| In higher plants, the products of photosynthesis is the source of development and storage energy materials of non-photosynthesis organs and energy materials, these assimilates are transported to sink-organs by phloem long-distance translocation, then unloaded and post-transported in sink-organs. There are several enzymes and sugar transporters involved in this pathway; it is of great significance to reveal this procedure for the study of sugar metabolism. So far, we know little about the pathway and metabolism of sucrose unloading, and we are also not clear about the enzymes, sugar contents and genes expression of sugar transporters in this pathway in cassava.In this study, we choose two cultivars of cassava-SC7 (Low starch content) and SC9 (High starch content) to analyze the activities of several enzymes (CWI, SAI, NIV, SuSy and SPS) related to sucrose unloading. The contents of soluble sugars (Sucrose, Glucose and Fructose) and six genes (MeCWI, MeHT, MeSUT-1, MeSUT-2, MeSUT4-1, MeSUT4-2) expression patterns, protein expression of CWI was detected using western blotting method. All the experiments above are expected to reveal the difference of sucrose unloading metabolism in various cassava cultivars.In study of enzymes activities, we found that CWI has opposite dynamics change in two cassava cultivars while SAI and NIV have the same trend, CWI activity is higher than activities of SAI and NIV, which inferred that CWI might have relation with differential of starch content. Activity of SPS is lower throughout the developmental stages which means that it might work together with other enzymes. Synthesis direction of SuSy is much higher than break down direction, which shows that sucrose is re-synthesized after hexose transported into cytoplasm.Three soluble sugars are detected in both cultivars. In SC7, The deposit pattern of soluble sugars in xylem is similar; they are lower in the early stages and reach to peak in the late stages. Soluble sugars in phloem are higher than that in xylem. In SC9, soluble sugars in xylem have high content in the middle stages and decreased gradually, soluble sugars content in phloem is contrary to xylem, to summarize, the difference of sugar content may caused the difference of ability in starch production in both cultivars.Gene expression showed that MeCWI has a balanced expression pattern except a little higher in both early and late stages. MeHT expression is higher in early stages of both xylem and phloem of SC7, while it is higher in xylem of SC7, which referred that there are diverse transportation pattern in time and position in both cultivars. The expression of MeSUT-1 is mainly in the late stages of tuber roots, while MeSUT-2, MeSUT4-1 and MeSUT4-2 have similar regular pattern which is higher in middle stages in xylem and is higher in early stages in phloem. It can be inferred that differential expression patterns of MeCWI and two sugar transporters might cause the discrepancy of starch content of two cassava cultivars.CWI plays a key role in apoplast unloading of sucrose, western blotting resulted that CWI protein expression is higher in late stages of xylem in both cultivars, while it is higher in early stages in phloem, additionally, CWI protein expression is higher in SC9 compare with SC7, which inferred that SC9 might has greater ability of sucrose unloading than SC7.These studies above will strengthen basis for further exploration of complete pathway and mechanism of sucrose unloading and provide fundamental base for cultivar improvement using biotechnology in the future. |
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