The irregular xylem3 locus of arabidopsis encodes a cellulose synthase required for secondary cell wall synthesis

The irregular xylem3 (irx3) mutant of Arabidopsis has a severe deficiency i n secondary cell wall cellulose deposition that leads to collapsed xylem ce lls. The irx3 mutation has been mapped to the top arm of chromosome V near the marker nga106. Expressed sequence tag clone 75G11, which exhibits seque nce similarity to cellulose synthase, was found to be tightly linked to irx 3, and genomic clones containing the gene corresponding to clone 75G11 comp lemented the irx3 mutation. Thus, the IRX3 gene encodes a cellulose synthas e component that is specifically required for the synthesis of cellulose in the secondary cell wall. The irx3 mutant allele contains a stop codon that truncates the gene product by 168 amino acids, suggesting that this allele is null, Furthermore, in contrast to radial swelling1 (rsw1) plants, irx3 plants show no increase in the accumulation of beta-1,4-linked glucose in t he noncrystalline cell wall fraction. IRX3 and RSW1 fall into a distinct su bgroup (Csa) of Arabidopsis genes showing homology to bacterial cellulose s ynthases.