Changes in flavonoids secreted by Phaseolus vulgaris roots in the presence of salt and the plant growth-promoting rhizobacterium Chryseobacterium balustinum

Root exudates affect the concentration of nutrients in the plant rhizosphere and indirectly enhance microbial activity. Flavonoids are polyphenolic compounds that play an important role as signal molecules in the early stages of the symbiosis between legumes and rhizobia. Flavonoids from root exudates of Phaseolus vulgaris cv. Bush Blue Lake obtained under control conditions, salt stress (50 mM), and/or the presence of the plant growth-promoting rhizobacterium (PGPR) Chryseobacterium balustinum Aur9 were analyzed by HPLC coupled to ESI-MS/MS. Six flavonoids were identified in the root exudates of P. vulgaris cv. BBL: naringenin, isoliquiritigenin, quercetin umbelliferone, 7′,4-dihydroxyflavone, and hesperetin. The latter three flavonoids have not previously been reported in bean root exudates. The presence of C. balustinum, but not salt stress, modified the pattern of flavonoids exuded by the bean roots. The capacity of flavonoids from root exudates to induce the expression of the nod genes and the production of lipochitooligosaccharides (LCOs) was investigated in two bean-nodulating rhizobia, Rhizobium tropici CIAT899 and Rhizobium etli ISP42. The R. etli ISP42 LCOs profile changed when it was induced by the exudates collected in the presence of the PGPR with and without salt. In the case of R. tropici CIAT899, changes in the LCOs profile were detected when the bacterium was induced with the different bean root exudates in comparison with the LCOs synthesized upon induction with apigenin.