Submitted to: Journal of the French Academy of Science
Publication Type: Review article
Publication Acceptance Date: 3/23/2004
Publication Date: 9/15/2004
Citation: Barrière, Y., Ralph, J., Méchin, V., Guillaumie, S., Grabber, J.H., Argillier, O., Chabbert, B., Lapierre, C. 2004. Genetic and molecular basis of grass cell wall biosynthesis and degradability. II. Lessons from brown-midrib mutants. Comptes Rend. Biologies 327(9):847-860. Interpretive Summary:
Technical Abstract: The brown midrib varieties of maize, which exhibit a reddish brown pigmentation of the leaf midrib and stalk pith, associated with lignified tissues, have been well known for a little less a century. Their under-lignification, and their higher feeding value, at least for some mutants, was evidenced about 50 years ago. These mutants thus progressively became models for lignification studies in maize and grasses, both based on genetic and biochemical topics. Comparisons at silage maturity of bm1, bm2, bm3, bm4 plants highlighted their reduced lignin, but also illustrated the biochemical specificities of each mutant, with a reduced p-coumarate content in bm1 and bm3, a reduced ferulate ester content in bm1, and a reduced etherified ferulate content in bm2, and to a lower extent in bm1 and bm4. The syringyl/guaiacyl monomer ratio after thioacidolysis was strongly reduced in bm3 plants, and possibly slightly increased in bm2 and bm4, whereas an important reduction in the release of these two monomers was also observed in bm1. From bm3 plant studies, it appears likely that, in maize, the bm3 locus encodes a 5-hydroxyconiferaldehyde O-methyltransferase (CaldOMT) rather than a caffeic acid O-methyltransferase (COMT) enzyme. Based on the current knowledge of the lignin pathway, and based on presently developed data and discussion, CCoAOMTs are probably a major hub in controlling cell wall lignification (and digestibility). It is also likely that ferulates come via the CCoAOMT pathway. Maize could be considered as the leading grass model plant for investigations on lignification and its relationships with cell wall digestibility. Targets considered of interest in maize could likely be extrapolated to other monocotyledon forages, including C3 plants.