Flavonoids play a crucial role as signal molecules in promoting the formation of nodules by symbiotic bacteria commonly known as rhizobia. The early interaction between flavonoids and NodD regulatory protein activates nod gene transcription and the synthesis of Nod factor that initiates nodule primordium. In this study, we assessed response to flavonoids as factors influencing competitiveness of rhizobia and their symbiotic activity. Rhizobium leguminosarum nodule isolates belonging to three biovars, trifolii, viciae and phaseoli characterized earlier as competitive or uncompetitive relative to native rhizobia, were used. Investigating nodA promoter induction using plasmid lacZ fusion, we found that competitive strains more readily responded to a wide range of synthetic flavonoids and seed exudates in comparison to uncompetitive strains, albeit some exceptions were noticed. Of all the synthetic flavonoids and seed exudates studied, naringenin, hespertin and clover and vetch exudates were the most effective inducers of nodA promoter in competitive strains. Only one of the nine examined uncompetitive strains was highly induced by clover seed exudate. Subsequently, the effect of preinduction of R. leguminosarum bv. trifolii with clover exudate was assessed. Out of 18 pre-activated strains, nine strains (including competitive ones) increased clover wet mass of shoots and nodule number when used as inoculants. Our results demonstrate a plausible approach of isolating and characterizing flavonoid-responsive field isolates that could be further developed into relevant legume inoculants.

The increase in legume production in sustainable agriculture depends not only on the effectiveness of the selected nitrogen-fixing inoculants but also on their competitiveness in a soil environment containing an indigenous rhizobial population. In this study, we investigated the effect of pretreatment of red clover seeds with specific Nod factor (LCOs) on germination, growth, and nodulation of clover growing under sterile conditions and in the soil. We demonstrated that, although the symbiotic ability and competitiveness of the inoculant strain RtKO17 was not improved under competitive soil conditions, LCOs treatment of clover seeds significantly enhanced clover nodulation and growth of plants.

The original article unfortunately contained a mistake. The authors regret overlooking and failing to cite a key laboratory study entitled "Nod-factor-treated Medicago truncatula roots and seeds show an increased number of nodules when inoculated with a limiting population of Sinorhizobium meliloti" by Macchiavelli, R. and BrellesMariño, G. (2004) J. Exp. Botany, 55: 2635-40. That paper, a pioneering contribution, showed that not only roots, but also seeds are able to perceive Nod Factors, as evidenced by higher number of nodules in the plant.The online version of the original article can be found at http://dx.doi.