Rice Genomics and Genetics 2024, Vol.15, No.6, 265-276 http://cropscipublisher.com/index.php/rgg 268 et al., 2016). Similarly, the regulation of anthocyanin biosynthetic mechanisms by MYB TFs is influenced by light, which modulates the complex formation of anthocyanins (Yan et al., 2021). The multilevel regulation of R2R3-MYB TFs in response to light has been extensively documented, underscoring its pivotal role in the transcriptional control of anthocyanin biosynthesis (Yang et al., 2022). 3.2 Temperature variations Temperature fluctuations have been demonstrated to exert a considerable influence on the regulation of anthocyanin biosynthesis. For instance, evidence indicates that cold stress can lead to an increase in the expression of R2R3-MYB genes, which in turn enhances the accumulation of anthocyanins. In wheat, TaPL1 expression is markedly elevated in response to cold stress, which is known to induce anthocyanin accumulation (Shin et al., 2016). The regulatory network of anthocyanin biosynthesis, which is mediated by R2R3-MYB activators, is subject to influence by temperature. This, in turn, affects the transcriptional regulation of anthocyanin biosynthetic genes (Yang et al., 2022). 3.3 Soil composition and nutrient availability The biosynthesis of anthocyanins is contingent upon the composition of the soil and the availability of nutrients. Nutrient stress, such as salt stress, has been demonstrated to induce the expression of R2R3-MYB TFs, which in turn has been shown to result in increased anthocyanin production. In wheat, the expression of TaPL1 is markedly elevated in response to salt stress, leading to elevated anthocyanin levels (Shin et al., 2016). The role of soil nutrients in the regulation of anthocyanin biosynthesis is also underscored in the context of metabolic engineering, where the availability of specific nutrients can influence the expression of anthocyanin-related genes (Zhu et al., 2017). 3.4 Drought and oxidative stress The availability of water and the effects of drought stress are significant environmental factors that influence the biosynthesis of anthocyanins. Drought stress can result in the up-regulation of R2R3-MYB TFs, which, in turn, enhances anthocyanin accumulation. The expression of OsC1, an R2R3-MYB transcriptional regulator in rice, is associated with increased anthocyanin production under oxidative stress conditions (Figure 2), which can be linked to drought stress (Upadhyaya et al., 2021). Nevertheless, the question of whether dark purple rice is more drought-resistant than white rice remains unanswered. The modulation of anthocyanin biosynthesis by water availability is of great consequence for the adaptation and stress tolerance of plants. 3.5 Biotic factors: pathogens and symbiotic relationships The biosynthesis of anthocyanins can be influenced by biotic factors, including pathogens and symbiotic relationships, through the regulation of R2R3-MYB TFs. A pathogen attack can prompt the expression of these TFs, resulting in augmented anthocyanin production as a defensive mechanism. The function of MYB TFs in the control of anthocyanin biosynthesis in response to biotic stress has been elucidated in numerous studies (Naing and Kim, 2018; Bao et al., 2021). Furthermore, symbiotic relationships can modulate the expression of genes involved in anthocyanin biosynthesis, thereby contributing to the overall regulation of this process (Yang et al., 2022). However, there is no empirical evidence to suggest that dark purple rice is more resistant to pests than white rice in paddy fields. 4 Regulation of R2R3-MYBGene Expression by Environmental Factors 4.1Light The expression of R2R3-MYB genes is subject to regulation by light, which in turn affects the biosynthesis of anthocyanins. In the case of petunia, the R2R3-MYB TFs DEEP PURPLE (DPL) and PURPLE HAZE (PHZ) are induced by high light conditions, which results in increased anthocyanin production in vegetative tissues (Albert et al., 2011). Conversely, under conditions of shade, the expression of PhMYB27, a putative R2R3-MYB active repressor, is upregulated, thereby reducing anthocyanin synthesis (Albert et al., 2011). Similarly, in wheat, the expression of TaPL1, an R2R3-MYB gene, is significantly upregulated by light, which enhances anthocyanin accumulation in coleoptiles (Shin et al., 2016).
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