Journal of Energy Bioscience 2024, Vol.15, No.6, 378-387 http://bioscipublisher.com/index.php/jeb 383 synthesis, but in high carbon dioxide (CO₂) concentration environments, this inhibitory effect may be alleviated, restoring isoflavone levels to control levels under normal growth conditions. This phenomenon may be related to the regulatory effect of CO₂ on carbon metabolism, which in turn affects the metabolic flow of the phenylpropane pathway. Mechanical damage can also promote the accumulation of isoflavones. In 2021, Barion et al. found that leaf damage activates secondary metabolic pathways and increases the synthesis rate of isoflavones, especially under sufficient light conditions, where this effect is more significant. This phenomenon suggests that isoflavones may play an important role in soybean defense responses to help plants cope with biotic and abiotic stresses. In addition to the above two factors, the expression of key genes involved in isoflavone synthesis is also affected by water and temperature. For example, chalcone synthase CHS7 and CHS8, as well as isoflavone synthase IFS1 and IFS2, are regulated by different environmental factors (Gutierrez Gonzalez et al., 2010; Chennupati et al., 2012). Among them, IFS2 is the main regulatory factor for the decrease in isoflavone accumulation under drought conditions, and the decrease in its expression level may be a key mechanism leading to limited metabolic flux. 6 The Role of Hormone Regulation in Isoflavone Biosynthesis 6.1 The role of auxin and cytokinin in the activation of the isoflavone pathway Auxin and cytokinin are two important plant hormones, both of which play an important role in the biosynthesis of soybean isoflavones. Kumar et al. (2021) found that the application of exogenous serotonin and melatonin is related to auxin signaling, which can increase the expression of isoflavone biosynthesis genes (such as chalcone synthase and isoflavone synthase) and increase the isoflavone content in soybean culture. Chu et al. (2017) found that the transcription factor GmMYB29 regulated by hormone signals can activate the promoters of isoflavone synthase 2 (IFS2) and chalcone synthase 8 (CHS8), further enhancing the biosynthesis of isoflavones. 6.2 Crosstalk between hormones and secondary metabolic pathways The relationship between plant hormones and secondary metabolic pathways is relatively complex, involving multiple regulatory networks. In 2021, Kumar et al. found that plant hormones such as ethylene (ET), jasmonic acid (JA) and abscisic acid (ABA) play an important role in regulating isoflavone biosynthesis. In soybean tissue culture systems, treatment with serotonin and melatonin can affect ethylene biosynthesis and regulate the expression of genes related to isoflavone biosynthesis. These studies all indicate that there is an interaction between plant hormone signaling pathways and secondary metabolic pathways. Plant hormones can also regulate the accumulation of isoflavones through specific transcription factors, such as GmZFP7, which can act as a C2H2 zinc finger protein transcription factor regulated by hormone signals, affect the expression of key enzyme genes in the phenylpropanoid pathway, and promote the synthesis of isoflavones through the redistribution of metabolic flux (Feng et al., 2022). 6.3 Hormone signals during soybean development Plant hormones play different roles in different stages of soybean growth and development, profoundly affecting the synthesis and accumulation of isoflavones in soybeans. The rhythmic regulation of root isoflavone biosynthesis is affected by the circadian rhythm, which is in turn regulated by plant hormone signals. Matsuda et al. (2020) found that the expression levels of isoflavone biosynthesis-related genes fluctuated significantly between day and night, and the expression level during the day was significantly higher than that at night, indicating that photoperiod signals and hormone pathways are jointly involved in the metabolic regulation of isoflavones. Chen et al. (2017) and Yang (2024) found that miRNA not only affected the transcription level of isoflavone synthesis-related genes, but also indirectly affected the accumulation of isoflavones by targeting hormone signaling pathways. 7 Applications in Agriculture and Nutritional Improvement 7.1 Breeding strategies for high-isoflavone soybean varieties The breeding strategy for high-isoflavone soybean varieties is a genetic and molecular mechanism for controlling isoflavone biosynthesis, with the goal of breeding soybean varieties with high isoflavone content. One of the most important steps is to identify the key gene GmMYB29, which can regulate isoflavone biosynthesis by activating
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