Perception of Rhizobium nodulation factors by tomato cells and inactivation by root chitinases.

The bacterial genera Rhizobium and Bradyrhizobium, nitrogen-fixing symbionts of legumes, secrete specific lipo-chitooligosaccharides that induce the formation of nodules on their host plants. When preparations of such nodulation-inducing factors (Nod factors) were added to suspension-cultured tomato cells, a rapid and transient alkalinization of the culture medium occurred. Lipo-oligosaccharide preparations from Rhizobium or Bradyrhizobium treated with flavonoids, known inducers of Nod factor synthesis, were up to 100 times more potent in inducing alkalinization than the ones from untreated bacteria. The activity was absent from preparations of the mutant strain Rhizobium sp. NGR234 delta nodABC, unable to produce any Nod factors. Preparations of Nod factors from various bacteria as well as individual, highly purified Nod factors from Rhizobium sp. NGR(pA28) induced alkalinization in the tomato cell cultures at nanomolar concentrations. This demonstrates that Nod factors can be perceived by tomato, a nonhost of rhizobia. Using the alkalinization response as a sensitive bioassay, Nod factors were found to be inactivated by plant chitinases. Root chitinases purified from different legumes differed in their potential to inactivate differently substituted Nod factors produced by Rhizobium sp. NGR(pA28). This indicates that the specificity of the bacterium-host plant interaction may be due, at least in part, to differential inactivation of Nod factors by root chitinases. IMAGES:

作为豆科植物固氮共生体的根瘤菌属（Rhizobium）与慢生根瘤菌属（Bradyrhizobium），会分泌特定的脂壳寡糖（lipo-chitooligosaccharides），诱导宿主植物形成根瘤。将此类结瘤因子（nodulation-inducing factors, Nod factors）的制备物添加至悬浮培养的番茄细胞中时，培养基会出现快速且瞬时的碱化现象。经黄酮类化合物（已知的结瘤因子合成诱导剂）处理的根瘤菌属或慢生根瘤菌属的脂寡糖制备物，其诱导碱化的活性较未处理细菌的制备物高出最多100倍。无法合成任何结瘤因子的突变菌株根瘤菌属（Rhizobium sp.）NGR234 ΔnodABC的制备物，则无此类活性。多种细菌来源的结瘤因子制备物，以及根瘤菌属（Rhizobium sp.）NGR(pA28)的单一高纯度结瘤因子，均可在纳摩尔浓度下诱导番茄细胞培养体系发生碱化。这表明结瘤因子可被非根瘤菌宿主的番茄所识别。以碱化响应作为灵敏的生物测定方法时，研究发现结瘤因子会被植物几丁质酶（chitinases）灭活。从不同豆科植物中纯化得到的根几丁质酶，其灭活根瘤菌属（Rhizobium sp.）NGR(pA28)所产生的不同取代型结瘤因子的能力存在差异。这表明，细菌-宿主植物互作的特异性至少部分源于根几丁质酶对结瘤因子的差异化灭活。IMAGES: