Annals of Botany

Carbs in the coffee cup

If I were asked to name the compounds that affect coffee flavour, polysaccharides would probably not feature in my list. However, this omission would be a mistake because in coffee (Coffea sp.), polysaccharides are laid down as nutrient reserves in the grain. Thus, as pointed out by Pré et al., at Tours, France (pp. 207–220) 25 % of the dry weight of a mature coffee bean consists of galactomannans, polysaccharides with a backbone of mannose units and side groups of single galactose units. They exist in the seed mainly as thickening material in the endosperm cell walls. They are important because of their own solubility profiles (which are affected by the degree to which the mannan backbone is modified by addition of galactose residues) and because of their effects on the solubility of other compounds. Because of the importance of these compounds in the coffee industry, the authors have studied the regulation of galactomannan synthesis at the level of gene expression. Two cDNAs encoding mannan synthases (ManS1 and 2) and two encoding galactomannan galactosyl transferases (GMGT 1 and 2) were identified. Gene sequence comparisons showed that ManS1 and 2 are related to two groups within the cellulose-synthase-like (Csl) gene family, CslA and CslC, whereas GMGT1 and 2 are members of the glycosyl transferase family 34. During development of the grain, ManS1 and GMGTI were expressed at a high level in the developing endosperm in both Coffea arabica and C. canaphora, suggesting that the ManS1 and GMGT1 enzymes synthesize specifically the grain galactomannans. There were differences in the timing and level of expression between species and between varieties within a species, but within any one variety the developmental patterns of expression of the two genes were very similar. This suggests that the two genes are coordinately regulated and thus the enzymes they encode are able to act together as required in galactomannan synthesis.