CesA proteins

6 CONSERVATION OF CesA PROTEIN FUNCTION IN OTHER SPECIES 

In order for information gained from studying Arabidopsis to be of use to industry it is important to establish that mechanisms of cellulose synthesis are conserved between Arabidopsis and commercially important species such as crop plants and trees. Studies on CesA genes in rice represent a powerful example of how well the mechanism of cellulose synthesis is conserved between diverse species. Three brittle-culm mutants of rice are the result of mutations in the rice orthologues of AtCesA4, 7 and 8 (Tanaka et al. 2003). This result suggests that in secondary cell walls a very similar mechanism of both cellulose synthesis and rosette organization is conserved between rice and Arabidopsis. 

A series of studies on both poplar and pine suggest that multiple CesA proteins are required in the secondary cell wall during wood formation (Kalluri and Joshi 2003 Djerbi et al. 2004 Kalluri and Joshi 2004 Liang and Joshi 2004 Naim and Haselkorn 2005). Further information on this topic may be found elsewhere in this issue. 

7 OTHER irx GENES REQUIRED FOR SECONDARY CELL WALL FORMATION 

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Peng L., Xiang F., Roberts E., Kawagoe Y, Greve L.C., Kreuz K., and Delmer D.P. 2001. The experimental herbicide CGA 325 615 inhibits synthesis of crystalline cellulose and causes accumulation of non-crystalline P-l,4-glucan associated with CesA protein. Plant Physiol 126 981 992. 

Taylor N.G., Howells R.M., Huttly A.K., Vickers K., and Turner S.R. 2003. Interactions among three distinct CesA proteins essential for cellulose synthesis. Proc Natl Acad Sci USA 100 1450-1455. 

Table 3-3. The cellulose synthase (CESA)proteins of Arabidopsis...

FUNCTION OF MULTIPLE CesA PROTEINS DURING CELLULOSE SYNTHESIS... 

Studies on mutants affecting cellulose synthesis in the primary cell wall have also led to the idea that three catalytic subunits are required for cellulose synthesis in these cell types. These genes are different from those required in the secondary cell wall (Arioli et al. 1998 Fagard et al. 2000 Scheible et al. 2001). Work on the rswl mutant of Arabidopsis demonstrates that in addition to their catalytic fimc-tion, CesA proteins are also essential for determining rosette structure, rswl-1 is caused by a comparatively small amino acid change (Ala to Val) in AtCesAl, yet at the restrictive temperature the hexameric rosettes are replaced by smaller structures (Arioli et al. 1998). 

The isolation of mutants deficient in secondary cell wall cellulose synthesis has led to the identification of a number of genes essential for cellulose production. The identification of these genes and the discovery that three CesA proteins are involved in the same protein complex has answered some of the questions regarding the complexity of the cellulose synthase complex. That fact that all three proteins are required for the correct assembly and targeting of the complex to the plasma membrane will allow us to further dissect the roles of these and other proteins in cellulose synthesis. In addition, the identification of a protein that is involved in both primary and secondary cell wall cellulose synthesis may allow us to identify other common components between the primary and secondary cell wall cellulose synthesizing machinery. 

Figure 5-2. Unrooted single most parsimonious tree of the CesA proteins from maize and Arabi-dopsis determined by the Branch and Bound algorithm of PAUP (Swofford 1998). Branch lengths are proportional to the inferred manber of amino acid substitutions, which are shown in bold font. Bootstrap values from 500 replicates (Felsenstein 1985) are represented as a percentage and are shown in parentheses. Reproduced with kind permission of Springer Science and Business Media from Appenzeller et al., 2004, Cellulose 11 287-299, Fig. 2. 2004 Kluwer Academic Publishers.

Dimerization of the CesA proteins has been proposed for the formation of a functional cellulose synthase complex (Scheible et al. 2001 Kurek et al. 2002). 

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