| Summary: | UDP-glucose, the precursor for cellulose biosynthesis, can be produced via the
catalysis of sucrose by sucrose synthase (SuSy) or through the phosphorylation of
glucose-I-phosphate by UDP-glucose pyrophosphorylase (UGPase). As such, these
genes, together with sucrose phosphate synthase (SPS) which recycles fructose (an
inhibitor of SuSy), are interesting targets for altering carbon allocation in plants.
In an attempt to alter cell wall biosynthesis in plants, targeted overexpression of
SuSy, UGPase and SPS independently and in a pyramiding strategy was assessed in
tobacco. All lines displayed enhanced growth and biomass production, and in the case
of double and triple transgenics, there was an additive effect. Despite the increased
growth rates, there was no consistent change in soluble carbohydrate pools.
Furthermore, only the triple transgenics had constant changes in structural
carbohydrates: with increased hemicellulose content and slight increases in cellulose.
Collectively, these results support the role of SPS, SuSy and UGPase in maintaining
sink strength, but suggest that the reallocation of carbon to cellulose production in
tobacco may not be possible by overexpressing these genes.
In contrast, transgenic poplar overexpressing UGPase produced significantly
more cellulose than wild-type trees. However, this was accompanied by a severe
reduction in growth and the production of a salicylic acid glucoside (SAG) in significant
quantities. The UDP-glucose generated by UGPase overexpression appeared to
participate in both the synthesis of cellulose and SAG, suggesting that cellulose
biosynthesis may be limited by the cellulose synthase complex.
Poplar transformed with SuSy and with SuSy x UGPase also had increased
cellulose production. The trees were phenotypically normal, with only minor reductions
in height growth in some lines. It appears that UDP-glucose may be channelled directly
to the cellulose synthase complex by SuSy. The increased cellulose content was
associated with an increase in cell wall crystallinity, but there was no change in
microfibril angle, confirming the re-allocation to cellulose synthesis was not the result of
tension wood formation, again supporting the hypothesis that the cellulose synthase
complex is the limiting factor.
Clearly, it is possible to alter cellulose deposition in trees by augmenting sucrose
metabolism to produce UDP-glucose, the precursor to cellulose biosynthesis.
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