Redox-active di-O-methylated coumarins exudation contributes to genotype-dependent iron deficiency tolerance in soybean

Iron (Fe) deficiency is a widespread disorder limiting global soybean (Glycine max (L.) Merr.) production. Although root exudation is a key adaptive mechanism for Fe scarcity in species like Arabidopsis, a detailed chemical characterization of soybean exudates is lacking. Here, we examined the accumulation and secretion of phenolic compounds in soybean roots and their correlation with intraspecific tolerance to Fe-deficiency chlorosis. Seven soybean genotypes with contrasting tolerance, derived from U.S. breeding programs, were analyzed. Root exudates from Fe-deficient soybean plants solubilized ferric oxide. We identified and quantified 28 coumarin-type phenolics, with catechol methylsideretin as the predominant component. Although the qualitative coumarin profile was consistent across all genotypes, Fe-efficient lines secreted these compounds at higher levels or earlier during Fe deficiency than Fe-inefficient lines. The efficient genotype A7 showed coordinated upregulation of coumarin biosynthesis and secretion, whereas this response was weaker in the Fe-inefficient genotype IsoClark. Catechol methylsideretin concentrations strongly correlated with the ability of root exudates to mobilize Fe from ferric oxide. The conserved phenolic profile, together with divergence from those reported in non-legume species, suggests lineage-specific adaptations and ecological roles beyond Fe mobilization. These results highlight genotype-dependent exudation as a determinant of soybean Fe-deficiency tolerance, with implications for breeding.