的。虽然 Nod 因子缺陷菌株在结节内总是更丰富，但琥珀聚糖缺陷菌株比仅产生未琥珀酰化琥珀聚糖的菌株更有效。总之，这些数据表明，琥珀酰化的琥珀聚糖对于蒺藜苜蓿的感染线形成是必不可少的，并且 MtLYK10 在这个共生过程中起着重要但不同的作用。这些数据还表明，对于结节内的细菌存活，琥珀聚糖比 Nod 因素更重要。