PMT, a Grass-Specific Acyltransferase Involved in Lignin Biosynthesis

 

Grass lignins contain substantial amounts of p-coumarate (pCA) acylating the sidechains of the phenylpropanoid polymer backbone. An acyltransferase, named p-coumaroyl-CoA:monolignol transferase (OsPMT), that could acylate monolignols with pCA in vitro was recently identified from rice. In planta, such monolignol-pCA conjugates become incorporated into lignin via oxidative radical coupling, thereby generating the observed pCA appendages. However, p-coumarates also acylate arabinoxylans in grasses. To test the authenticity of PMT as a lignin biosynthetic pathway enzyme, we examined Brachypodium distachyon plants misregulated in the putative BdPMT gene. Using newly developed cell wall analytical methods, we determined that the transferase was specifically involved in lignin acylation. A sodium azide-generated Bdpmt-1 missense mutant had no (<0.5%) residual pCA on lignin, and BdPMT RNAi plants had levels as low as 10% of wild-type, whereas the amounts of pCA acylating arabinosyl units on arabinoxylans in these PMT-misregulated plants remained unchanged. pCA acylation of lignin from BdPMT overexpressing plants was found to be more than three-fold higher than that of wild-type, but again the level on arabinosyl units remained unchanged. Taken together, these data are consistent with a defined role for grass PMT genes in encoding BAHD acyltransferases that specifically acylate monolignols with pCA, producing monolignol p-coumarate conjugates that are used for lignification in planta.