CAD2 deficiency causes both brown midrib and gold hull and internode phenotypes in Oryza sativa L. cv. Nipponbare

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Several brown midrib (bm) mutants have so far been isolated from the C4 grasses, maize, sorghum and pearl millet, but have not been detected in C3 grasses including rice (Oryza sativa). In the present study we characterized the cad2 (cinnamyl alcohol dehydrogenase 2) null mutant isolated from retrotransposon Tos17 insertion lines of Oryza sativa L. ssp. japonica cv. Nipponbare. This mutant exhibited brown-colored midribs in addition to hulls and internodes, clearly indicating both bm and gold hull and internode (gh) phenotypes. The enzymatic saccharification efficiency in the culm of cad2 null mutant was increased by 16.1% than that of the control plants. The lignin content of the cad2 null mutant was 14.6% lower than that of the control plants. Thioacidolysis of the cad2 null mutant indicated the presence of cinnamaldehyde structures in the lignin. Taken together, our results show that deficiency of OsCAD2 causes the bm phenotype in addition to gh, and that the coloration is probably due to the accumulation of cinnamaldehyde-related structures in the lignin. Additionally, this cad2 null mutant was useful to silage purposes and biofuel production.