Cloning and characterization of a soluble acid invertase-encoding gene from muskmelon

Soluble acid invertase (S-AIV; EC 3.2.1.26) is thought to play a critical role in sucrose hydrolysis in muskmelon (Cucumis melo L.) fruit. A full-length cDNA clone encoding S-AIV was isolated from muskmelon by RT-PCR and RACE. The clone, designated as CmS-AIV1, contains 2178 nucleotides with an open reading frame of 1908 nucleotides. The deduced 636 amino acid sequence showed high identities with other plant soluble acid invertases. Northern blot analysis indicated that CmS-AIV1 was expressed in flowers and fruit, but was not detected in roots, stems or leaves. Moreover, the mRNA accumulation of CmS-AIV1 showed its maximum level at 10 days after pollination (DAP) and decreased gradually during fruit development until its minimum level at mature fruit. Interestingly, the sucrose content was very low in fruit before 20 DAP but increased dramatically between 20 and 30 DAP during fruit development. In contrast to sucrose content, the activities of S-AIV was very high in fruit before 20 DAP and decreased apparently between 20 and 30 DAP, suggesting that sucrose metabolism may be linked to the CmS-AIV1 transcript level in muskmelon fruit.