Variation Mechanism Of Two Plant Height Mutants In Rice

Rice plant height is one of the important agronomic traits, which is closely related to grain yield, photosynthesis efficiency and lodging resistance. Gibberellin (GA) plays an important role in controlling plant height. GA regulated growth and development by converting the GA signal into alterations in expression of the genes controlling plant growth and development. When GA biosynthesis was blocked or GA signaling was impaired, the plant growth and development could be affected and plant morphology could be altered. Thus, the plant height mutants were not only used to identify novel genes related to GA biosynthesis and GA signaling, but also to understand the mechanism of plant height morphogenesis, which had important significance in plant ideotype breeding.The rice tall mutant,9311HR-T, was obtained from a natural occurrence in a semidwarf strain,9311HR. Because plant height mutants can be used to understand phytohormone metabolism, phytohormone signal transduction and the mechanism of plant height morphogenesis, this study aimed to reveal the mechanism that governed the tall phenotype in9311HR-T. The final height of9311HR-T was210.8±4.9cm from the ground to the top of the panicle, which was1.8times as long as wild type. Compared to wild type, the extra plant height of9311HR-T was mainly caused by an increase in length of each elongated internode. The length of the first, second, third, fourth, fifth, and sixth internodes in9311HR-T was1.4,1.8,1.6,2.1,2.8and4.5times longer than those in9311HR, respectively. Microscopic observation of internode was made to find that the cell length in the mutant enhanced compared to the wild type, which indicated that the mutant was related to GA. To investigate the response of9311HR-T to GA, the role of GA in second leaf sheath elongation and in shoot elongation and agar plate assays for amylases were examined. The sensitivity of mutant to GA3application was found to be similar to that of wild type. To determine whether or not the tall phenotype of9311HR-T was caused by abnormal endogenous GA level, uniconazole, inhibitor for GA biosynthesis, was used to pretreat the plants, and then the response to exogenous GA3was investigated. The wild type phenotype of9311HR-T could be rescued by uniconazole treatment. The dwarf phenotype of9311HR-T retarded by uniconazole treatment can be restored by exogenous GA3. These results suggested that the tall phenotype of9311HR-T may be caused by elevating endogenous bioactive GA levels. To determine the specific cause of the elongating of the internodes length in9311HR-T, we measured endogenous GA1levels in the elongating internodes of both wild type and9311HR-T plants. The results showed that endogenous GA1levels in mutant9311HR-T was2.6times over that in wild type9311HR, which directly supported the conclusion that the mutant9311HR-T displayed a tall phenotype, owing to enhanced endogenous bioactive GA levels. To explore the intrinsic reasons for enhanced endogenous GA levels in9311HR-T, the expression of key GA metabolism genes were analyzed by semi-quantitative RT-PCR. The results show that the expression of two key GA metabolism genes was not regulated by endogenous GA. The expression of GA20ox2was obviously up-regulated, while the expression of GA2ox3was obviously down-regulated. Sequence analysis revealed a C to G transversion at position1026in GA20ox2coding region in9311HR, which led to the production of a stop codon TAG and translation termination. A novel G to C transversion was observed at position1026in GA20ox2coding region in9311HR-T, which restored translation of the full-length gene product. However, the coding sequence of GA2ox3in9311HR-T was identical to that in9311HR. Taken together, up-regulation and restorer mutation of GA20ox2and down-regulation of GA2ox3led to elevated GA levels in9311HR-T, which made9311HR-T exhibit a tall phenotype.The rice strain,02428h derived from a semidwarf material02428in somatic cell culture was a recessive tall mutant, and the recessive tall culm was controlled by elongated uppermost internode (eui).02428ha derived from02428h was a semi-dwarf mutant in eui background. Because semidwarf varieties possess high yield potential, the present study aimed to further characterize the dwarfing mechanism in the mutant. The plant height of02428ha was significantly shorter than that of the wild-type02428h plants, and the final height from the ground to the top of the panicle was127.3±2.6cm, which was29.7cm shorter than that of02428h. The dwarfed plant phenotype of02428ha was mainly caused by a decrease in length of the first internode. To investigate the shorten internode of02428ha was caused by abnormal cell elongation and/or cell division, five equally divided sections of the uppermost internode in wild-type and mutant plants were subjected to microscopic observation. In each section, the cell length of02428ha was shorter than that of the wild type, and the cell length decreased ratios were36.4%,37.5%,37.1%,40.3%and29.4%from bottom to top, respectively. The uppermost internode length decreased ratio was24.1%. Therefore, the shortening of the uppermost internode length in02428ha was due to longitudinally decreased cell lengths. To determine whether or not the dwarf phenotype of02428ha was caused by abnormal endogenous GA level or altered sensitivity to GA, the response to exogenous GA3was investigated. When the second leaf sheaths of wild type and02428ha plants were treated with various concentrations of GA3, the wild type elongated at GA3concentrations of1x10-8M or higher, but the mutant02428ha did at GA3concentrations of1×10-6M or higher. Importantly, the dwarf phenotype of02428ha could be incompletely rescued by1x10-4M GA3treatment. The results suggested that mutant02428ha showed decreased sensitivity to exogenous GA3. Endogenous GA1levels in mutant02428ha were15.96ng.g-1fresh weight, which was6times over that in wild type02428h. The results also supported the conclusion that the mutant altered sensitivity to GA. Because02428ha showed a reduced sensitivity to GA, it was necessary to investigate whether or not the GA signaling genes are affected in02428ha. The expression of the F-box gene GID2was down-regulated in02428ha, which was responsible for the reduced sensitivity to GA in the mutant. Especially, gene clone and sequence analysis revealed that a15base pair insertion between base594and595in its coding region from the translation start site was found in the mutant. The insertion mutation just altered the LSL domain, which was essential to GID2. Thus, the insertion mutation might altered its function. The results firstly suggested that the down-regulation.expression was responsible for the phenotype of the mutant02428ha, and that sequence variation of GID2gene might be related to it. The utilization potential of02428ha in hybrid rice breeding was also discussed.