Fulvic Acid Promotes Legume–Rhizobium Symbiosis by Stimulating Endogenous Flavonoids Synthesis and Secretion

Fulvic acid (FA) promotes symbiosis
between legumes
and rhizobia.
To elucidate from the aspect of symbiosis, the effects of root irrigation
of water-soluble humic materials (WSHM) or foliar spraying of its
highly active component, FA, on soybean root exudates and on rhizosphere
microorganisms were investigated. As a result, WSHM/FA treatments
significantly altered root exudate metabolite composition, and isoflavonoids
were identified as key contributors in both treatments compared to
the control. Increased expression of genes related to the isoflavonoid
biosynthesis were validated by RT-qPCR in both treatments, which notably
elevated the synthesis of symbiotic signals genistein, daidzin, coumestrol,
and biochanin A. Moreover, the WSHM/FA treatments induced a change
in rhizosphere microbial community, coupled with an increase in the
relative abundance of rhizobia. Our findings showed that WSHM/FA promotes
symbiosis by stimulating the endogenous flavonoid synthesis and leads
to rhizobia accumulation in the rhizosphere. This study provides new
insights into mechanisms underlying the FA-mediated promotion of symbiosis.