Enhanced Salt Tolerance of Rhizobia-inoculated Soybean Correlates with Decreased Phosphorylation of the Transcription Factor GmMYB183 and Altered Flavonoid Biosynthesis.

Soybean (Glycine max (L.) Merrill) is an important component of the human diet and animal feed, but soybean production is limited by abiotic stresses especially salinity. We recently found that rhizobia inoculation enhances soybean tolerance to salt stress, but the underlying mechanisms are unaddressed. Here, we used quantitative phosphoproteomic and metabonomic approaches to identify changes in phosphoproteins and metabolites in soybean roots treated with rhizobia inoculation and salt. Results revealed differential regulation of 800 phosphopeptides, at least 32 of these phosphoproteins or their homologous were reported be involved in flavonoid synthesis or trafficking, and 27 out of 32 are transcription factors. We surveyed the functional impacts of all these 27 transcription factors by expressing their phospho-mimetic/ablative mutants in the roots of composite soybean plants and found that phosphorylation of GmMYB183 could affect the salt tolerance of the transgenic roots. Using data mining, ChIP and EMSA, we found that GmMYB183 binds to the promoter of the soybean GmCYP81E11 gene encoding for a Cytochrome P450 monooxygenase which contributes to the accumulation of ononin, a monohydroxy B-ring flavonoid that negatively regulates soybean tolerance to salinity. Phosphorylation of GmMYB183 was inhibited by rhizobia inoculation; overexpression of GmMYB183 enhanced the expression of GmCYP81E11 and rendered salt sensitivity to the transgenic roots; plants deficient in GmMYB183 function are more tolerant to salt stress as compared with wild-type soybean plants, these results correlate with the transcriptional induction of GmCYP81E11 by GmMYB183 and the subsequent accumulation of ononin. Our findings provide molecular insights into how rhizobia enhance salt tolerance of soybean plants.