Bradyrhizobium japonicum with higher motility and its efficiency to colonize and nodulate soybean roots

ALTHABEGOITI, M.J.; LÓPEZ GARCÍA, S.; MONGIARDINI, E.; PÉREZ GIMÉNEZ, J.; QUELASJ.I.; PERTICARI, A.; LODEIRO, A.

Mérida, México

Congreso; XII International Congress on Molecular Plant-Microbe Interactions; 2005

International Society of Plant-Microbe Interactions

Motility of Bradyrhizobium japonicum in soil or vermiculite has been shown as scarce in the vertical direction. This scarce vertical motility may prevent nodulation by an inoculant applied to the seeds if a naturalized plant-compatible rhizobial population is established in the soil. In this situation, most of the inoculant remains in the non-infectable upper root areas while emergent root hairs toward root apical regions are colonized by the soil rhizobia. We observed that in soils with naturalized soybean-nodulating rhizobia, the application of inoculants directly to the soil below the seeds enhanced nodule occupation by the inoculated strain in relation to an inoculant applied onto the seeds. Here we evaluated whether this could be further improved using a rhizobial strain with higher motility. By means of a recurrent selection procedure we obtained a B. japonicum isolate that formed colonies in swarm plates with 2.4-fold the diameter of the wild type. This phenotype, which we called Mot++, was stable after passage through nodules. Growth rate and soybean nodulation ability of the Mot++ isolate were not affected. As the parent strain, the Mot++ strain had single polar flagella, with equal flagellin contents. When these strains were inoculated onto soybean seeds, root colonization was similar for the Mot++ strain and the wild type, being rhizobia more concentrated in the basal one-third of the roots and almost absent at the apical one-third. However, the Mot++ strain displayed a higher rate to infect the emergent root hairs of young soybean plants. When applied to soybean seeds in competition with an isogenic rhizobial population established in the vermiculite substrate since 30 days before (107 cfu/ml free solution), a small increment in nodule occupation by the Mot++ strain was observed in relation to the wild-type. These results suggest that the increase in motility may be applied to improve inoculant competitiveness in field situations. Field experiments are in process to quantify nodule occupation by each strain when inoculated onto the seeds or in the soil, in the presence of naturalized soybean-nodulating rhizobia