Role of bifunctional ammonia lyase in grass cell wall biosynthesis
L Phenylalanine ammonia lyase (PAL) is the first enzyme in the biosynthesis of phenylpropanoid derived plant compounds such as flavonoids, coumarins and the cell wall polymer lignin. The cell walls of grasses possess higher proportions of syringyl (S) rich lignins and high levels of esterified coumaric acid compared with those of dicotyledonous plants, and PAL from grasses can also possess tyrosine ammonia lyase (TAL) activity, the reason for which has remained unclear. Using phylogenetic, transcriptomic and in vitro biochemical analyses, we identified a single homotetrameric bifunctional ammonia lyase (PTAL) among eight BdPAL enzymes in the model grass species Brachypodium distachyon. 13C isotope labelling experiments along with BdPTAL1 downregulation in transgenic plants showed that the TAL activity of BdPTAL1 can provide nearly half of the total lignin deposited in Brachypodium, with a preference for S lignin and wall bound coumarate biosynthesis, indicating that PTAL function is linked to the characteristic features of grass cell walls. Furthermore, isotope dilution experiments suggest that the pathways to lignin from L phenylalanine and L tyrosine are distinct beyond the formation of 4 coumarate, supporting the organization of lignin synthesis enzymes in one or more metabolons.
Nature Plants