Drought stress modifies the source-sink dynamics of nitrogen-fixing soybean plants prioritizing roots and nodules

Soybean plants are one of the most cultivated legume crops worldwide. Their ability to establish nitrogen-fixing symbiosis with rhizobium bacteria allows the reduction of molecular nitrogen to ammonium, contributing to a reduction in the dependence on nitrogen fertilizers. However, nitrogen fixation is highly sensitive to environmental stresses, such as water deficit, and the regulatory mechanisms underlying this inhibition remain debatable. In the current study, we analyzed carbon (C) allocation dynamics in drought-stressed soybean plants following the application of [U-C-13]-sucrose to source leaves. Three sets of plants were analyzed: well-watered plants, mild drought, and severe drought-stressed plants. C-13 distribution was monitored for up to 6 h post-application. Under optimal water conditions, C-13 was mainly allocated to young (sink) leaves. During drought stress, transport trends changed, prioritizing C allocation primarily to the roots and nodules to a lesser extent. Metabolite profiling identified drought- and tissue-specific variations in the levels of the major C and N compounds.