Metabolic diversion of the phenylpropanoid pathway causes cell wall and morphological changes in transgenic tobacco stems

Studies involving transgenic plants with modifications in the lignin pathway reported to date, have received a relatively preliminary characterisation in relation to the impact on vascular integrity, biomechanical properties of tissues and carbon allocation to phenolic pools. Therefore, in this study transgenic tobacco plants (Nicotiana tabacum cv XHFD 8) expressing various levels of a bacterial 4-hydroxycinnamoyl-CoA hydratase/lyase (HCHL) gene have been characterised for cell wall and related morphological changes. The HCHL enzyme converts p-coumaroyl-CoA to 4-hydroxybenzaldehyde thereby rerouting the phenylpropanoid pathway. Plants expressing high levels of HCHL activity exhibited reduced lignin deposition, impaired monolignol biosynthesis and vascular integrity. The plants also exhibited reduction in stem toughness concomitant with a massive reduction in both the cell wall esterified and soluble phenolics. A notable result of redirecting the carbon flux was the wall-bound accretion of vanillin and vanillic acid, probably due to the shunt pathway. Intracellular accumulation of novel metabolites such as hydroxybenzoic and vanillic acid derivatives also occurred in the transgenic plants. A line with intermediate levels of HCHL expression conferred correspondingly reduced lignin deposition, toughness and phenolics. This line displayed a normal morphology but distorted vasculature. Coloration of the xylem has been previously attributed to incorporation of alternative phenolics, whereas results from this study indicate that the coloration is likely to be due to the association of low molecular weight phenolics. There was no evidence of increased growth or enhanced cellulose biosynthesis as a result of HCHL expression. Hence, rerouting the phenylpropanoid biosynthetic pathway quantitatively and qualitatively modifies cell wall-bound phenolics and vascular structure.