Biogenesis of Cellulose Nanofibrils by a Biological Nanomachine - The Nanoscience and Technology of Renewable Biomaterials - page 53

2.4.8 Control Crystallization?

Niklas (1992) points out that the parallel chain conformation within cellulose nanofibrils
is an essential basis of the high elastic modulus and tensile strength of cellulose within
cell walls. As explained before, the organized CSC is a necessary facilitator of cellulose
I crystallization through its extrusion of numerous ß-1,4-glucan chains in close prox-
imity. In addition, the nanostructure of the CSC seemingly regulates fine differences
in the allomorph of cellulose I synthesized by different species. Spectral analysis via
solid-state 13 C NMR showed that native celluloses include species-specific mixtures of
two allomorphs, I α and I ß , which differ primarily in their hydrogen bonding patterns.
Typically, higher plants with rosette type CSCs produce 60-80% of the I ß form despite
variances in the size and crystallinity of nanofibrils (62, 63). Currently, it is not known if
or how differences in CSC geometry lead to such subtle differences in chain associations
and crystallinity.
Other proteins may also impact cellulose crystallinity, including specialized endo-ß-
1,4-glucanases (i.e. CMCax or Korrigan) associated with cellulose synthesis in prokary-
otes and plants (64-66). It is not known how an endo-ß-1,4-glucanase is functionally
integrated with the operation of the CSC.

2.4.9 Move in the Plasma Membrane as it Spins out Cellulose Nanofibrils

Both live cell imaging of labeled CesA protein (36) and biophysical modeling (67)
supported the early hypothesis that the force of crystallization causes the CSC to move
in the plasma membrane as it spins cellulose fibrils. Microscopy showed an average rate
of movement for labeled CesA protein of 330 nm/min during primary wall synthesis in
Arabidopsis hypocotyls. The role of the cytoskeleton in determining the orientation of
this movement, possibly via a feedback mechanism also involving pre-existing nanofibrils
as controlling entities, is beyond the scope of this chapter.

2.5 Phylogenetic Analysis

Phylogenetic analysis is valuable for assessing essential elements and possible points of
functional variation in the CSC nanomachine. Phylogenetic analysis is a computational
method allowing the estimation of evolutionary relationships among groups of divergent
CesA gene sequences. When sequence relationships are viewed in the context of the
timing of evolution of different groups, plant morphology, and CSC structure, we can
generate hypotheses about control of basic function and possible functional diversification
of the CSC. This approach is the most powerful in the context of fully sequenced
genomes that allow complete gene families to be identified. Therefore, the discussion
below emphasizes plant species with sequenced genomes, and this approach will become
even more informative as the number of sequenced genomes increases.

2.5.1 Possible Functional Diversification of CS Proteins

The presence of orthologs, from other seed plants, of the functionally distinct Arabidopsis
CesAs within clades P1-3 and S1-3 indicates that heterotrimeric rosette CSCs specialized