Evidence for a role of AtCAD 1 in lignification of elongating stems of Arabidopsis thaliana

The cinnamyl alcohol dehydrogenase (AtCAD) multigene family in Arabidopsis is composed of nine genes. Our previous studies focused on the two isoforms AtCAD C and AtCAD D which show a high homology to those related to lignification in other plants. This study focuses on the seven other Arabidopsis CAD for which functions are not yet elucidated. Their expression patterns were determined in different parts of Arabidopsis. Only CAD 1 protein can be detected in elongating stems, flowers, and siliques using Western-blot analysis. Tissue specific expression of CAD 1, B1, and G genes was determined using their promoters fused to the GUS reporter gene. CAD 1 expression was observed in primary xylem in accordance with a potential role in lignification. Arabidopsis T-DNA mutants knockout for the different genes CAD genes were characterized. Their stems displayed no substantial reduction of CAD activities for coniferyl and sinapyl alcohols as well as no modifications of lignin quantity and structure in mature inflorescence stems. Only a small reduction of lignin content could be observed in elongating stems of Atcad 1 mutant. These CAD genes in combination with the CAD D promoter were used to complement a CAD double mutant severely altered in lignification (cad c cad d). The expression of AtCAD A, 131, 132, F, and G had no effect on restoring a normal lignin profile of this mutant. In contrast, CAD 1 complemented partly this mutant as revealed by the partial restoration of conventional lignin units and by the decrease in the frequency of beta-O-4 linked p-OH cinnamaldehydes.