QUASIMODO1 is Expressed in Vascular Tissue of Arabidopsis Thaliana Inflorescence Stems, and Affects Homogalacturonan and Xylan Biosynthesis

An insertion in the promoter of the Arabidopsis thaliana QUA1 gene (qua1-1 allele) leads to a dwarf plant phenotype and a reduction in cell adhesion, particularly between epidermal cells in seedlings and young leaves. This coincides with a reduction in the level of homogalacturonan epitopes and the amount of GalA in isolated cell walls (Bouton et al., Plant Cell 14: 2577 2002). The present study was undertaken in order to investigate further the link between QUA1 and cell wall biosynthesis. We have used rapidly elongating inflorescence stems to compare cell wall biosynthesis in wild type and qua1-1 mutant tissue. Relative to the wild type, homogalacturonan alpha-1-4-D-galacturonosyltransferase activity was consistently reduced in qua1-1 stems (by about 23% in microsomal and 33% in detergent-solubilized membrane preparations). Activities of beta-1-4-D-xylan synthase, beta-1-4-D-galactan synthase and beta-glucan synthase II activities were also measured in microsomal membranes. Of these, only beta-1-4-D-xylan synthase was affected, and was reduced by about 40% in qua1-1 stems relative to wild type. The mutant phenotype was apparent in inflorescence stems, and was investigated in detail using microscopy and cell wall composition analyses. Using in situ PCR techniques, QUA1 mRNA was localized to discrete cells of the vascular tissue and subepidermal layers. In mutant stems, the organization of these tissues was disrupted and there was a modest reduction in homogalacturonan (JIM5) epitopes. This study demonstrates a specific role for QUA1 in the development of vascular tissue in rapidly elongating inflorescence stems and supports a role of QUA1 in pectin and hemicellulose cell wall synthesis through affects on alpha-1,4-D-galacturonosyltransferase and beta-1,4-D-xylan synthase activities.