Cloning of two isoforms of soluble acid invertase of Japanese pear and their expression during fruit development.

Soluble acid invertase (S-AIV; EC 3.2.1.26) in Japanese pear fruit has an important role in accumulating hexoses during fruit enlargement and regulates the sucrose-to-hexose ratio in the vacuole. Full-length cDNA of PsS-AIV1 and PsS-AIV2 isoforms were cloned from Japanese pear fruit and their amino acid sequences share 40% identity; PsS-AIV1 was confirmed to code S-AIV isozyme purified previously. The roles of PsS-AIV1 and PsS-AIV2 genes throughout fruit development and in sugar composition were investigated by semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis using specific primers of their transcripts. PsS-AIV1 transcript had a maximum level at 34 days after full bloom (DAFB) and decreased rapidly during fruit development; PsS-AIV2 transcript increased gradually during fruit growth from 34 DAFB, had its maximum level at 79 DAFB and remained high until 107 DAFB at active fruit enlargement. The activity of S-AIV was highest at 34 DAFB, decreased during fruit growth until 66 DAFB, remained almost the same during early fruit enlargement until 79 DAFB and then decreased again. Soluble sugars fructose and glucose began accumulating predominantly during fruit enlargement from 66 DAFB; sucrose began increasing rapidly during fruit maturation from 121 DAFB. High expression of PsS-AIV1 transcript and high enzyme activity in the young fruit stage seems to have an important role in supplying a lot of substrate for energy needed for cell division and growth by hydrolyzing sucrose to hexoses. Increasing PsS-AIV2 expression during fruit enlargement may lead to rapid cell expansion through increased osmotic pressure by accumulation of a large amount of hexose in the vacuole.