Cinnamyl alcohol dehydrogenase deficiency causes the brown midrib phenotype in rice

Abstract

Because lignin encrusts lignocellulose polysaccharides, it presents obstacles to chemical pulping, forage digestion, and enzymatic hydrolysis of plant cell wall polysaccharides for biorefining. Hence, it would be beneficial for plant materials to either contain less lignin or to have lignin that is easier to remove for these processes. Grass mutants known as brown midrib (bm) mutants generally show a reduced lignin content and higher in vitro digestibility compared with wild-type plants. Several bm mutants have been isolated only from the C4 grasses, maize, sorghum, and pearl millet, but have not been detected in C3 grasses including rice (Oryza sativa). Recently, the cad2 (cinnamyl alcohol dehydrogenase 2) null mutant isolated from retrotransposon Tos17 insertion lines of O. sativa ssp. japonica cv. Nipponbare was observed to exhibit brown-colored midribs in addition to hulls and internodes, clearly showing both bm and gold hull and internode (gh) phenotypes. In addition, chemical analysis of the mutant indicated that the coloration was probably due to the accumulation of cinnamaldehyde-related structures in the lignin. The lignin content of the cad2 null mutant was lower than that of the control plants, while the enzymatic saccharification efficiency in the culm of cad2 null mutant was increased compared with that of the control plants. This mutation could be applied to breed forage paddy rice cultivars and other grass biomass plants that are suitable for use as fodder and industrial feedstock.