microRNA Regulation of Auxin-Cytokinin Balance during Soybean Nodule Development

Legume nodules result from coordinated interactions between the plant and nitrogen-fixing rhizobia bacteria. The phytohormone cytokinin is essential for and promotes nodule primordium formation. Recent findings suggest that the phytohormone auxin can inhibit nodule formation. Therefore, the auxin-cytokinin ratio is likely to be an important determinant of legume nodule primordium formation, but the mechanism has yet to be defined. We identified that microRNA160 (miR160) is a key signaling element that regulates auxin-cytokinin balance during nodule development in soybean. We used transgenic approaches to over-express or suppress miR160 activity, and to localize auxin, cytokinin, and miR160 activities. Our results indicate that miR160 orchestrates proper nodule formation by reducing auxin sensitivity and enhancing cytokinin sensitivity early during nodule development; and by promoting nodule maturation later by enhancing auxin sensitivity.