The isoflavonoids are essential for different types of plant-microbe interactions. They are important chemo-attractants and signal molecules for symbiotic Rhizobium bacteria. The nodD gene-encoded proteins of Rhizobia have been shown to physically bind to flavonoids and isoflavonoids, and this ligand association initiates transcription of the nod operon leading to root nodule formation. Different isoflavonoids are also either precursors to, or are themselves the major phytoalexins in legumes, which play key roles in non-specific plant defense against bacterial and fungal pathogens. The activation of isoflavonoid synthesis during the disease resistance response is important for providing these many defense compounds.

The pathway that leads to isoflavonoid synthesis is a branch of the general phenylpropanoid pathway that exists in all higher plants. The phenylpropanoid pathway leads to the synthesis of many important secondary metabolites such as lignins, flavanols, and anthocyanins. Genes encoding enzymes of this pathway are developmentally and tissue-specifically regulated and may be induced by environmental stresses such as nutrient deficiency, prolonged cold, exposure to intense UV light, and pathogen attack. 

An enzyme found almost exclusively in the legumes, isoflavone synthase (IFS), converts the phenylpropanoid pathway intermediates tetrahydroxy-chalcone (naringenin) and trihydroxy-chalcone (isoliquiritigenin) into the isoflavones genistein and daidzein, respectively. The gene encoding this key enzyme, a cytochrome P450 monoxygenase, has recently been identified from a variety of legumes by three groups independently.  Two IFS genes from soybean with more than 97% similarity have been discovered. In our yeast microsome assay, both enzymes could convert the two substrates to isoflavones efficiently.

We have demonstrated that expression of soybean IFS1 in non-legume plants resulted in the production of genistein. However, the accumulation of genistein in both monocot and dicot transgenic tissues was closely related to the activity of the phenylpropanoid pathway. We discovered that isoflavones were produced at significant levels only in tissues where the phenylpropanoid pathway activity was elevated, such as in floral tissues, in UV-treated tissues, and in tissues where expression of a heterologous transcription factor was used to activate genes of the phenylpropanoid pathway. Thus, when this pathway is activated, the IFS1 enzyme is able to capture a portion of the naringenin intermediate produced and convert it to genistein.

In legumes, the production of isoflavones is involved in at least two distinct physiological processes, nodulation and plant defense. The objective of this project is to answer some of the fundamental questions:

• What transcriptional regulation mechanisms govern the expression of key enzyme isoflavone synthase (IFS) and other isoflavone synthesis genes?
• What are the signal transduction pathways/networks resulting from pathogen attacks and Rhizobia perception that lead to the increased production of isoflavonoids?
• How does transcriptional regulation affect the isoflavonoid synthesis pathway at the systematical level and its relationship to general phenopropanoid pathway regulation?
• How is the flow of substrate among the upper part of phenopropanoid pathway, the isoflavonoid biosynthesis pathway, and the flavonoid biosynthesis pathway controlled during biotic stress and nodulation process?

[Return to top]