Early interactions of Rhizobium leguminosarum bv. phaseoli and bean roots: specificity in the process of adsorption and its requirements of Ca2+ and Mg2+ Ions.

LODEIRO A.R.; LAGARES A.; MARTÍNEZ E.N.; FAVELUKES G.

APPLIED AND ENVIRONMENTAL MICROBIOLOGY

American Society of Microbiology

Año: 1995 vol. 61 p. 1571 - 1579

0099-2240

Roots of Phaseolus vulgaris L. were incubated with dilute suspensions (1 3 103 to 3 3 103 bacteria ml21) of an antibiotic-resistant indicator strain of Rhizobium leguminosarum bv. phaseoli in mineral medium and washed four times by a standardized procedure prior to quantitation of adsorption (G. Caetano-Anolle´s and G. Favelukes, Appl. Environ. Microbiol. 52:371–376, 1986). The population of rhizobia remaining adsorbed on roots after washing was homogeneous, as indicated by the first-order course of its desorption by hydrodynamic shear. Rhizobia were maximally active for adsorption in the early stationary phase of growth. The process leading to adsorption was rapid, without an initial lag, and slowed down after 1 h. Adsorption of the indicator strain at 103 bacteria ml21 was inhibited to different extents in the presence of 103 to 108 antibiotic-sensitive competitor rhizobia ml21. After a steep rise above 104 bacteria ml21, inhibition by heterologous competitors in the concentration range of 105 to 107 bacteria ml21 was markedly less than by homologous strains, while at 108 bacteria ml21 it approached the high level of inhibition by the latter. At 107 bacteria ml21, all of the heterologous strains tested were consistently less inhibitory than homologous competitors (P < 0.001). These differences in competitive behavior indicate that in the process of adsorption of R. leguminosarum bv. phaseoli to its host bean roots, different modes of adsorption occur and that some of these modes are specific for the microsymbiont (as previously reported for the alfalfa system [G. Caetano-Anolle´s and G. Favelukes, Appl. Environ. Microbiol. 52:377–381, 1986]). Moreover, whereas the nonspecific process occurred either in the absence or in the presence of Ca21 and Mg21 ions, expression of specificity was totally dependent on the presence of those cations. R. leguminosarum bv. phaseoli bacteria adsorbed in the presence of Ca21 and Mg21 were more resistant to desorption by shear forces than were rhizobia adsorbed in their absence. These results indicate that (i)  ymbiotic specificity in the P. vulgaris-R. leguminosarum bv. phaseoli system is expressed already during the early process of rhizobial adsorption to roots, (ii) Ca21 and Mg21 ions are required by R. leguminosarum bv. phaseoli for that specificity, and (iii) those cations cause tighter binding of rhizobia to rootsPhaseolus vulgaris L. were incubated with dilute suspensions (1 3 103 to 3 3 103 bacteria ml21) of an antibiotic-resistant indicator strain of Rhizobium leguminosarum bv. phaseoli in mineral medium and washed four times by a standardized procedure prior to quantitation of adsorption (G. Caetano-Anolle´s and G. Favelukes, Appl. Environ. Microbiol. 52:371–376, 1986). The population of rhizobia remaining adsorbed on roots after washing was homogeneous, as indicated by the first-order course of its desorption by hydrodynamic shear. Rhizobia were maximally active for adsorption in the early stationary phase of growth. The process leading to adsorption was rapid, without an initial lag, and slowed down after 1 h. Adsorption of the indicator strain at 103 bacteria ml21 was inhibited to different extents in the presence of 103 to 108 antibiotic-sensitive competitor rhizobia ml21. After a steep rise above 104 bacteria ml21, inhibition by heterologous competitors in the concentration range of 105 to 107 bacteria ml21 was markedly less than by homologous strains, while at 108 bacteria ml21 it approached the high level of inhibition by the latter. At 107 bacteria ml21, all of the heterologous strains tested were consistently less inhibitory than homologous competitors (P < 0.001). These differences in competitive behavior indicate that in the process of adsorption of R. leguminosarum bv. phaseoli to its host bean roots, different modes of adsorption occur and that some of these modes are specific for the microsymbiont (as previously reported for the alfalfa system [G. Caetano-Anolle´s and G. Favelukes, Appl. Environ. Microbiol. 52:377–381, 1986]). Moreover, whereas the nonspecific process occurred either in the absence or in the presence of Ca21 and Mg21 ions, expression of specificity was totally dependent on the presence of those cations. R. leguminosarum bv. phaseoli bacteria adsorbed in the presence of Ca21 and Mg21 were more resistant to desorption by shear forces than were rhizobia adsorbed in their absence. These results indicate that (i)  ymbiotic specificity in the P. vulgaris-R. leguminosarum bv. phaseoli system is expressed already during the early process of rhizobial adsorption to roots, (ii) Ca21 and Mg21 ions are required by R. leguminosarum bv. phaseoli for that specificity, and (iii) those cations cause tighter binding of rhizobia to roots