Rhizosphere soil aggregation and plant growth promotion of sunflowers by an exopolysaccharide-producing Rhizobium sp strain isolated from sunflower roots

Root-adhering soil (RAS) forms the immediate environment where plants take up water and nutrients for their growth. We report the effect of an exopoly saccharide (EPS)-producing rhizobacterium (strain YAS34) on the physical pr operties of sunflower (Helianthus annuus L,) RAS, associated with plant gro wth promotion, under both water stress and normal water supply conditions. Strain YAS34 was isolated as a major EPS-producing bacterium from the rhizo plane of sunflowers grown in a French dystric cambisol, Strain YAS34 was as signed to the Rhizobium genus by 16S ribosomal DNA gene sequencing. Inocula tion of sunflower seeds and soil with strain YAS34 caused a significant inc rease in RIS per root dry mass (dm) (up to 100%) and a significant increase in soil macropore volume (12 to 60 mu m in diameter). The effect of inocul ation on sunflower shoot dm (up to +50%) and root dm (up to +70%) was signi ficant under both normal and water stress conditions. Inoculation with stra in YAS34 modified soil structure around the root system, counteracting the negative effect of water deficit on growth. Using [N-15]nitrate, we showed that inoculation made the use of fertilizer more effective by increasing ni trogen uptake by sunflower plantlets.