Early phenylpropanoid biosynthetic pathway genes are responsible for flavonoid accumulation in the leaves of three crabapple ( Malus spp.) cultivars

SUMMARYFlavonoids are a class of plant secondary metabolites, some of which are pigments that contribute to the colour of various organs including flowers and leaves. It is known that phenylpropanoid metabolism is coupled to flavonoid biosynthesis, but the mechanism by which the expression of early genes in the phenylalanine biosynthetic pathway affect flavonoid accumulation remains unclear. In this study, levels of expression of the genes encoding phenylalanine ammonia lyase (McPAL), cinnamic acid 4-hydroxylase (McC4H), and 4-coumarate:CoA ligase (Mc4CL) in the leaves of three cultivars of crabapple (Malus spp.) [‘Royalty’ (red leaves), ‘Flame’ (green leaves), and ‘Radiant’ (red buds, green mature leaves)] were measured, together with flavonoid concentrations. Levels of gene expression were consistent with the concentrations of flavonols in the leaves, with transcript levels being significantly higher in mature leaves than in young leaves. Reduced light and low temperature treatments resulted in the up-regulation of early gene expression in the phenylpropanoid biosynthetic pathway, which may have been responsible for the increase in flavonoid concentration. Expression of the R2R3 transcription factor, McMYB2, was positively correlated with the levels of expression of McPAL, McC4H, and Mc4CL. These results indicate that early genes in the phenylpropanoid biosynthetic pathway are important for the accumulation of flavonoids, especially flavonols, and that McMYB2 may be involved in regulating their expression in crabapple leaves.