Investigation of downstream signals of the soybean autoregulation of nodulation receptor kinase GmNARK.

The Glycine max nodule autoregulation receptor kinase (GmNARK) plays a central role in the systemic signal transduction pathway controlling nodulation in soybean. We used transcriptional profiling to identify potential downstream signals of this receptor kinase. These studies revealed that GmNARK-mediated signaling controls the expression of genes involved in the jasmonic acid (JA) pathway. Genes encoding the key enzymes controlling JA biosynthesis as well as JA-response genes were regulated systemically but not locally by root inoculation with Bradyrhizobium japonicum. This systemic regulation was abolished in Gmnark mutant plants, indicating that their expression was specifically controlled by signaling events associated with this receptor kinase. Foliar application of a JA biosynthesis inhibitor significantly reduced nodulation specifically in supernodulating mutant plants. These results indicate that the receptor-mediated regulation of JA signaling plays an important role in the AON signal transduction pathway. A second class of genes was identified that were controlled by GmNARK in a rhizobia-independent manner. These candidates provide insight on additional, nonsymbiotic signaling pathways that are likely regulated by GmNARK, such as those involved in root growth and defense. The discovery of downstream components of the GmNARK receptor kinase advances our understanding of the systemic control of nodule development and its association with other signaling networks.