Rice Genomics and Genetics 2024, Vol.15, No.6, 265-276 http://cropscipublisher.com/index.php/rgg 266 purple rice, rendering it a valuable system for elucidating the molecular mechanisms underlying anthocyanin regulation and the role of environmental factors in modulating this process. This review aims to elucidate the regulatory mechanisms by which R2R3-MYB TFs control anthocyanin biosynthesis in dark purple rice, examine the role of environmental factors in modulating the expression of R2R3-MYB genes and their downstream targets, and assess the impact of altered anthocyanin biosynthesis on the stress tolerance and overall adaptation of dark purple rice. The attainment of these objectives will facilitate the acquisition of invaluable insights into the intricate regulatory networks that govern anthocyanin biosynthesis and the potential for genetic manipulation to enhance crop quality and stress resilience. 2 R2R3-MYB Gene Family in Dark Purple Rice 2.1 Genome-wide characteristics and classification of R2R3-MYBgenes The R2R3-MYB TFs constitute a large family of proteins that play a pivotal role in regulating a multitude of physiological processes in plants, including the biosynthesis of anthocyanins. These TFs are distinguished by the presence of two MYB domains (R2 and R3) at their N-terminus, which are involved in DNA binding. The R2R3-MYB genes are classified into distinct subfamilies based on their sequence homology and functional characteristics (Blanco et al., 2022; Li et al., 2023; Shi et al., 2024). In rice, R2R3-MYB genes have been demonstrated to regulate flavonoid production, including the biosynthesis of anthocyanins and proanthocyanins (Li et al., 2016; Upadhyaya et al., 2021; Yang et al., 2023). 2.2 Identification and characterization of R2R3-MYBgenes in dark purple rice In black rice, several R2R3-MYB genes have been identified and characterized concerning their roles in anthocyanin biosynthesis. For example, the OsC1 gene, which encodes an R2R3-MYB TF, has been demonstrated to regulate the expression of late ABP genes, resulting in the accumulation of anthocyanins, particularly cyanidin 3-glucoside, in the panicle stage of black rice (Upadhyaya et al., 2021). OsMYB3/OsKala3, an MYB TF, was identified as the R2R3-MYB gene that plays a pivotal role in anthocyanin biosynthesis in rice pericarps (Chen et al., 2023; Kim et al., 2021; Zheng et al., 2021). Comprehensive bioinformatic analyses have identified numerous R2R3-MYB genes in various plant species, including rice, and have provided insights into their gene structure, motif conservation, and chromosomal localization (Yang et al., 2023). 2.3 Phylogenetic analysis and evolutionary insights A phylogenetic analysis of R2R3-MYB genes in rice and other plant species has revealed that these genes are highly conserved and can be grouped into distinct subfamilies. For example, a study on Ananas comosus var. bracteatus identified 99 R2R3-MYB genes, which were classified into 33 subfamilies. The gene structures and protein motifs within each subfamily were found to be conserved (Yang et al., 2023). Such analyses indicate that segmental duplication events have played a significant role in the expansion of the R2R3-MYB gene family in plants. Furthermore, phylogenetic studies have demonstrated that R2R3-MYB genes involved in anthocyanin biosynthesis are evolutionarily conserved across diverse plant species, suggesting their indispensable role in plant adaptation and survival (Albert et al., 2011). 2.4 Expression patterns of R2R3-MYBgenes in different tissues and developmental stages The expression patterns of R2R3-MYB genes in dark purple rice exhibit variability across different tissues and developmental stages. For example, the OsC1 gene is highly expressed during the panicle stage, which correlates with increased anthocyanin accumulation (Upadhyaya et al., 2021) (Figure 1). In other plants, such as Arabidopsis thaliana, R2R3-MYB genes like PAP1, PAP2, and MYB113 demonstrate differential expression in juvenile and adult leaves, indicating stage-specific regulation of anthocyanin biosynthesis (Koo and Poethig, 2021). Similarly, in Ananas comosus var. bracteatus, ten R2R3-MYBgenes have been observed to exhibit tissue-specific expression patterns, with some genes being highly expressed in flowers, bracts, and leaves. This suggests their potential involvement in the spatial and temporal regulation of anthocyanin biosynthesis (Yang et al., 2023). The expression patterns are frequently influenced by environmental factors, including light, temperature, and stress conditions, which serve to modulate the activity of R2R3-MYB TFs (Albert et al., 2011; Shin et al., 2016; Zhang et al., 2019; Yang et al., 2022).
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