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 HCTindependent. The growth phenotype of HCT-silenced plants was shown to be controlled by light and to depend on chalcone synthase expression. Histochemical analysis of silenced stem tissues demonstrated altered tracheary elements. The level of plant growth reduction of HCT-deficient plants was correlated with the inhibition of auxin transport. Suppression of flavonoid accumulation by chalcone synthase repression in HCT-deficient plants restored normal auxin transport and wildtype plant growth. By contrast, the lignin structure of the plants simultaneously repressed for HCT and chalcone synthase remained as severely altered as in HCT-silenced plants, with a large predominance of nonmethoxylated H units. These data demonstrate that the reduced size phenotype of HCT-silenced plants is not due to the alteration of lignin synthesis but to flavonoid accumulation.

Root hairs and rhizoids are cells with rooting functions in land plants. We describe two basic helix-loop-helix transcription factors that control root hair development in the sporophyte (2n) of the angiosperm Arabidopsis thaliana and rhizoid development in the gametophytes (n) of the bryophyte Physcomitrella patens. The phylogeny of land plants supports the hypothesis that early land plants were bryophyte-like and possessed a dominant gametophyte and later the sporophyte rose to dominance. If this hypothesis is correct, our data suggest that the increase in morphological complexity of the sporophyte body in the Paleozoic resulted at least in part from the recruitment of regulatory genes from gametophyte to sporophyte.

The hydroxyl group in the 3-position of the phenylpropanoid compounds is introduced at the level of coumarate shikimate/ quinate esters, whose synthesis implicates an acyltransferase activity. Specific antibodies raised against the recombinant tobacco (Nicotiana tabacum) acyltransferase revealed the accumulation of the enzyme in stem vascular tissues of tobacco, in accordance with a putative role in lignification. For functional analysis, the acyltransferase gene was silenced in Arabidopsis thaliana and N. benthamiana by RNA-mediated posttranscriptional gene silencing. In Arabidopsis, gene silencing resulted in a dwarf phenotype and changes in lignin composition as indicated by histochemical staining. An indepth study of silenced N. benthamiana plants by immunological, histochemical, and chemical methods revealed the impact of acyltransferase silencing on soluble phenylpropanoids and lignin content and composition. In particular, a decrease in syringyl units and an increase in p-hydroxyphenyl units were recorded. Enzyme immunolocalization by confocal microscopy showed a correlation between enzyme accumulation levels and lignin composition in vascular cells. These results demonstrate the function of the acyltransferase in phenylpropanoid biosynthesis.