Submitted to: Toxin Reviews
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 11, 2012
Publication Date: June 4, 2012
Citation: Bellaloui, N., Mengistu, A., Zobiole, L.H., Shier, W.T. 2012. Resistance to toxin-mediated fungal infection: role of lignins, isoflavones, other seed phenolics, sugars and boron in the mechanism of resistance to charcoal rot disease in soybean. Toxin Reviews. 31(1-2):16-26. Interpretive Summary: Charcoal rot is a major soybean disease in the Early Soybean Production System, causing major production loss, seed quality reduction, and phytosanitary challenges for soybean export. Charcoal rot is caused by the fungus Macrophomina phaseolina, and is more virulent under warm temperatures and drought. Since there are no commercial resistant cultivars to charcoal rot, finding alternatives to control the disease such as phenotypic traits that are related to the disease to increase the resistance is important. Phenolics including phenol, lignin, and isoflavones are natural compounds that have been thought to be involved in plant defense against diseases. Therefore, the objective of this research was to investigate the effect of charcoal rot on soybean seed phenolic compounds (phenol, lignin, and isoflavones) in infested soil under irrigated and nonirrigated conditions. Seed phenolic compounds were higher in moderately resistant genotype than in susceptible genotypes under irrigated and nonirrigated conditions and in infested and noninfested soils. The higher phenolic compounds in moderately resistant genotype were accompanied by higher seed sucrose, raffinose, and stachyose. Seed sucrose was higher in susceptible genotypes under irrigated-noninfested than under nonirrigated-infested conditions. The opposite trend was observed in raffinose and stachyose in susceptible genotypes, where raffinose and stachyose were higher under nonirrigated-infested conditions, suggesting a positive response with raffinose and stachyose to disease infection and drought. This research demonstrated that the level of phenolic compounds and sugars in seed depended on the level of resistance of a genotype to charcoal rot infection and the sensitivity of the genotype to drought. The research may explain the role of phenolics in disease resistance, and this knowledge may enable soybean breeders to use these biochemical compounds as biomarkers to select for seed with high nutritional qualities of phenol, isoflavones, and sugars under disease infection and drought conditions.
Technical Abstract: The objective of this research was to investigate the combined effects of charcoal rot and drought on total seed phenol, isoflavones, sugars, and boron in susceptible (S) and moderately resistant (MR) soybean genotypes to charcoal rot pathogen. A field experiment was conducted for two years under irrigated-inoculated and irrigated-noninoculated conditions, and under nonirrigated-inoculated and nonirrigated-noninoculated conditions. The results showed that phenolic compounds (total phenol, lignin, and isoflavones) were significantly (P=0.05) higher in MR genotype than in S genotypes under all treatments conditions. Phenol concentration was higher under inoculated than under noninoculated conditions in S genotypes. Seed sucrose, raffinose, and stachyose were higher in MR genotype than in S genotypes in all treatments. In S genotypes, seed sucrose was higher under irrigated-noninoculated and nonirrigated-noninoculated conditions than under irrigated-inoculated and nonirrigated-inoculated conditions, but raffinose and stachyose were higher under irrigated-inoculated and nonirrigated-inoculated conditions than under irrigated-noninoculated and nonirrigated-noninoculated conditions. Total boron was higher in MR genotypes than in S genotypes in all treatments, and total B was higher under noninoculated than under inoculated conditions in all genotypes. Cell wall boron percentage was higher in S genotypes than those of MR genotype. This research demonstrated that phenolic compounds and sugars levels depended on the level of charcoal rot infection and drought. The higher levels of raffinose, stachyose, and cell wall boron in seed in S genotypes than in MR genotype under inoculated conditions indicated a possible association of these compounds with disease resistance. The current knowledge is beneficial for understanding the role of phenolics in the plant defense mechanisms, and for selecting seed with high nutritional qualities of phenol, isoflavones, and sugars under stress environments of diseases and drought.