Klein S, Hirsch A M, Smith C A, Signer E R
Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.
Mol Plant Microbe Interact. 1988 Feb;1(2):94-100. doi: 10.1094/mpmi-1-094.
Among the genes of Rhizobium meliloti SU47 that affect nitrogen-fixing symbiosis with alfalfa are nod genes, in which mutations block nodule induction, and exo genes, in which mutations allow nodule formation but block rhizobial exopolysaccharide production as well as nodule invasion and nitrogen fixation. To investigate whether an exo+ bacterium can "help" (that is, reverse the symbiotic defect of) an exo mutant in trans, we have coinoculated alfalfa with pairs of rhizobia of different genotypes. Coinoculant genotypes were chosen so that the exo+ helper strain was nif while the exo "indicator" strain was nif+, and thus any fixation observed was carried out by the exo coinoculant. We find that a nod exo+ coinoculant can help an exo mutant both to invade nodules and to fix nitrogen. However, a nod+ exo+ coinoculant cannot help an exo mutant: Few exo bacteria are recovered from nodules, some bacteroids differentiate into bizarre aberrant forms, and the nodules fail to fix nitrogen. In a triple coinoculation, the effect of nod+ helper supersedes that of nod helper. Implications of these results for interaction of nod and exo gene products are discussed.
在苜蓿中华根瘤菌SU47中，影响与苜蓿固氮共生的基因包括结瘤基因（nod基因），其中的突变会阻断根瘤诱导；以及外多糖基因（exo基因），其中的突变会使根瘤形成，但会阻断根瘤菌胞外多糖的产生以及根瘤侵染和固氮过程。为了研究exo⁺细菌是否能在反式作用中“帮助”（即逆转）exo突变体的共生缺陷，我们用不同基因型的根瘤菌对共同接种苜蓿。选择共接种基因型时，使exo⁺辅助菌株为固氮基因缺陷型（nif⁻），而exo“指示”菌株为固氮基因正常型（nif⁺），因此观察到的任何固氮作用都是由exo共接种菌进行的。我们发现，结瘤基因正常且外多糖基因正常（nod exo⁺）的共接种菌能帮助exo突变体侵染根瘤并固氮。然而，结瘤基因正常且外多糖基因正常（nod⁺ exo⁺）的共接种菌不能帮助exo突变体：从根瘤中回收的exo细菌很少，一些类菌体分化成奇异的异常形态，并且根瘤不能固氮。在三重共接种中，结瘤基因正常的辅助菌的作用取代了结瘤基因缺陷辅助菌的作用。讨论了这些结果对结瘤基因和外多糖基因产物相互作用的意义。
