Characterization of sugar accumulation and enzyme activities in jackfruit

Abstract: 【Objective】Sugars are a key component in fruits, and their composition and contents affect theflavor of fruits. Sugars not only provide energy for plant development, but also contribute to the quality ofsink tissues. Artocarpus heterophyllus Lam., commonly known as jackfruit, produces the largest tree-bornefruit known thus far. The fruit contains many sugar-derived compounds including carotenoids, flavonoids,sterols and terpenes, which contribute the unique flavor to jackfruit. The degradation and synthesis systemof sucrose determines the sugar composition in the fruit. In order to characterize sugar composition and accumulation in jackfruit, the concentrations of sucrose, glucose, fructose and starch were determined. Mean⁃while, the activities of sucrose and starch metabolism-related enzymes were analyzed in order to providethe important information on sugar metabolism and provide a useful reference for further research of fruitdevelopment in jackfruit.【Methods】Fruit of‘Malaysia-1’cultivated in an experimental field at Xinglongtropical garden (Wanning, Hainan, China) were collected from three trees at different developmental stag⁃es for the experiment. Fruit were tagged on the day of pollination. Perianth tissues were removed carefullyfrom the fruit samples at different developmental stages (at 50, 70, 90, 110, 117, and 120 days after polli⁃nation). Frozen powdered tissue (3 g) was homogenized in 10 mL of 80% alcohol with ribitol as an internalstandard, and centrifuged at 10 000 r for 15 min at 4 ℃. The supernatant was transferred into a new tube,vacuum dried under ambient temperature and then re-dissolved. The solution was filtered through a 0.45μm membrane filter prior to analysis. The contents of sucrose, glucose and fructose in jackfruit were mea⁃sured by high performance liquid chromatography (HPLC). For enzyme activity assay, 0.1 g of powderedsample was homogenized in 5 mL of Mops-NaOH buffer (50 mmol·L^-1, pH 7.2) containing 5 mmol·L-1MgCl2, 1 mmol·L^-1 EDTA, 20 mmol·L^-1 β-mercaptoethanol, 5% PVPP, and 1% BSA. After incubation inice for 30 min, the homogenates were centrifuged at 10 000 g for 15 min. The supernatant was used to determine enzyme activity. The content of starch and the activities of related enzymes were measured usingspectrophotography. Sugar metabolism was characterized by evaluating correlations between enzyme activ⁃ity and sugar accumulation during fruit development.【Results】The sucrose concentration was less than 1g·kg^-1 before 90 DAP, but it increased significantly to 165 g·kg^-1 at 120 DAP. In mature fruit, sucroseconcentration was the dominant sugar accounting for approximately 70% of total sugars. Glucose and fruc⁃tose contents were similar, and they were approximately 13 g·kg^-1 from 50 DAP to 90 DAP, before a twofolddecrease at 110 DAP, and then increased to about 20 g·kg^-1 at 117 and 120 DAP. Each of them ac⁃counted for approximately 10% of the total sugars. Starch was an important non-structural carbohydratein jackfruit. Starch concentration increased from 50 DAP (12.8 g·kg^-1) to a highest level at 110 DAP (148g·kg^-1), and then decreased as the fruit matured. The enzyme AI (acid invertase) was active from 50 to 90DAP and its activity decreased from 110 to 120 DAP. Its activity was significantly negatively correlated tosucrose concentration (r=-0.602*). The activities of sucrose synthase (SUSY) and sucrose phosphate syn⁃thase (SPS) were high at 117 and 120 DAP. Their activities were positively correlated with sucrose content(r=0.915**, 0.997**, respectively). Adenosine diphosphoglucose pyrophosphprylase (AGPase), an importantenzyme in starch metabolism, had the highest activity at mature stage, and had a positive correlation withstarch content during fruit development (R=0.900**). The activity of α-amylase was high at repining stage.【Conclusion】Sucrose is the main sugar in jackfruit, which has a negative correlation with AI activity andpositive correlations with SUSY and SPS activities. Starch has a positive correlation with AGPase. We suggest that developmental changes in sugar and starch concentrations appear to result from the coordinatedactions of three factors. First, during the early fruit development, AI and SUSY are involved in sucrose hy⁃drolysis and unloading. However, as the primary demand for sucrose at this stage is for growth, there is lit⁃tle excess glycogen for sugar accumulation, resulting in low concentrations of sucrose, glucose, fructoseand starch. Second, as the fruit matures, starch accumulation reaches a peak level. Third, during fruit ripening (after 110 DAP), starch breaks down and sucrose begins to accumulate, leading to increase in theconcentration of total soluble sugars and thus the sweetness of the fruit. From an evolutionary perspective,the sweetness of the mature fruits attracts animals for seed dispersal. These results suggest that sugarmechanism is intimately associated with fruit development.