Sorbitol dehydrogenase expression and activity during apple fruit set and early development

Apple (Malus domestica Borkh.) fruit set and early development is very sensitive to carbohydrate availability, but details on carbohydrate metabolism during this phase are limited. The objective of this work was to determine if sorbitol dehydrogenase (SDH, EC 1.1.1.14), the primary enzyme responsible for metabolism of the major phloem-transported carbohydrate sorbitol, is present and active during apple fruit set and early development. Nine different genes encoding SDH were determined from analysis of cDNA- and genomic-clones obtained from an apple cDNA library or PCR using genomic DNA and degenerate oligonucleotide primers, respectively. The presence of multiple genes was confirmed from Southern analysis. Comparing three apple cultivars with different maturation dates, Northern analysis showed that SDH transcript was present during the first 5 weeks of fruit development after bloom. Using antibody developed against purified recombinant SDH protein from a fragment of apple cDNA expressed in Escherichia coli, SDH was detected in whole fruit extracts at approximately 37 kDa during each of the 5 weeks after fruit set with the greatest intensity at 1-3 weeks after bloom. ELISA analysis of SDH protein indicated that SDH comprised 7-8% of the total extractable protein at this time. SDH activity was highest at 2-3 weeks after bloom in each of the three cultivars. Thus, SDH activity during apple fruit set and early development may play a primary role in defining fruit sink activity. The present work is the first to show that SDH is expressed and active in apple fruit during the first 5 weeks after bloom.