Department of Agronomy, Ohio State University, Columbus, Ohio 43210-1086.
Plant Physiol. 1992 Mar;98(3):1181-9. doi: 10.1104/pp.98.3.1181.
Inoculum droplets of approximately 10 nanoliter volume and containing about 10 Rhizobium meliloti cells were placed onto the root surface of alfalfa seedlings in plastic growth pouches at either the root tip, the position of the smallest emergent root hairs, or at a site midway between these points. The droplets were initially confined to an area of about 0.2 square millimeter at the point of application. By 48 and 96 hours after inoculation, the inoculum bacteria and their progeny were distributed over several centimeters of the root between the initial site of deposition and the growing root tip, reaching densities of 10(3) to 10(4) bacteria per centimeter near the site of initial deposition and decreasing exponentially from that point toward the root tip. Graphite particles deposited on the root surface close to the growing tip were similarly distributed along the root length by 48 and 96 hours, suggesting that passive displacement by root cell elongation was primarily responsible for the spread of bacteria. A nonmotile mutant of R. meliloti colonized alfalfa roots to the same extent as the wild type and was usually distributed in the same manner, indicating that bacterial motility contributed little under these conditions to long distance spread of the bacteria. However, when applied in low numbers, R. meliloti mutants defective in motility or chemotaxis were considerably less efficient in initiating nodules near the point of inoculation than the wild type. This implies that motility and/or chemotaxis contribute significantly to local exploration for suitable infection sites. Almost all nodules on the primary root formed within a few millimeters of the spot-inoculation site, indicating that, under our experimental conditions, movement and multiplication of R. meliloti on the root surface were not sufficient to maintain an adequate population in the infectible region of the root during root growth.
将大约 10 纳升体积且含有约 10 个根瘤菌 meliloti 细胞的接种物液滴放置在塑料生长袋中的紫花苜蓿幼苗的根表面上，要么放在根尖处，即最小的新生根毛的位置，要么放在这两点之间的中间位置。接种物最初局限于施加点周围约 0.2 平方毫米的区域。在接种后 48 小时和 96 小时，接种物细菌及其后代分布在根的几个厘米之间，从最初沉积的位置到生长的根尖，在初始沉积点附近的密度达到 10(3)到 10(4)个细菌/厘米，并从该点向根尖呈指数递减。靠近生长根尖沉积在根表面上的石墨颗粒在 48 小时和 96 小时也沿着根长分布，这表明根细胞伸长的被动位移是细菌传播的主要原因。R. meliloti 的非运动突变体与野生型一样定植于紫花苜蓿根，通常以相同的方式分布，这表明在这些条件下细菌的运动对细菌的远距离传播贡献很小。然而，当以低数量应用时，R. meliloti 运动或趋化性缺陷的突变体在接种点附近起始结瘤的效率比野生型低得多。这意味着运动和/或趋化性对局部探索合适的感染部位有重要贡献。主根上的几乎所有结节都形成在接种点附近的几毫米内，这表明，在我们的实验条件下，R. meliloti 在根表面上的运动和增殖不足以在根生长过程中维持根可感染区域内的足够种群。
