Arabidopsis cyt1 mutants are deficient in a mannose-1-phosphate guanylyltransferase and point to a requirement of N-linked glycosylation for cellulose biosynthesis

Arabidopsis cyt1 mutants have a complex phenotype indicative of a severe de fect in cell wall biogenesis. Mutant embryos arrest as wide, heart-shaped s tructures characterized by ectopic accumulation of callose and the occurren ce of incomplete cell walls. Texture and thickness of the cell walls are ir regular, and unesterified pectins show an abnormally diffuse distribution. To determine the molecular basis of these defects, we have cloned the CYT1 gene by a map-based approach and found that it encodes mannose-1-phosphate guanylyltransferase. A weak mutation in the same gene, called vtc1, has pre viously been identified on the basis of ozone sensitivity due to reduced le vels of ascorbic acid. Mutant cyt1 embryos are deficient in N-glycosylation and have an altered composition of cell wall polysaccharides. Most notably , they show a 5-fold decrease in cellulose content. Characteristic aspects of the cyt1 phenotype, including radial swelling and accumulation of callos e, can be mimicked with the inhibitor of N-glycosylation, tunicamycin. Our results suggest that N-glycosylation is required for cellulose biosynthesis and that a deficiency in this process can account for most phenotypic feat ures of cyt1 embryos.