| Muskmelon is one of the most important horticultural plants. Due to its good taste and rich nutrition, it is so deeply loved by the masses and cultivated widely throughout our country. However, the quality of melon is relatively poor and the economic efficiency is very low. We had become the member of the World Trade Organization, and the farm produce market will open widely, and the competition will become more acute. So it is very urgent to provide more high-quality muskmelon.Sugar content is one of the most important indicators for the melon quality. It is not only influenced the taste and pigmentation, but also the basic material composition for synthetic organic acids, vitamins, and carotenoids and other nutrients. Therefore, it is so significant to unravel the mechanism of sugar accumulation of muskmelon. The soluble sugar of muskmelon includes sucrose, fructose and glucose, and there are about more than 60% sucrose in mature muskmelon fruits, so sucrose determines the sugar content of muskmelon. In sucrose metabolism, there are three enzymes participated in: sucrose phosphate synthase (SPS), sucrose synthase (SS), acid invertase (AI) and neutral invertase (NI). Increasing SPS activity or decreasing AI activity can accelerate the sucrose synthase. Changing the SPS activity on molecular level may influence the sucrose metabolism. SPS regulated the carbonhydrate partitioning between sucrose and starch. Furthermore, SPS plays an important role in photosynthesis. Therefore, using modern molecular biology method to change the sugar metabolism enzymes activities is great significance to quality breeding of muskmelon. In this study, based on the previous work (included SPS gene cloning, expression vector construction, transformation and so on) , we obtained two antisense SPS gene transgenic lines and three overexpression SPS gene transgenic lines, and using the later regenerated plants we identified the SPS gene function, and it laid the foundation of improving melon quality. The main results are as follows:1. We determined the growth trend of all the transgenic lines and the control plant in different developmental stages. These results show that the growth of the antisense SPS gene transgenic lines are significantly weaker than control plants, while the overexpressing SPS gene transgenic lines are stronger than control plants, indicating that SPS gene affects melon normal growth.2. We measured the net photosynthetic rate of all the transgenic lines and the control plants in different developmental stages. These results show that the net photosynthetic rate of the antisense SPS gene lines are lower than control plants. We also observed the chloroplast ultrastructure of transgenic and control plants, and the results show that a large part of chloroplasts are damaged in antisense transgenic plants, which cause the net photosynthetic rate decreased. The net photosynthetic rate of the overexpressing SPS gene transgenic lines are higher than the control plants, especially the T-5 lines. There are more chloroplasts in T-5 line than control plants, so the photosynthetic performance is better than control.3. We determined the content of sucrose, fructose, glucose and the activity of SPS of all transgenic lines and control plants in different fruit developmental stages. These results show that the SPS activity of the antisense transgenic lines are lower than the control plants, and the sucrose content are also decreased. While the SPS activity of overexpressing SPS gene transgenic plants are higher than the control plants, and the sucrose content are increased to varying degrees in all transgenic lines, especially in mature fruit of T-5 line, the sucrose content is increased by 22% compared with the control plants. These results suggested that the level of SPS activity is the key factor and necessary prerequisite in sucrose accumulation.4. Compared with the control plants, the fruit growth rate is slower in antisense fruit during the whole fruit growth period, and the fruit weight of transgenic lines T-1 and T-2 are decreased by 25% and 18%, respectively. While in overexpressing SPS gene transgenic plants, the fruit growth rate is much faster than control, and the mature fruit weight of the T-5 line is increased by 26.4%. Sucrose is the basis of plant growth substances and a substrate for many metabolic reactions, indicating that the level of sucrose content in leaves determines the melon fruit development.5. We illustrated the role of SPS in carbohydrate partitioning by measuring the leaf sucrose / starch ratio of diurnal variation in the antisense lines and control plants. The results show that: SPS activity of leaves changes associated with the irradiance and there is a peak activity at 12:00 both in transgenic and control plants, and the sucrose content in leaves is parallel with SPS activity. While the trend of the starch content is: starch content of T-1, T-2 lines has been raising between 8:00 to 18:00, but the starch content of control first lower slightly and then rising, so the sucrose / starch rates are increased first and then decreased, while the sucrose / starch ratio of control is higher than transgenic plants. It was suggested that SPS plays an important role in the regulation of the direction of sucrose and starch synthesis, and SPS promots the sucrose synthesis, but starch accumulation inhibited. |
