Convergence between light quality and methyl jasmonate signaling in the regulation of betalain biosynthesis

Betalains are nitrogen-containing pigments that produce red-violet and yellow-orange coloration in plants of the order Caryophyllales. Owing to their antioxidant, anti-inflammatory and anticancer activities, they have attracted considerable interest as natural food colourants and functional ingredients. Betalain biosynthesis is governed by both developmental programmes and environmental inputs, among which light quality and jasmonate signalling are key regulators. This review systematically examines the independent and synergistic roles of these two signals in controlling betalain production. Exogenous methyl jasmonate (MeJA), a widely used elicitor, is converted intracellularly to the bioactive ligand JA-Ile, which triggers SCF(COI1)-dependent degradation of JAZ repressors, thereby releasing MYC2 and MYC3 to activate downstream transcription. Light quality, operating through specific photoreceptors and the COP1-HY5 module, modulates betalain accumulation in a wavelength- and species-specific manner. Accumulating evidence suggests that both pathways converge on R2R3-MYB transcription factors, which bind MBS motifs in the promoters of the key structural genes CYP76AD1 and DODA. The betalain biosynthetic pathway proceeds through four principal enzymatic steps to yield betacyanins and betaxanthins, catalysed sequentially by ADH, CYP76AD1, DODA and cDOPA5GT. We propose a MYC2-HY5-MYB regulatory axis as a working model for the coordinate regulation of betalain biosynthesis, and identify key questions that remain to be resolved through functional studies in betalain-producing species.