The promoters of two isoflavone synthase genes respond differentially to nodulation and defense signals in transgenic soybean roots.

Isoflavonoids are a group of secondary metabolites common to leguminous plants that play roles in nodulation and defense responses. Isoflavone synthase (IFS) catalyzes the key entry point step of isoflavone biosynthesis from the general phenylpropanoid pathway. We have cloned the 5' upstream regions of the genes encoding the two isoflavone synthase isoforms from soybean. We characterized the tissue-specific expression patterns of IFS1 and IFS2 genes of soybean by quantitative RT-PCR. Isoflavone synthase transcripts were detected primarily in the roots and seeds. In transgenic soybean plants in which the IFS1 promoter drove expression of the bacterial uidA (GUS) gene, the expression was localized to the root epidermis and root hairs. We detected differential tissue-specific expression of IFS1:GUS in response to nodulation and defense signals. Bradyrhizobium japonicum, a diazotropic symbiont of soybean, induced the expression of IFS1:GUS in root hairs and xylem poles in the young differentiation zone of the root. This observation suggests novel roles for isoflavonoids in nodulation. The defense related hormone salicylic acid induced the expression of IFS1:GUS in all cell types of the root. We also noticed corresponding alterations in the transcript levels of IFS1 and IFS2 in response to B. japonicum or salicylic acid as identified by quantitative RT-PCR. Using hairy root transformed soybean composite plants, we have identified regions of the IFS1 and IFS2 promoters mediating response to B. japonicum. We have also shown conserved root-specific expression of IFS1 in rice and Arabidopsis. The expression pattern of IFS1 in soybean is consistent with the physiological roles of isoflavonoids as defense compounds against pathogens and signal molecules to symbiotic bacteria in soybean.