Molecular characterization of two Arabidopsis thaliana glycosyltransferase mutants, rra1 and rra2, which have a reduced residual arabinose content in a polymer tightly associated with the cellulosic wall residue

Two putative glycosyltransferases in Arabidopsis thaliana, designated reduced residual arabinose-1 and -2 (RRA1 and RRA2), are characterized at the molecular level. Both genes are classified in CAZy GT-family-77 and are phylogenetically related to putative glycosyltranferases of Chlamydomonas reinhardtii. The expression pattern of the two genes was analyzed by semi-quantitative RT-PCR using mRNA extracted from various organs of bolting Arabidopsis thaliana plants. In addition, promoter::gusA analysis of transgenic Arabidopsis thaliana containing a fusion between either the RRA-1 or -2 promoter fragment and the gusA reporter gene showed that whereas the RRA1 promoter was primarily active in the apical meristem, the expression pattern of the RRA2 promoter was more diverse but also highly active in the meristematic region. In addition, T-DNA mutant insertion lines of both RRA-1 and -2, were identified and characterized at the molecular and biochemical level. Monosaccharide compositional analyses of cell wall material isolated from the meristematic region showed a ca. 20% reduction in the arabinose content in the insoluble/undigested cell wall residue after enzymatic removal of xyloglucan and pectic polysaccharides. These data indicate that both RRA-1 and -2 play a role in the arabinosylation of cell wall component(s).