Regulation of Local Auxin Metabolism during Soybean Nodule Development

Legume nodules result from symbiotic interactions between the plant and compatible members of rhizobia bacteria. There are two major types of nodules formed in legume roots: indeterminate and determinate. Indeterminate nodules are oblong and have a persistent nodule meristem (e.g. a Medicago nodule) where as determinate nodules are spherical and lack a persistent nodule meristem (e.g. a soybean nodule). While auxin activity has been associated with the development of both these types of nodules, it was shown that inhibition of auxin transport at the site of nodule initiation is crucial for indeterminate nodule formation, but not determinate nodule formation. What mechanisms regulate auxin levels/output during determinate nodule formation? We evaluated the expression of auxin biosynthesis and signaling components in global transcriptomes of emerging and mature nodules in soybean. We identified a number of genes associated with auxin metabolism that were specifically enriched in nodules versus lateral roots indicating functional diversification. Functional analyses using gene silencing in soybean composite plants demonstrated that regulation of auxin metabolism plays a key during soybean nodule development.