Rice Genomics and Genetics 2024, Vol.15, No.6, 265-276 http://cropscipublisher.com/index.php/rgg 270 4.2 Temperature Temperature has been demonstrated to exert a considerable influence on the expression of R2R3-MYB genes. In wheat, TaPL1 expression is responsive not only to light but also to cold stress, which further promotes anthocyanin biosynthesis (Shin et al., 2016). Conversely, elevated temperatures have been demonstrated to impede anthocyanin biosynthesis by diminishing the expression of pivotal MYB activators (Yang et al., 2022). This indicates that R2R3-MYB genes may be involved in a more expansive stress response mechanism, whereby temperature fluctuations induce anthocyanin synthesis as a protective strategy. 4.3 Nutrients The expression of R2R3-MYB genes can be influenced by soil nutrients, particularly the availability of essential minerals. In Ananas comosus var. bracteatus, the expression of multiple R2R3-MYB genes is regulated by hormonal treatments, which are frequently associated with nutrient availability. For example, the application of abscisic acid (ABA), salicylic acid (SA), and methyl jasmonate (MeJA) has been demonstrated to induce the expression of specific R2R3-MYB genes, thereby enhancing anthocyanin biosynthesis (Yang et al., 2023). This suggests that soil nutrient status, via its impact on hormonal levels, may regulate R2R3-MYB gene expression indirectly. 4.4 Water stress Another environmental factor that affects R2R3-MYBgene expression is water stress. In rice, the R2R3-MYBgene OsC1 has been demonstrated to enhance oxidative stress tolerance by regulating anthocyanin biosynthesis (Figure 2). In plants subjected to water stress, the elevated anthocyanin levels observed in OsC1-overexpressing plants appear to mitigate oxidative damage, indicating a protective role for anthocyanins in stressful conditions (Upadhyaya et al., 2021). This underscores the significance of R2R3-MYB genes in plant acclimation to water stress through the modulation of anthocyanin concentrations. 4.5 Biotic factors The expression of R2R3-MYB genes can also be influenced by biotic factors, such as pathogen attacks. In the species Capsicum annuum, the insertion of a non-long terminal repeat retrotransposon in the promoter region of the CaAn2 gene, which encodes an R2R3-MYB TF, results in its activation. This activation is probably mediated by the recruitment of TFs to the retrotransposon sequence, which enhances anthocyanin biosynthesis and provides a defense mechanism against biotic stress (Jung et al., 2019). This example demonstrates how biotic interactions can result in genetic modifications that upregulate R2R3-MYB gene expression, thereby enhancing plant resilience. In summary, the expression of R2R3-MYB genes in dark purple rice and other plants is subject to intricate regulation by a multitude of environmental factors, including light, temperature, soil nutrients, water stress, and biotic interactions. These factors collectively influence the biosynthesis of anthocyanins, thereby contributing to the adaptation and survival of plants in diverse environmental conditions. 5 Molecular Mechanisms Linking R2R3-MYBGenes to Anthocyanin Biosynthesis 5.1 Signal transduction pathways involved inR2R3-MYBregulation R2R3-MYB TFs play a crucial role in the regulation of anthocyanin biosynthesis, responding to a multitude of environmental and internal signals. It has been demonstrated that light, temperature, and hormonal signals, including MeJA and ethylene, can influence the activity of R2R3-MYB proteins. For example, the expression of R2R3-MYB genes in petunia is subject to strict regulation by light conditions, with high-light environments inducing the expression of MYB factors that promote anthocyanin synthesis (Albert et al., 2011). Similarly, in wheat, environmental stresses such as cold and salt have been demonstrated to upregulate the expression of R2R3-MYBgenes, resulting in increased anthocyanin accumulation (Shin et al., 2016). These findings underscore the intricate interplay between environmental cues and R2R3-MYB-mediated anthocyanin biosynthesis.
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