Expression of a UDPglucose pyrophosphorylase cDNA during fruit ripening of banana (Musa acuminata)

We report the isolation of a banana cDNA, designated MWUGPA, encoding uridine diphosphoryl (UDP)-glucose pyrophosphorylase (UGPase, EC. 2.7.7.9) that catalyses the reversible conversion between glucose 1-phosphate and UDPglucose in plants and animals. Furthermore, UGPase expression in fruit during ripening and in response to exogenous ethylene and sugars was also investigated. MWUGPA encodes a polypeptide of 467 amino acid residues and shares a high degree of sequence similarity (85-90%) with other plant UGPase homologs. In northern blot analysis, a 1.7-kb UGPase transcript was detected in both the vegetative and reproductive organs, but the former was considerably less abundant than the latter. In fruit, the level of accumulated transcripts was higher in pulp than peel at all ripening stages. Transcript abundance in both fruit tissues was relatively constant during ripening, but pulp transcripts surged in the 'more green than yellow' category fruit when ethylene also increased. Further analysis revealed that UGPase expression in fruit was ethylene-inducible, but the response was tissue-specific, as evidenced by the promoting effect of exogenous ethylene on accumulation of UGPase transcripts in pulp but not peel. Exogenous application of sucrose and fructose also increased UGPase transcript abundance in leaf and fruit tissues, especially pulp, whereas exogenous glucose had little or no effect. The results of this study indicate that ethylene and soluble sugars may play a regulatory role in UGPase expression during ripening in banana fruit.