| Cassava(Manihot esculenta Crantz) is belonged to Euphorbiaceous Manihot. It is one of three potato crop. Cassava can live in a barren soil, which has powerful strong stress tolerance. Cassava would be affected by the drought stress easily, which was cultivated in field more than8months of growth. People would pay more attention to the mechanism of leaves and roof drought resistance. Photosynthesis take place in leaves and nutrition was manufactured. Roof root has economic value. Stem was ignored which is the channel of material transport between leaf and storage root. The stem is the "seed" of cassava cultivation. There is a lot of starch and sugars in stele and epidermis of cassava stem, which were used to build morphology of cassava, and whether a certain role play in drought resistance of cassava, no systematic study was found. Cassava SC124which is drought resistant variety and cassava Arg7which is drought sensitive variety was planted in pot in rainproof shed to do drought stress research. Under continuous drought treatment condition, to observe plant (especially-stem) morphological structure with optical and scanning electron microscope, to analysis of the changes of enzyme activity in the metabolism of carbohydrate and the content of starch and sugar, to analysis of transcriptome and verify key enzymes of glucose metabolism gene expression with QRT-PCR. To study the sugar in stem of cassava and its drought resistance function. The main research results as follows.1. Drought resistance cassava varieties SC124had been formed genetic characteristics to adapt drought tolerance in the evolution of the arid environment, had effective regulation of water use. These genetic characteristics included more hairs on the blade, thicker leaves, thicker spongy tissue, higher stomata density, the stem lenticels smaller and higher density, more developed myeloid tissue to improve the water transport capacity. Drought sensitive varieties Arg7did not have these characteristics or relatively weak.2. Drought resistance cassava varieties SC124had more sensitive ability to regulate the stomata. Drought resistance varieties SC124changed stomata to coordinate contradiction between CO2absorption and loss water. The new leaf stomata density was more concentrated. Parenchyma cells of stomata protruded outward to sensitively feel the change of water in the air. Drought sensitive varieties Arg7had these drought adaptation characteristics, while active response was relatively insensitive.3. Structural observations show that the number of starch grains decreased in stele and cortex of cassava varieties SC124and Arg7stem under drought stress. The number of starch grains reduced under continuous drought stress, and starch grains appeared "small hole" and surface was irregularity. Which means cassava plant stalk storage nutrient was involved in the physiological regulation.4. The content of glucose, fructose and sucrose was increased in varying degrees in stalk of cassava varieties SC124and Arg7, and a new sugar, trehalose, was synthesized. Those sugars may be involved in the regulation of osmotic response to drought stress. Higher sugar content of some small molecules in drought resistance varieties SC124than drought sensitive varieties Arg7. Drought resistance varieties had stronger osmotic regulatory effect.5. Under drought stress, part starch in stem had decomposition trend to degrade into glucose, fructose and sucrose to improve the osmotic regulation in cells. Starch decomposition reaction was faster in drought resistance varieties SC124than drought sensitive varieties Arg7, which was a mechanism of adaptation to drought. In the case of drought stress inhibited photosynthesis, stem storage starch can give priority in leaf and stem cell energy required.6. Under drought stress, the key enzymes activity of starch degradation and sucrose synthesis was raised in stem of SC124and Arg7, especially in drought resistance varieties SC124. These proofs fitted with the number of starch grains decreased, small hole appeared in starch grains, and the content of glucose, fructose, sucrose and trehalose was increased. The biological process of decomposition of starch and sucrose synthesis was confirmed to exist once more in stem of cassava under drought stress to provide energy for growth and development of plants.7. Transcriptome expression analysis indicated the transcriptional regulation network system of carbohydrate metabolism in Arg7stem was steered into carbohydrate catabolism, SC124was affected weakly. Gene expression changes in photo system Ⅱ and photo system I was synchronized in drought resistance varieties sc124, but out of synchronization in drought sensitive varieties Arg7. In drought resistance varieties sc124, the gene expression of UDP-glucose pyrophosphorylase (UGPase) was up regulated, the gene expression of hexokinase and ADP-glucose pyrophosphorylase (AGPase) kept unchanged almost, and up regulation of the gene expression of starch degrading enzymes was not obvious. In drought sensitive varieties, the gene expression of UGPase, hexokinase and AGPase was down regulated, and the gene expression of starch degrading enzymes was up regulated. Differential gene express indicated expression of starch degrading enzyme genes in stem of cassava was up regulated. Expression of starch synthase gene was down regulated, and the down regulated gene of starch synthase was more in Arg7than SC124. The network of transcriptome systems was plant response adaptive to the degree of tolerance cassava plants to drought stress.Transcriptional expression is closely related to the tolerance levels and drought stress.8. Expression results of key enzyme gene of starch decomposition and sucrose synthesis with QRT-PCR was tested. The gene expression of amylophosphorylase (SP), sucrose synthase (SuSy) and Alpha amylase increased2to9times in drought resistance varieties SC124. The gene expression of the there enzyme was lower in drought sensitive varieties Arg7. The result of QRT-PCR was mutual corroboration with transcriptome results and enzyme activity. Under drought stress, starch catabolism pathway was started in stem, and related gene expression was up regulation. The gene expression of sucrose synthase was cassava important adaptive regulation at the level of transcription. |
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