Sucrose metabolism during fruit development in coffea racemosa.

Sucrose is one of the compounds in the raw coffee bean that has been identified as an important precursor of coffee flavour and aroma. In order to increase our knowledge of sucrose metabolism in coffee species, sucrose and reducing sugar content were investigated in the fast fruit-ripening coffee species Coffea racemosa. Fruits were harvested regularly from anthesis, until the point of complete fruit maturation and tissue development, followed by measurement of fruit tissue weight. Pericarp was the most abundant tissue, and always represented around 70?80% of fruit fresh weight. The perisperm present in young fruits was rapidly replaced by the endosperm at around 40 days after flowering (DAF). In the latter tissue, total and reducing sugars decreased during development. However, sucrose content was maintained at a relatively high level throughout fruit development, with a peak at 40 DAF that coincided with the highest level of sucrose synthase (SUS) activity detected in this tissue. For all endosperm developmental stages analysed, very low invertase activity was observed, suggesting a limited role for this enzyme in sucrose metabolism. Northern blot experiments using SUS1 and SUS2 cDNA sequences from Coffea arabica as probes were carried out to study the expression of SUS-encoding genes. The SUS1 transcripts of C. racemosa overlapped with the peak of SUS activity in the endosperm, whereas SUS2 transcripts accumulated mainly during the latest stages of endosperm development. Altogether, these results suggest that the SUS1 isoform of SUS is essential for sucrose accumulation in the fruits of C. racemosa. Introduction Coffea racemosa Lour. (Rubiaceae) originates from East Africa and is characterised by abundant branching, high leaf drop during the dry season and rapid fruit maturation (Guerreiro Filho, 1992). C. racemosa produces a pale coloured aromatic