Fruit taste depends upon sugars, organic acids, firmness, amino acids, aromatic compounds and so on. Of these components, sugars are one of the most important components of fruit taste. The amount of sucrose, glucose, fructose and sorbitol plays a key role in sweetness of pear fruits. The differences in sugar composition within cultivars exist in Japanese pear. There is much difference in sucrose content and commercially important cultivars accumulate much sucrose. Cultivars with high sucrose accumulation exhibit a rapid increase in sucrose at later stage, while those with low sucrose accumulation exhibit no increase even at later stage. To study whether sucrose content is related to changes in the gene expression of sucrose metabolizing enzymes (sucrose synthase: SUS, soluble acid invertase: AIV and sucrose phosphate synthase: SPS), we examined the expression of five genes such as SUS (PpSUS1), AIV (PpAIV1 and PpAIV2) and SPS (PpSPS1) during development. PpAIV2 is specifically expressed in fruit at later stage by cultivars with low sucrose content. Moreover, we used RFLP analysis to identify PpAIV2 gene linked to sucrose content in ripening fruits. When total DNA was probed with PpAIV2, we identified two bands of 4.4kb and 2.8kb, which were specific to cultivars with low sucrose content. The absence of these bands enabled the identification of high or moderate sucrose content. We have converted these markers in to PCR-based markers. The markers identified here, which are linked to low sucrose content, should be useful for the selection of seedlings with good taste.
