Rhizobium spp. show chemotaxis to plant root exudates. Both legumes and non-legume root exudates attract the different rhizobia studied. However, the bacteria show a differential response in that they are attracted to the root exudates of some plants and show no attraction toward others. An example of negative chemotaxis was also observed.The trefoil strain of Rhizobium shows chemotaxis which is qualitatively different from that observed in other bacteria in that simple sugars, diand trisacchandes, dextrans, and amino acids do not attract this bacterium.The bacteria which nodulate legumes, Rhizobium spp., are specific in that a single strain of the bacterium can only infect certain species, or even certain varieties within a species, of legume (4). This specificity is the basis of species differentiation in Rhizobium as well as the cross inoculation groups in legumes (4,6,16), that is groups of legume species nodulated by a single species of Rhizobium. The mechanistic basis of bacteria-host specificity is unknown. It has been suggested that the basis for the specificity of Rhizobium is governed by the ability of the bacterium to induce polygalacturonase specifically in the roots of compatible hosts (11). This induced polygalacturonase would partially hydrolyze the root hair cell wall allowing penetration of the bacteria. Others (10, 21) failed to demonstrate enhanced polygalacturonase or pectinase production in several legumes inoculated with either infective or noninfective strains of Rhizobium.Recently, it has been suggested that legume lectins may play a part in the specificity of Rhizobium-legume root nodule symbiosis. Bohlool and Schmidt (5) showed that the lectin from soybean bound specifically with 22 strains of Rhizobium japonicum which nodulate soybean. The lectins did not bind to any of 23 other strains of rhizobia which do not nodulate soybean. This suggests that lectins on the surface of the legume root might interact specifically with a polysaccharide on the surface of the Rhizobium which nodulates that legume.It has also been observed that a leguminous plant causes accumulation of its own particular nodule bacterium in the rhizosphere of the plant (18,19). Many more bacteria accumulate on the root surface than in the soil nearby. Before nodulation, the surface of the root is covered with a matrix of bacteria (7). Groups of bacteria at the tips of roots have also been noted (23,25 various legumes. This work is also concerned with both nodulation and effectiveness of nitrogen fixation.
MATERIALS AND METHODSBacteria. Five strains of Rhizobium were received from Nitragen (3101 W. Custer Ave., Milwaukee, Wis. 53209): a trefoil strain (95C13), a sainfoin strain (116A12), a Rhizobium japonicum strain specific for soybean (61A76), and two R. meliloti strains specific for sweet clover and alfalfa (102F66) and 102F51). A cowpea strain (32A1) was received from R. Valentine, University of California, Davis. These cultures were maintained on slants of yeast extract-mannitol media (14), containing 1.5% ...
Rhizobium spp. show chemotaxis to plant root exudates. A glycoprotein has been isolated from the root exudates of birdsfoot trefoil, Lotus corniculatus, which, at micromolar concentrations, attracts six strains of rhizobia. This glycoprotein has been given the trivial name trefoil chemotactin and contains approximately twice as much protein as carbohydrate. Gel filtration of trefoil chemotactin on a Bio-Gel A-1.5m column gave a molecular weight of approximately 60,000. Trefoil chemotactin represents a new class of chemoattractants for bacteria.
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