Biosynthesis of hydroxycinnamate conjugates: implications for sustainable biomass and biofuel production

Hydroxycinnamic acids constitute a large class of phenylpropanoid metabolites that are distributed ubiquitously in terrestrial plants. They occur most frequently as esters, amides or glycosides within the cytosol, the particular subcellular compartments such as the vacuole or the cell wall. Hydroxycinnamate conjugates play a vital role in the plant's growth and development and in its defense responses against biotic- and abiotic-stresses. Furthermore, the incorporation of hydroxycinnamate conjugates into the cell wall is a major factor attenuating the wall's biodegradability. Understanding the biosyntheses of hydroxycinnamate conjugates and its molecular regulation may well facilitate the sustainable production of cell wall biomass, and the efficient conversion of lignocellulosic materials. This paper reviews our current molecular and biochemical understandings on the formation of several classes of hydroxycinnamate esters and amides, including the soluble conjugates and the 'wall-bound' phenolics. It also discusses the emerging biotechnological applications in manipulating hydroxycinnamates to improve the degradability of the cell wall biomass and enhance the production of valuable chemicals and biomaterials.