Flavonoid accumulation in Arabidopsis repressed in lignin synthesis affects auxin transport and plant growth

Abstract

In Arabidopsis thaliana, silencing of hydroxycinnamoyl-CoA shikimate/qulnate 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 flavonolds through chalcone synthase activity. Several flavonol glycosides and acylated anthocyanin were shown to accumulate in higher emounts in silenced plants. By contrast, slnapoylmalate levels were barely affected, suggesting that the synthesis of that phenylpropenoid compound might be HCT-independent. The growth phenotype of HCT-silenced plants was shown to be controlled by light and to depend on chalcone synthase expression. Histochemlcal analysis of silenced stem tissues demonstrated altered tracheary elements. The level of plant growth reduction of HCT-deficiant plants was correlated with the inhibition of auxin transport. Suppression of flavonold accumulation by chalcone synthase repression in HCT-deficient plants restored normal auxin transport and wild-type plant growth. By contrast, the lignin atructure of the plants simultaneoualy repressed for HCT and chalcone aynthaae remained as severely altered as in HCT-allenced plants, with a large predominance of nonmethoxylated H units. These data demonstrate thst the reduced size phenotype of HCT-silenced plants is not due to the alteration of lignin synthesis but to flavonold accumulation. © 2007 American Society of Plant Biologists.