Once initial flowering occurs, the papaya tree flowers and sets fruit continuously, requiring an uninterrupted supply of carbohydrates for fruit development and a full flavor. Developing fruit have a very large demand for sugars that has to be supplied from the leaves. Papaya lacks stored sugar reserves such as starch found in bananas, and must remain attached to the plant to accumulate sugars. During papaya fruit development, total sugar content increased slowly during the first two thirds of fruit development, then increases rapidly during fruit ripening. Variety, timing of harvest and ripening stage of fruit are major factors influencing final sugar composition of papaya.

To understand the relationship and regulation of sugar accumulation and fruit development, an expressed gene library from immature green papaya fruit was constructed and a putative complete invertase gene and a sucrose synthetase gene fragment isolated and characterized. The relationship between sugar accumulation in papaya fruit and the expression of papaya invertase and SS gene was investigated. The pattern of the gene expression during fruit development was compared with invertase activity of the enzyme extracted in the presence and absence of sodium chloride (NaCl) as an extractant to break cell wall ionic bonds. The complete deduced amino acid sequence of papaya invertase had an open reading frame that encoded a polypeptide chain of 582 residues and calculated molecular weight of 65, 684 Da. The protein was highly homologous to known plant cell wall invertase and 67% identical at the amino acid level with carrot cell wall invertase. The cloned 720 bp SS fragment was highly homologous to A. glutinosa (X92378, 81% identical) and SS gene of many other species. Invertase gene was expressed at a higher level in late fruit development stage than in other papaya plant tissues. The pattern of increased mRNA expression during late fruit developmental stage paralleled invertase protein level and in vitro enzyme activity. SS gene expression was higher in young fruit and petioles tissues, and lower in stem, flower and root tissue but significantly higher than in developing fruit flesh tissue and seed. The results indicated that invertase and SS genes are differential and developmental expressed and had different roles in papaya plant development. Southern blot analysis indicated that both invertase and SS genes were coded by a low copy number gene. The data confirmed that apoplastic invertase may have an important function in phloem unloading during papaya fruit sugar accumulation period and the activity was regulated at both transcriptional and translational levels.

Sugar accumulation by fruit is determined by the activity of three enzymes. The activity of these enzymes has been studied in papaya fruit. The predominate enzyme during early fruit development is sucrose synthase, while sucrose phosphate synthase remains very low throughout fruit development. The enzyme invertase is apparently the major enzyme contributing to sugar accumulation in papaya fruit during the last stage of development. The availability of this invertase gene opens the possibility for altering invertase expression and hence fruit sugar accumulation in papaya by genetic engineering.