Monolignol Compositional Determinants in Loblolly Pine: Aromatic Amino Acid Metabolism and Associated Rate-Limiting Steps

Phenylpropanoid and phenylpropanoid-acetate pathways are major repositories of organic carbon in vascular plants, affording the related lignins, lignans, suberins, flavonoids, coumarins, and numerous other phenolics. Together these constitute about 30-40% of all organic plant matter. Their formation was central to the evolutionary adaptation of plants to land, this being achieved mainly by elaboration of complex biochemical pathways from phenylalanine and, to a lesser extent, tyrosine. A component of our research program is involved in determining how monolignol formation, leading to lignins and lignans, is controlled in terms of both the ratios and relative amounts of monolignols formed. Thus, loblolly pine ( Pinus taeda L.) cell suspension cultures were grown under conditions which induced formation of its monolignol precursors, p -coumaryl and coniferyl alcohols. Increased phenylalanine availability to the cells resulted in a steady increase in amounts of p -coumaryl alcohol relative to that of coniferyl alcohol, until essentially equimolar levels were reached. Monolignol precursor administration experiments and metabolite analysis (pool sizes and end-product accumulation) revealed that there were three operationally slower steps during monolignol formation: these included the overall nitrogen recycling process from phenylalanine, and the two hydroxylation steps affording p -coumarate and caffeate, respectively. No evidence was found that cinnamyl alcohol dehydrogenase functions in a rate-limiting capacity, in contrast to previous assertions by others.