Accumulation of specific flavonoids in soybean (Glycine max (L.) Merr.) as a function of the early tripartite symbiosis with arbuscular mycorrhizal fungi and Bradyrhizobium japonicum (Kirchner) Jordan

This study is the first report assessing the effect of soil inoculation on the signalling interaction of Bradyrhizobium japonicum, arbuscular mycorrhizal fungi (AMF) and soybean plants throughout the early stages of colonisation that lead to the tripartite symbiosis. In a study using soil disturbance to produce contrasting indigenous AMF treatments, the flavonoids daidzein, genistein and coumestrol were identified as possible signals for regulating the establishment of the tripartite symbiosis. However, it was unclear whether soil disturbance induced changes in flavonoid root accumulation other than through changing the potential for AMF colonization. In this study, soil treatments comprising all possible combinations of AMF and B. japonicum were established to test whether (1) modifications in root flavonoid accumulation depend on the potential for AMF colonization, and (2) synthesis and accumulation of flavonoids in the roots change over time as a function of the early plant–microbial interactions that lead to the tripartite symbiosis. The study was comprised of two phases. First, maize was grown over 3-week periods to promote the development of the AM fungus Glomus clarum. Second, the interaction between soybean, G. clarum and B. japonicum was evaluated at 6, 10, 14 and 40 days after plant emergence. Root colonization by G. clarum had a positive effect on nodulation 14 days after emergence, producing, 30% more nodules which were 40% heavier than those on roots solely inoculated with B. japonicum. The tripartite symbiosis resulted in 23% more N2 being fixed than did the simpler symbiosis between soybean and B. japonicum. The presence of both symbionts changed accumulation of flavonoids in roots. Daidzein and coumestrol increased with plant growth. However, development of the tripartite symbiosis caused a decrease in coumestrol; accumulation of daidzein, the most abundant flavonoid, was reduced in the presence of AMF.