| Potato ranges the forth of food crops in the world while it is of increasing importance in Chinese food, vegetable, feed and processing. Potato degeneration caused by virus diseases usually results in a serious loss of both tuber yield and quality, which has been a main hinder in potato production for long time. Since 1970's, virus-elimination through meristem culture has been widely used in potatoes and the techniques have been improved as the development of biotechnology. The potato microtuber, which is induced from axillary buds of the plantlets in vitro, has the same genetic stability, morphological and physiological characteristics compared with conventional tubers. Perspectives of using potato microtubers in seed potato system has, therefore, drawn great interests in research. However, the productivity of microtubers is relatively low since there is little information about the tuberization mechanism. With aims to look into this aspect, the experiments were laid out under various tuber inducing conditions and investigations of variation in endogenous hormones and expression of tuber-forming-relative-genes (TFRG) were carried out. The main results obtained are listed bellow:1. GA3, ABA and JA are known to relate to potato tuber initiation. Variation in contents of these hormones were observed in the experiments with various treatments of day length during incubation and concentrations of growth substances and sucrose of the medium. The results showed that there was no obvious relationship between tuber formation and the content of either GA3 or ABA, but closely related to the ratio of GA3/ABA, indicating the involvement of these two substances in tuber initiation but they play different role in the process. Only when a certain range of the value of GA3/ABA ratio (1.1-1.3 in the experiment) was achieved could the plantlets produce more tubers. In the most favourate treatment, the highest tuber production was 2.3 tubers/plant which was about 2 times higher than the previous reports.2. In different tuber-inducing conditions, the content of total RNA was increased with the treatments capable to form tubers when tuber was initiating which demonstrated that tuberization was related to the expression of some new genes. With further mRNA differentiation analysis, it was proved that some genes expressed while some others closed when tuber initiation, indicating that tuber formation was a result of activation of many genes.3. It was obvious that patatin gene played role in potato tuberization. Patatin was expressed at stolons and axillary buds, where had potentials of forming tubers, during tuber initiation in the treatments produced tubers. The results also indicated that expression of patatin gene was sucrose inducible while light seemed to have an indirect regulation to the expression. Tuber formation was only relate to the expression of patatin but not to its strength, which predicted other roles of patatin gene in potato physiological process in addition to its function of tuber induction.4. Microtubers had the same tuber tissues as normal tubers, of which the perimedulla tissue took over 60% of the tuber volume at any stage of tuber growth and was therefore, the most important part of the tuber to make up its final size. Cell division and cell expansion had a relationship with tuber weight during tuber growth that could be described with a power function (Y = aWb). The rate of cell division was always higher than that of cell expansion, indicating that cell division contributed more to the tuber growth than cell enlargement. It was notable that tuber formation and tuber growth were controlled by two independent regulation systems, but desire sized tubers could be obtained under favourable tuber inducing conditions.5. The conditions suitable for patatin gene expression could also keep the balance of endogenous GA3 and ABA and content of JA that promoting tuber initiation. The results suggested that a similar regulation mechanism was existed for expression of patatin and other TFRGs.
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