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.

Evidence for the Involvement in Nodulation of the Two Small Putative Regulatory Peptide-Encoding Genes MtRALFL1 and MtDVL1

Nod factors are key bacterial signaling molecules regulating the symbiotic interaction between bacteria known as rhizobia and leguminous plants. Studying plant host genes whose expression is affected by Nod factors has given insights into early symbiotic signaling and development. Here, we used a double supernodulating mutant line that shows increased sensitivity to Nod factors to study the Nod factor-regulated transcriptome. Using microarrays containing more than 16,000 70-mer oligonucleotide probes, we identified 643 Nod-factor-regulated genes, including 225 new Nod-factor-upregulated genes encoding many potential regulators. Among the genes found to be Nod factor upregulated, we identified and characterized MtRALFL1 and MtDVL1, which code for two small putative peptide regulators of 135 and 53 amino acids, respectively. Expression analysis confirmed that these genes are upregulated during initial phases of nodulation. Overexpression of MtRALFL1 and MtDVL1 in Medicago truncatula roots resulted in a marked reduction in the number of nodules formed and in a strong increase in the number of aborted infection threads. In addition, abnormal nodule development was observed when MtRALFL1 was overexpressed. This work provides evidence for the involvement of new putative small-peptide regulators during nodulation.

Newly featured infection events in a supernodulating soybean mutant SS2-2 by Bradyrhizobium japonicum

Supernodulating soybean (Glycine max L. Merr.) mutant SS2-2 and its wild-type counterpart, Sinpaldalkong 2, were examined for the microstructural events associated with nodule formation and development. SS2-2 produced a substantially higher percentage of curled root hairs than the wild type, especially at 14 days after inoculation with Bradyrhizobium japonicum. In addition, there was new evidence that in SS2-2, B. japonicum also entered through fissures created by the emerging adventitious root primordia. Early steps of nodule ontogeny were faster in SS2-2, and continued development of initiated nodules was more frequent and occurred at a higher frequency than in the wild type. These data suggest that the early expression of autoregulation is facilitated by decreasing the speed of cortical cell development, leading to the subsequent termination of less-developed nodules. The nodules of SS2-2 developed into spherical nodules like those formed on the wild type. In both the wild type and supernodulating mutant, vascular bundles bifurcate from root stele and branch off in the nodule cortex to surround the central infected zone. These findings indicate that SS2-2 has complete endosymbiosis and forms completely developed nodule vascular bundles like the wild type, but that the speed of nodule ontogeny differs between the wild type and SS2-2. Thus, SS2-2 has a novel symbiotic phenotype with regard to nodule organogenesis.Key words: Bradyrhizobium japonicum, early nodule development, Glycine max, root hair curling, supernodulation.