Flavonoid Accumulation in Arabidopsis Repressed in Lignin Synthesis Affects Auxin Transport and Plant Growth

In Arabidopsis thaliana, silencing of hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyl transferase (HCT), a lignin biosynthetic gene, results in a strong reduction of plant growth. We show that, in HCT-silenced plants, lignin synthesis repression leads to the redirection of the metabolic flux into flavonoids through chalcone synthase activity. Several flavonol glycosides and acylated anthocyanin were shown to accumulate in higher amounts in silenced plants. By contrast, sinapoylmalate levels were barely affected, suggesting that the synthesis of that phenylpropanoid compound might be HCT-independent. The growth phenotype of HCT-silenced plants was shown to be controlled by light and to depend on…