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Isoflavones Impact Human Health, But Soybean Plant Health, Too

Guohong Mao, PhD and Oliver Yu, PhDAmong the many unique chemical components of soybean, isoflavonoids are probably the best known. They have been intensely studied for their potential health benefits for more than 20 years. However, they also play key roles in the survival of the soybean.

The three isoflavones (genistein, daidzein and glycitein) are not widely distributed in other food crops, but can accumulate 

 

For plant defense, daidzein appears to be the most important compound because it is the precursor of a group of complex isoflavonoids called glyceollins. These are potent fungicides. Even at minute concentrations, glyceollins can kill a broad spectrum of fungal pathogens. Graham et al. at Ohio State University demonstrated that when glyceollin biosynthesis was blocked by silencing daidzein synthetic enzymes, the disease symptoms of Phytophthora sojae (the pathogen causing “root rot” and “stem rot” disease) became much more severe.4,5 These data suggest that isoflavones are important anti-microbial compounds and that their accumulation in soybean seeds is associated with growth conditions.

Interestingly, sometimes isoflavones can be friendly to microbes as well. Soybean roots can interact with the nitrogen fixation bacteria called Rhizobia and form root nodules. Root nodules generate nitrogen fertilizer for crops and improve soil fertility. To form root nodules successfully, specific chemical signal exchanges have to occur prior to symbiosis. Isoflavones are an essential part of this chemical dialog between the soybean and Rhizobia. Biochemical analysis of soybean roots showed that isoflavone levels increased in response to Bradyrhizobium japonicum treatment.6 Subramanian et al. reported that isoflavones are essential for nodulation of soybean roots because of their ability to induce the nodulation genes of B. japonicum, and suppression of isoflavonoid genes in soybean leads to severely reduced nodulation.7

In addition to pathogenic and beneficial microbes, abiotic and genetic factors affect isoflavone levels in soybeans as well. Abiotic factors, such as drought and changes in temperature, can alter isoflavone levels in seed. Also different soybean cultivars vary markedly in their isoflavone content. Dr. Henry Nguyen’s laboratory at the University of Missouri screened many soybean cultivars and progenies of soybean hybrids and identified a set of genetic markers that are related to seed isoflavone levels.8 In summary, isoflavones play multiple roles in plants, and therefore are under a complex regime of regulations. To control the accumulation of this group of compounds, scientists need to investigate several aspects of isoflavonoid biosynthesis, including catalytic enzymes, regulating factors, storage and release, and genetic backgrounds. This information will in turn allow scientists to develop soybean lines with differing isoflavone content.

References

Ahuja. I., Kissen. R., Bones A.M., Phytoalexins in defense against pathogens, Trends in Plant Science 17 (2012) 73-90.

Graham. T.L., Graham M.Y., Glyceollin elicitors induce major but distinctly different shifts in isoflavonoid metabolism in proximal and distal soybean cell populations, Molecular Plant-Microbe Interactions 4 (1991) 60-68.

Graham. T.L., Graham M.Y., Signaling in soybean phenylpropanoid responses, Plant Physiology 110 (1996) 1123-1133.

Subramanian. S., Graham. M.Y., Yu. O., Graham T.L., RNA interference of soybean isoflavone synthase genes leads to silencing in tissues distal to the transformation site and to enhanced susceptibility to phytophthora sojae, Plant Physiology 137 (2005) 1345-1353.

Graham. T.L., Graham. M.Y., Subramanian. S., Yu O., RNAi silencing of genes for elicitation or biosynthesis of 5-deoxyisoflavonoids suppresses race-specific resistance and hypersensitive cell death in phytophthora sojae infected tissues, Plant Physiology 144 (2007) 728-740.

Cho. M.-J., Harper J.E., Effect of Inoculation and Nitrogen on Isoflavonoid Concentration in Wild-Type and Nodulation-Mutant Soybean Roots, Plant Physiology 95 (1991) 435-442.

Subramanian. S., Stacey. G., Yu O., Endogenous isoflavones are essential for the establishment of symbiosis between soybean and Bradyrhizobium japonicum, The Plant Journal 48 (2006) 261-273.

Gutierrez-Gonzalez. J.J., Wu. X., Gillman. J.D., Lee. J.-D., Zhong. R., Yu. O., Shannon. G., Ellersieck. M., Nguyen. H.T., Sleper D.A., Intricate environment-modulated genetic networks control isoflavone accumulation in soybean seeds, BMC Plant Biology 10 (2010) 105-120.

About the Author

Oliver Yu, PhD, is co-founder of Conagen Inc., a biotechnology company. The company’s focus is on engineering of plant metabolic compounds. Formerly, he was a principal investigator and associate member of the Danforth Plant Science Center.

Guohong Mao, PhD, is a group leader in Conagen Inc. His research has focused on molecular mechanisms of plant abiotic and biotic stress response, especially the regulation of phytoalexin biosynthesis in plants. Currently, his research focuses on plant secondary metabolism during defense responses.

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