Minisymposium 11: Plant-symbiont interactions

Abs # 24003: 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, USA (B): E.I. du Pont de Nemours & Company, Inc., Wilmington, DE, USA (C): Department of Plant Microbiology and Pathology, University of Missouri, Columbia, MO, USA
Web Site:	http://www.danforthcenter.org/yu

Isoflavonoids are a group of secondary metabolites common to leguminous plants that are thought to play roles in plant-microbe interactions. They act as signal molecules in the process of nodulation and as general defense compounds against various pathogens. 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 or chalcone reductase (CHR) 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. Both the IFS genes were primarily expressed in the root and seeds. Inoculation with 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 symbiotic association between soybean and B. japonicum, such as providing positional cues or inhibition of auxin transport. We have analyzed the expression of IFS1:GUS in hairy roots of a few nodulation mutants of soybean. Preliminary results showed that the expression of IFS1 was higher in the roots of a hypernodulating mutant (nts382) and lower in a non-nodulating mutant (nod139) in response to B. japonicum. 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 performing additional experiments such as testing the effect of over-expression of IFS1 on nodulation and the analysis of expression of the auxin-inducible marker DR5:GUS to understand the role of isoflavones in the nodulation of soybean.