Phenoxyacetic acid enhances nodulation symbiosis during the rapid growth stage of soybean.

Root exudates are known signaling agents that influence legume root nodulation, but the molecular mechanisms for nonflavonoid molecules remain largely unexplored. The number of soybean root nodules during the initial growth phase shows substantial discrepancies at distinct developmental junctures. Using a combination of metabolomics analyses on root exudates and nodulation experiments, we identify a pivotal role for certain root exudates during the rapid growth phase in promoting nodulation. Phenoxyacetic acid (POA) was found to activate the expression of and thereby facilitate rhizobial infection and the formation of infection threads. Furthermore, POA exerts regulatory control on the miR172c-NNC1 module to foster nodule primordia development and consequently increase nodule numbers. These findings collectively highlight the important role of POA in enhancing nodulation during the accelerated growth phase of soybeans.