Biogenesis of Cellulose Nanofibrils by a Biological Nanomachine - The Nanoscience and Technology of Renewable Biomaterials - page 54

for primary and secondary cell wall biosynthesis are common among angiosperms. For
example, orthologs of AtCesA 4, 7 ,and 8 are expressed in wood and cotton fibers
during secondary wall deposition (9, 68, 69). The use of a different family of CesA
genes for primary vs. secondary wall synthesis could cause differences in the fine
structure of cellulose nanofibrils that are not yet fully known. Phylogenetic analyses
(cited above) generally agree that the divergence of primary and secondary wall types
of CesAs occurred prior to the diversifications that produced the three different primary
and secondary CesAs. Therefore, rosettes composed of P1-P3 or S1-S3 CesA subunits
evolved independently in the context of both primary and secondary wall deposition.
Among the sequenced and annotated angiosperm genomes, CesA genes in the S clades
are less diversified than those in the P clades, which is consistent with unknown greater
constraints on functionality in the S clades.
The P3 clade of Arabidopsis is unique in having four members ( AtCesA2 , 5 , 6 and
9) with origins that can be traced to the two most recent duplications of the Arabidop-
sis genome (70). Selective gene retention in this lineage is consistent with selection
favoring functional specialization, and there is evidence for tissue-specific roles of the
AtCesA6-like genes in Arabidopsis (17, 71). Whereas AtCesA1 and AtCesA3 null muta-
tions are embryo lethal, mutations in CesA2,5,6 and/or 9 yield phenotypes related to
cellulose deficiency in certain organs. However, the genome duplications that gave rise
to these paralogs occurred after the divergence of the crucifer lineage. (Paralogs are
genes within a genome that share a common ancestor but differ in function.) There-
fore, the particular case of functional specialization among AtCesA2,5,6 ,and 9 cannot
be generalized to other angiosperms groups. Accounting for all 10 of the Arabidopsis
CesA genes, AtCesA10 arose from a recent duplication of AtCesA1 , and it has limited
expression within the Arabidopsis plant.
As the CesA gene families in other species are understood in more detail, similar
cases of diversification and functional specialization may well be discovered. Addi-
tional research is needed to reveal whether CesA diversification within specific clades
and taxa underpin adaptive changes in the regulation of cellulose synthesis and/or the
nanoscale properties of cellulose that enhance fitness in relation to particular growth
habits and/or ecological niches. For example, among the sequenced and annotated
angiosperm genomes, Populus trichocarpa has the most numerous CesA genes, with
18 (25). We can speculate that 18 P. trichocarpa CesA genes (vs. 10 in Arabidopsis)
might support the massive scale of cellulose synthesis in wood as well as developmental
specializations required for large trees to survive stress, such as the ability to synthesize
cellulose-rich tension wood with larger cellulose nanofibrils upon bending (61).
The CesA gene families of nonvascular plants (the moss, Physcomitrella patens) and
early-divergent vascular plants (the lycophyte or spikemoss, Selaginella moellendorfii )
can shed light on the significance of CesA diversification in seed plants. Phylogenetic
analyses support the contention that the common ancestor of P. patens and S. moellen-
dorfii shared a single CesA gene (27), as did the common ancestor of lycophytes and
seed plants (A. Roberts, unpublished). Although S. mollendorfii is a vascular plant, it
has no orthologs of angiosperm P1-P3 or S1-S3 CesA genes. Therefore, divergence of
the P and S CesA clades was not a precondition for evolution of vascular cells with sec-
ondary walls. The number of CesA genes increased and S1-S3 clades diverged before the
divergence of gymnosperms and angiosperms within the euphyllophyte lineage, which