Regulation of Flavonoid Biosynthesis by the MYB-bHLH-WDR (MBW) Complex in Plants and Its Specific Features in Cereals

Flavonoids are a large group of secondary metabolites, which are responsible for pigmentation, signaling, protection from unfavorable environmental conditions, and other important functions, as well as providing numerous benefits for human health. Various stages of flavonoid biosynthesis are subject to complex regulation by three groups of transcription regulators-MYC-like bHLH, R2R3-MYB and WDR which form the MBW regulatory complex. We attempt to cover the main aspects of this intriguing regulatory system in plants, as well as to summarize information on their distinctive features in cereals. Published data revealed the following perspectives for further research: (1) In cereals, a large number of paralogs of MYC and MYB transcription factors are present, and their diversification has led to spatial and biochemical specialization, providing an opportunity to fine-tune the distribution and composition of flavonoid compounds; (2) Regulatory systems formed by MBW proteins in cereals possess distinctive features that are not yet fully understood and require further investigation; (3) Non-classical MB-EMSY-like complexes, WDR-independent MB complexes, and solely acting R2R3-MYB transcription factors are of particular interest for studying unique regulatory mechanisms in plants. More comprehensive understanding of flavonoid biosynthesis regulation will allow us to develop cereal varieties with the required flavonoid content and spatial distribution.