The Cellulose Synthase Genes
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,
Zm
CesA10–12 cluster with the
Arabidopsis CesAs that are shown to be involved in
secondary wall cellulose synthesis. Zm
CesA10, Zm
CesA11, and Zm
CesA12 group
with At
CesA4 (
irx5), At
CesA8 (
irx1), and At
CesA7 (
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, Os
CesA7,
Os
CesA4, and Os
CesA9 are the orthologous genes in rice, as are barley Hv
CesA4,
Hv
CesA5/7, and Hv
CesA8 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, Ptt
CesA1,
Ptt
CesA3–1, Ptt
CesA3–2, and Ptt
CesA9 were shown to exhibit xylem-specific
expression, with the derived amino acid sequences and expression profiles of
Ptt
CesA3–1 and Ptt
CesA3–2 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. Ptt
CesA1 is most similar to At
CesA4,
Ptt
CesA3–1, and Ptt
CesA3–2 are closest to At
CesA8, and Ptt
CesA9 is closest to
At
CesA7 (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).