Cellulose microfibrils, para-crystalline arrays of c. 36 chains of unbranched (1→4)-β-D-glucan chains, form the major scaffolding component in the cell wall (Carpita & McCann, 2000). Cellulose is synthesized by terminal complexes in the plasma membrane arranged in hexagonal rosette structures at the ends of microfibrils (Mueller & Brown, 1980; Herth, 1983). Each subunit of the rosette may contain six cellulose synthase (CesA) proteins that synthesize six (1→4)-β-D-glucan chains, which co-crystallize into the 36-glucan chain microfibril, with an estimated diameter of 8–10 nm (Doblin et al., 2002).
Arabidopsis thaliana has 10 CesA genes that encode proteins with 64% average sequence identity (Holland et al., 2000; Richmond, 2000). Maize (Zea mays) has at least 12 CesA genes (Appenzeller et al., 2004), while barley (Hordeum vulgare) has eight (Burton et al., 2004) and poplar (Populus trichocarpa) 18 (Djerbi et al., 2005). Functional genetic and co-expression analyses indicate that the A. thaliana CesA genes can be subdivided into two distinct groups. Group I contains genes required for synthesis of cellulose in primary (growing) cell walls: CesA1, CesA3 and CesA6, for which there are mutant alleles radial-swelling1 (rsw1; Arioli et al., 1998), isoxaben resistant1 (ixr1; Scheible et al., 2001), and procuste1 (prc1; Fagard et al., 2000), respectively. Three additional genes CesA2, CesA5 and CesA9 have sequence similarity to CesA6 and may have functions that show partial redundancy with those of CesA6 (Desprez et al., 2007; Persson et al., 2007). The expression patterns of the CesA2, CesA5, CesA6 and CesA9 genes are regulated according to developmental stage and tissue type. Group II consists of the genes CesA4, CesA7 and CesA8, for which there are mutant alleles irregular xylem5 (irx5; Taylor et al., 2003), irx3 (Taylor et al., 1999) and irx1 (Taylor et al., 2000), respectively. Group II genes are required for cellulose deposition in the thick secondary walls of vascular cells. Mutants are mainly characterized by the collapse of xylem vessels that are unable to withstand the negative pressure generated during water transport. The expression patterns of the secondary wall CesA genes show tight co-regulation, but this is less clear for primary wall CesA genes (Hamann et al., 2004; Brown et al., 2005; Persson et al., 2005). The similar effects on cellulose deposition of mutations in various single CesA genes have demonstrated that at least three different CesA gene products are required for a functional cellulose synthase complex in either primary (Desprez et al., 2007; Persson et al., 2007) or secondary walls (Taylor et al., 2000, 2003). Although numerous mutant alleles of A. thaliana CesA genes have been identified, a dominant growth phenotype is observed only upon overexpression of the fragile fiber5 (fra5) mutant allele of CesA7 (Zhong et al., 2003).
In this paper, we report the isolation of a novel missense mutation of AtCesA3 with a dominant-negative growth phenotype. The mutation occurs in the second cytoplasmic domain affecting the proline578 residue that is conserved among evolutionarily diverse, cellulose-biosynthetic organisms. In both homozygous and heterozygous seedlings, growth is impaired, dramatically so in the homozygous seedlings, consistent with an impact on primary-walled cells rather than on secondary-walled cells. Cellulose content is reduced in heterozygous and further reduced in homozygous plants, showing gene-dosage dependence. As the mutation is predicted to drastically change the secondary structure of the CesA3 subunit, we propose that the incorporation of a mis-folded CesA3 subunit into the cellulose synthase complex may stall or prevent the formation of heteromeric rosette complexes.