Effects of High-Temperature Stress on Soybean Isoflavone Concentration and Expression of Key Genes Involved in Isoflavone Synthesis

Isoflavones have been reported to have putative health-beneficial properties, which has led to increased interest and demand for soybeans and soy-based products. This study was conducted to determine the effects of high-temperature stress on isoflavone concentration and expression of four key genes involved in isoflavone synthesis (i.e., CHS7, CHS8, IFS1, and IFS2) in both soybean pods and seeds during their late reproductive stage (i.e., R5-R8). Isoflavone concentrations were quantified using high-performance liquid chromatography (HPLC), and gene expression was studied using quantitative real-time (qRT)-PCR. High-temperature stress [33/25 °C (day/night temperatures)] imposed at the late reproductive stage (R5-R8) reduced total isoflavone concentration by 46-86 and 20-73% in seeds and pods, respectively, the reduction depending on the stage of maturity. At stage R5, the reduction in total isoflavone concentration was greater in seeds than in pods, whereas at subsequent stages, the reverse was observed. High-temperature stress had a large impact on the expression of CHS7, CHS8, IFS1, and IFS2 in both seeds and pods. In seeds, temperature stress reduced the expression of one gene at the R5 stage (CHS8), two genes at the R6 stage (CHS7 and IFS1), and all four genes at the R7 stage, the reduction ranging between 35 and 97%. In pods, high-temperature stress affected the expression of two genes at the R6 stage (CHS7 and IFS2) and all four genes at the R7 stage. Unlike in seeds, at the R6 stage, high temperature increased the expression of CHS7 and IFS2 by 72 and 736%, respectively, whereas at R7 stage the expression of all genes was reduced by an average of 97%. The present study reveals that high-temperature stress initiated at the R5 stage and maintained until maturation (i.e., R8 stage) has a rapid and sustained negative effect on isoflavone concentration in both seeds and pods. High temperature also affects gene expression; however, there was no clear correlation between isoflavone concentration and gene expression.