| Heading date is an important agronomic trait related to the regional and seasonal adaptability of rice varieties, which has been one of the main objective traits in rice breeding. Rice heading date is mainly decided by the photoperiod sensitivity and basic vegetative growth. Our laboratory found a mutant lbvp (long basic vegetative growth period), which shows a long growth period, from the progeny of radiation mutated seeds of japonica cultiar Nipponbare. Genetic analysis suggested that the mutant was generated due to the recessive mutation of a single gene, which is temporarily named as LBVP(t). It is preliminarily inferred that LBVP(t) is an important gene controlling the basic vegetative growth period and development. So far no similar mutants have been reported. Therefore, deep study of this mutant will be meaningful for revealing the genetic mechanism of heading date. In this study, the morphology and genetic characteristics of the mutant were analyzed and the LBVP(t) gene was mapped, which will facilitate the cloning of this gene. The main results are as follows.1. Morphological traits of lbvp mutant: lbvp mutant stayed in vegetative growth stage for a long time. Its heading date was at least 75 days later than wild-type. Because of the extended vegetative growth phase, lbvp mutant showed obviously increased plant height, more tillers, and more and longer leaves. Paraffin section observation of shoot apex at different developmental stages showed that during the vegetative growth period of wild-type plants, there was no difference between the mutant and wild-type in shoot apex morphology and leaf primordium development; when wild-type entered reproductive development stage, the mutant remained in the vegetative growth phase and continued to generate new leaf primordia; the mutant began reproductive development 80 days after wild-type initiated reproductive development.2. Characteristics of lbvp mutant’s vegetative growth: In order to understand whether the long growth period phenotype of lbvp mutant is mainly decided by the photoperiod sensitivity or by the basic vegetative growth, 4 photoperiods (10, 12, 14 and 16h) were used to deal with lbvp mutant and wild-type with natural photoperiod as control and the relationship between photoperiods and rice growth duration was observed. The results showed that lbvp mutant had no obvious photoperiod sensitivity in growth duration, suggesting that the mutant phenotype was mainly decided by a long basic vegetative growth period .3. Genetic behavior of lbvp mutant: The selfed progeny of lbvp heterozygote were planted in Fuzhou (early and late season) and Hainan, respectively. The results showed a good fit to 3:1 ratio of normal plants to mutant plants in all the environmental conditions tested, suggesting that lbvp mutant trait is controlled by a single recessive gene. 4. Primary mapping of LBVP(t): An F2 population was constructed by crossing lbvp mutant with an indica cultivar Acc8558. A mutant DNA pool and a normal pool DNA were constructed with 10 wild-type plants and 10 mutants randomly selected from the F2 population. 150 SSR markers showing polymorphism between the two parents were used to analyze the two DNA pools, from which two SSR markers on chromosome 6 were found to be possibly linked to LBVP(t). This was verified by further analysis on 212 mutants selected from the F2 population. As a result, LBVP(t) was mapped between SSR markers RMYC1 and RMYC2, with genetic distances of 4.8 cM to RMYC1 and 14.4 cM to RMYC22, respectively. Based on this result, 23 new SSR markers were designed according to the rice genome sequence, from which 4 markers showing polymorphism between the two parents were obtained. By analyzing the 212 F2 mutants with these 4 markers, LBVP(t) was finally mapped in a 3.5 cM region. |
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