Multiple cellulose synthase catalytic subunits are required for cellulose synthesis in Arabidopsis

The irregular xylem 1 (irx1) mutant of Arabidopsis has a severe deficiency in the deposition of cellulose in secondary cell walls, which results in co llapsed xylem cells. This mutation has been mapped to a 140-kb region of ch romosome 4. A cellulose synthase catalytic subunit was found to be located in this region, and genomic clones containing this gene complemented the ir x1 mutation. IRX1 shows homology to a previously described cellulose syntha se (IRX3). Analysis of the irx1 and irx3 mutant phenotypes demonstrates tha t both IRX1 and IRX3 are essential for the production of cellulose in the s ame cell. Thus, IRX1 and IRX3 define distinct classes of catalytic subunits that are both essential for cellulose synthesis in plants. This finding is supported by coprecipitation of IRX1 with IRX3, suggesting that IRX1 and I RX3 are part of the same complex.