Poster: Plant-symbiont interactions

Abs # 591: Isoflavones are essential for the establishment of symbiosis between soybean and Bradyrhizobium japonicum

Presenter:	Subramanian, Senthil , ssubramanian@danforthcenter.org
Authors	Subramanian, Senthil  (A)   Menne, Chris  (A)   Odell, Joan T (B)   Stacey, Gary  (C)   Yu, Oliver  (A)
Affiliations:	(A): Donald Danforth Plant Science Center, St Louis, MO (B): E.I. du Pont de Nemours & Company, Inc., Wilmington, DE, USA (C): Department of Plant Microbiology and Pathology, University of Missouri, Columbia, MO
Web Site:	http://www.danforthcenter.org/yu/

Isoflavonoids are a group of secondary metabolites common to leguminous plants that are thought to play roles as signal molecules in the process of nodulation. Isoflavone synthase (IFS) catalyzes the key entry point step of isoflavonoid biosynthesis from the general phenylpropanoid pathway. To establish the role of isoflavones in the process of symbiotic association, we generated composite hairy root transgenic soybean plants expressing RNA-i constructs to suppress IFS and chalcone reductase transcripts. Transgenic hairy roots had isoflavone levels that varied between 0.1 and 5% of that of the wild-type. When inoculated with B. japonicum, these roots had significantly reduced nodule numbers, suggesting that isoflavones are essential for nodulation of soybean by B. japonicum. We characterized the tissue-specific expression patterns of IFS1 and IFS2 genes of soybean by quantitative RT-PCR and using transgenic roots expressing promoter:GUS fusions. B. japonicum induced the expression of IFS1:GUS in root hairs and xylem poles in the root hair zone. This expression pattern was distinct from those induced by salicylic acid or jasmonic acid. We also noticed a 10-fold increase in the transcript levels of IFS1 and IFS2 in response to B. japonicum in the root. Taken together, these observations suggest novel roles for isoflavones in the symbioitic association between soybean and B. japonicum, such as providing positional cues or inhibition of auxin transport. In addition, we have identified regions of the IFS1 and IFS2 promoters mediating the response to B. japonicum using hairy root transformed soybean composite plants. We are further testing the role of isoflavones in nodulation by extending our studies to soybean and B. japonicum mutants impaired in nodulation.