Annals of Botany

Sink signals source in shaded sugar

Following a small decline in 2000–01, world sugar production has risen steadily to reach 163 million tonnes in 2006–07. About 70 % of this comes from sugar cane (Saccharum spp.), providing a major source of revenue for many less-developed countries. It is thus important to understand more about the factors that influence sucrose synthesis and deposition, as discussed by a South African–Australian group, McCormick et al. (pp. 89–102). They disrupted normal source–sink relationships by reducing the source to just one leaf; all other leaves being shaded. This led to decreased hexose concentrations in the unshaded leaf, while sucrose concentrations were little affected; the reverse was true for culm (the sink tissue). There was a marked increase in photosynthetic efficiency of the unshaded leaf, especially evident in the assimilation rate and the electron transport rate. Then, in a very careful and thorough study, the authors used a reverse northern macroarray to look at gene expression in the unshaded leaf. Of the genes studied, 27 showed changes in expression greater than two-fold; the majority of these were up-regulated. The cohort of up-regulated genes showed a remarkable correlation with the changes in leaf metabolism; for example, expression of PEP carboxylase, representing the C₄ phase of photosynthesis, increased by between 2.2- and 4.3-fold in 14 days, while that of the large subunit of Rubisco increased 3-fold over the same period. Such changes were typical of the range of genes involved in photosynthesis. Of the down-regulated genes, the authors note particularly hexokinase (HXK) and fructokinase (FK). Down-regulation of HXK was strongly correlated with the decline in leaf hexose concentration, consistent with a role for HXK as a sensor in hexose signalling. Overall, the results provide clear evidence that an increased carbon demand at the sink (because of decreased supply from the source) leads to signalling, possibly via a hexose-based pathway, thus up-regulating C-fixation via increased photosynthesis in the source leaf.