GROWTH AND MOVEMENT OF SPOT INOCULATED RHIZOBIUM-MELILOTI ON THE ROOTSURFACE OF ALFALFA

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, t he position of the smallest emergent root hairs, or at a site midway b etween these points. The droplets were initially confined to an area o f about 0.2 square millimeter at the point of application. By 48 and 9 6 hours after inoculation, the inoculum bacteria and their progeny wer e 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 depositi on and decreasing exponentially from that point toward the root tip. G raphite particles deposited on the root surface close to the growing t ip were similarly distributed along the root length by 48 and 96 hours , suggesting that passive displacement by root cell elongation was pri marily 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 bacte rial motility contributed little under these conditions to long distan ce spread of the bacteria. However, when applied in low numbers, R. me liloti mutants defective in motility or chemotaxis were considerably l ess efficient in initiating nodules near the point of inoculation than the wild type. This implies that motility and/or chemotaxis contribut e significantly to local exploration for suitable infection sites. Alm ost all nodules on the primary root formed within a few millimeters of the spot-inoculation site, indicating that, under our experimental co nditions, movement and multiplication of R. meliloti on the root surfa ce were not sufficient to maintain an adequate population in the infec tible region of the root during root growth.