The irregular xylem3 Locus of Arabidopsis Encodes a Cellulose Synthase Required for Secondary Cell Wall Synthesis

Abstract

The irregular xylem3 (irx3) mutant of Arabidopsis has a severe deficiency in secondary cell wall cellulose deposition that leads to collapsed xylem cells. The irx3 mutation has been mapped to the top arm of chromosome V near the marker nga106. Expressed sequence tag clone 75G11, which exhibits sequence similarity to cellulose synthase, was found to be tightly linked to irx3, and genomic clones containing the gene corresponding to clone 75G11 complemented the irx3 mutation. Thus, the IRX3 gene encodes a cellulose synthase 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 the noncrystalline cell wall fraction. IRX3 and RSW1 fall into a distinct subgroup (Csa) of Arabidopsis genes showing homology to bacterial cellulose synthases.

Expression of a Mutant Form of Cellulose Synthase AtCesA7 Causes Dominant Negative Effect on Cellulose Biosynthesis

“…The reduced cellulose synthesis in primary walls is associated with a decrease in cell elongation, which was shown in some of these mutants to be correlated with an increased production of jasmonate and ethylene (Ellis et al, 2002). Two other CesA genes, AtCesA7 (IRX3) and AtCesA8 (IRX1), have been shown to be essential for cellulose biosynthesis in secondary walls (Taylor et al, 1999(Taylor et al, , 2000. Mutations in these gene as seen in the irx mutants cause dramatic reductions in cellulose content and secondary wall thickness, leading to a collapsed xylem phenotype (Turner and Somerville, 1997).…”

Patterning of plant cell wall deposition by cortical microtubules

“…Whereas libriform fibres have a uniformly deposited secondary cell wall, tracheary elements have patterned secondary cell wall thickenings. Besides structural support for the plant as a whole, these patterned secondary wall thickenings resist forces during transport, preventing collapse of the vessel as mentioned previously 6,[9][10][11][12] . Secondary cell wall thickenings in tracheary elements are deposited in different patterns, such as rings, spirals, reticulated or pitted patterns 1,13-15 ( Fig.…”

“…Mutants in these secondary CesAs result in dwarfed Arabidopsis plants, caused by collapsed vessel elements indicating the importance of the secondary cell wall thickenings [9][10][11][12] . Tracheary elements are characterized by typical patterned secondary cell wall thickenings.…”

Structure, organization, and functions of cellulose synthase complexes in higher plants

“…In fact, orthologs of AtCesA1, AtCesA3, and AtCesA6 appear to be required for the synthesis of cellulose deposited in the primary cell walls (Fagard et al, 2000;Scheible et al, 2001;Burn et al, 2002;Doblin et al, 2002). In contrast, orthologs of AtCesA4, AtCesA7, and AtCesA8 seem to be essential for cellulose synthesis in the secondary cell walls (Turner and Somerville, 1997;Taylor et al, 1999Taylor et al, , 2000Taylor et al, , 2003Zhong et al, 2003). The latter three CESA isoforms are specific to secondary cell wall formation and they actually physically interact with each other so it is highly probable that they are located in the same CelS as discussed previously (Taylor et al, 2003).…”