Dylan T, Nagpal P, Helinski D R, Ditta G S
Department of Biology and Center for Molecular Genetics, University of California, San Diego, La Jolla 92093.
J Bacteriol. 1990 Mar;172(3):1409-17. doi: 10.1128/jb.172.3.1409-1417.1990.
Nodule development (ndv) mutants of Rhizobium meliloti cannot invade alfalfa to establish a nitrogen-fixing symbiosis and instead induce the formation of small, white, unoccupied nodules on alfalfa roots. Such mutants also fail to produce the unusual cyclic oligosaccharide beta-(1----2)-glucan and show defects in several aspects of vegetative growth and function. Here we show that ndv mutants are severely reduced, although not totally deficient, in the ability to attach to and initiate infection threads on alfalfa seedlings, and we demonstrate that the symbiotic deficiency can be separated from the rest of the mutant phenotype by isolating second-site pseudorevertants. Pseudorevertants selected for restoration of motility, a vegetative property, regained a substantial amount of attachment capability but only slight infection thread initiation and symbiotic ability. Such strains also regained partial tolerance to growth at low osmolarity, even though they did not recover the ability to synthesize periplasmic beta-(1----2)-glucan. Pseudorevertants selected on alfalfa for restoration of symbiosis were unrestored for beta-(1----2)-glucan production or any other vegetative property and regained little or no attachment or infection thread initiation capability. We take these data to indicate that wild-type R. meliloti normally has considerable excess capability for both attachment and infection thread initiation and that the symbiotic block in ndv mutants lies further along the developmental pathway than either of these processes, probably at the level of infection thread extension. Further, the fact that neither type of pseudorevertant recovered the ability to produce periplasmic beta-(1----2)-glucan raises the possibility that this oligosaccharide is not directly required for nodule development.
苜蓿根瘤菌的结节发育（ndv）突变体无法侵入苜蓿以建立固氮共生关系，反而会在苜蓿根上诱导形成小的、白色的、未被占据的结节。这类突变体也无法产生异常的环状寡糖β-(1→2)-葡聚糖，并且在营养生长和功能的几个方面表现出缺陷。在这里我们表明，ndv突变体在附着于苜蓿幼苗并启动感染丝的能力上严重降低，尽管并非完全缺失，并且我们证明通过分离第二位点假回复体，可以将共生缺陷与突变体表型的其他部分分开。为恢复运动性（一种营养特性）而选择的假回复体恢复了大量的附着能力，但仅轻微恢复了感染丝启动和共生能力。这类菌株也恢复了对低渗透压生长的部分耐受性，尽管它们没有恢复合成周质β-(1→2)-葡聚糖的能力。在苜蓿上选择以恢复共生能力的假回复体在β-(1→2)-葡聚糖产生或任何其他营养特性方面没有恢复，并且几乎没有或没有恢复附着或感染丝启动能力。我们根据这些数据表明，野生型苜蓿根瘤菌通常在附着和感染丝启动方面具有相当大的过剩能力，并且ndv突变体中的共生阻断位于比这两个过程中任何一个都更远的发育途径上，可能在感染丝延伸水平。此外，两种类型的假回复体都没有恢复产生周质β-(1→2)-葡聚糖的能力，这一事实增加了这种寡糖不是结节发育直接所需的可能性。
