As of June 2006, CesA and Csl gene sequences have been identified in 252 plant species (http://cellwall.stanford.edu/). In Arabidopsis, 10 CesA and 30 Csl genes have been identified. Similar numbers of CesA and Csl genes have been identified in other plants as well. In rice, at least 12 CesA genes have been identified by analysis of cDNA, ESTs, and genome sequencing (http://cellwall.stanford.edu/). Twelve members of the CesA gene family are identified in maize (Appenzeller et al., 2004). In most cases, the CesA genes are found to be dispersed on different chromosomes and have similar numbers of exons and introns. The CesA genes identified in maize from cDNA analysis and mapping studies were found to be distributed to different chromosomes, similar to the Arabidopsis CesA genes (Holland et al., 2000). In Arabidopsis, the genes range in size from 3.5 to 5.5 kbp and contain 9–13 introns and the CesA transcripts range in size from 3.0 to 3.5 kb, encoding proteins that are 985–1,088 amino acids in length (Richmond, 2000). Orthologs of the Arabidopsis CesA genes have been identified in a number of plants by phylogenetic analysis using the CesA protein sequences. Three maize CesAs, ZmCesA10–12 cluster with the Arabidopsis CesAs that are shown to be involved in secondary wall cellulose synthesis. ZmCesA10, ZmCesA11, and ZmCesA12 group with AtCesA4 (irx5), AtCesA8 (irx1), and AtCesA7 (irx3), respectively and are probable orthologs of these genes. Based on expression patterns, these three genes appear to be coordinately expressed (Appenzeller et al., 2004). Likewise, OsCesA7, OsCesA4, and OsCesA9 are the orthologous genes in rice, as are barley HvCesA4, HvCesA5/7, and HvCesA8 genes, respectively (Burton et al., 2004; Tanaka et al., 2003).
Orthologs of the Arabidopsis CesA genes required for secondary wall cellulose synthesis have also been identified by expression analysis of normal wood undergoing xylogenesis in hybrid aspen (Djerbi et al., 2004). Four CesAs, PttCesA1, PttCesA3–1, PttCesA3–2, and PttCesA9 were shown to exhibit xylem-specific expression, with the derived amino acid sequences and expression profiles of PttCesA3–1 and PttCesA3–2 being very similar, suggesting that they represent redundant copies of a CesA with the same function. Phylogenetic analysis indicates that the xylem-specific CesAs from hybrid poplar cluster with similar CesAs from other poplars and Arabidopsis. PttCesA1 is most similar to AtCesA4, PttCesA3–1, and PttCesA3–2 are closest to AtCesA8, and PttCesA9 is closest to AtCesA7 (Djerbi et al., 2004). Although it has been possible to identify orthologs of CesAs required for secondary wall cellulose synthesis in various plants, the relationship between the CesAs involved in primary wall cellulose synthesis from different plants is not as clear. From phylogenetic analysis, it appears that the genes for primary wall cellulose synthesis have duplicated relatively independently in dicots and monocots (Appenzeller et al., 2004).
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