Chemotaxis of Rhizobium spp. to Plant Root Exudates

Abstract

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% ...

Chemotaxis of Rhizobium meliloti to the plant flavone luteolin requires functional nodulation genes

“…A number of sugars and amino acids commonly present in root exudates can serve as chemoattractants for rhizobia (e.g., see references 5 and 12), although the concentrations of these readily metabolized compounds might be quite low in a well-populated rhizosphere. There have also been reports that rhizobia are selectively attracted to certain proteins present in host root exudates (6,7). It now appears that certain phenolic compounds in host root exudates can serve as potent and specific chemoattractants.…”

Two host-inducible genes of Rhizobium fredii and characterization of the inducing compound

“…Recently, evidence has accumulated from several laboratories indicating that some form of bacterium-plant communication is important for the early symbiotic steps (3,4,7,8,13,14,17,18,29,37; R. M. Kosslak, R. Bookland, J. Barkei, H. E. Paaren, and E. R. Applebaum, Proc. Natl.…”

“…Recently, evidence has accumulated from several laboratories indicating that some form of bacterium-plant communication is important for the early symbiotic steps (3,4,7,8,13,14,17,18,29,37 (7), cause a phenotypic reversion in slow-to-nodulate B. japonicum HS111 (18), induce symbiosis-associated genes in Rhizobium fredii (29), and increase the competitiveness of some B. japonicum strains (3). Both positive and negative interactions of root-or seed-derived compounds (flavanones, flavanols, flavones, and isoflavones) have been shown with the common nodulation genes of Rhizobium meliloti (28, 30), Rhizobium trifolii (22), Rhizobium leguminosarum (49), and B. japonicum (Kosslak et al, in press).…”

Motility and chemotaxis of a Xanthobacter sp. isolated from roots of rice

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