Abstract: 【Objective】In order to shorten the conventional back-cross breeding cycle, a rapid back-cross way was studied to improve crops using tobacco as a model plant.【Method】Using flue-cured tobacco variety FC8, which is immune to TMV, as the resistant donor parent, and flue-cured tobacco variety Zhongyan 100, which is susceptible to TMV but with good comprehensive properties, as the receptor parent, the following experiments were conducted. First, the Arabidopsis thaliana FT gene was cloned and connected to plant expression vector P22, which was then transferred into Agrobacterium, and leaf-disc method was adopted for gene transformation to create FC8 positive plants containing FT. These positive plants were hybridized with Zhongyan 100 respectively, and according to the separation ratio between early-flowering and non early-flowering plants in F₁ progeny, the positive plants containing single FT gene were identified. Second, the early-flowering plants were selected from the target F₁ progeny to backcross with Zhongyan 100. According to the separation ratio of FT and N gene (TMV resistance gene) marker in BC₁ progeny, the positive plants containing single FT gene and in which FT gene was not linked to N gene were identified. Next, continuous backcross with Zhongyan 100 was conducted using the specific positive plants, and plants with FT and N gene marker at each earlier backcross generation were selected. In the final generation, the plants containing N gene marker but no FT marker were selected, and identified whether they contained genetically modified components such as FT, Ubi promoter, Nos terminator and Hyg selection marker. Plants free of those components were continuously self-crossed to make Ngene isozygoty. 【Result】A total of 8 positive plants containing FT were obtained by transgenic technology, one of which was identified to contain single FT gene, and which FT gene was not linked to TMV resistance gene. Using it as the early-flowering TMV resistance donor parent and Zhongyan100 as the recurrent parent, Zhongyan 100 was improved through backcross. The BC₄F₁ progeny were obtained in more than one year, and the time was shortened by 450-500 d compared with conventional backcross breeding. It was found that the transgenic FT gene could be expressed stably in hybridization and backcross progenies. The plants containing FT gene budded at the 50-60 d in the age of seedling, and when they budded, they had 3-4 leaves and the plant height was about 35 cm on average, while the height of non early-flowering plants was about 27 cm and still at the vegetative growth stage. The normal plants budded at the 130-140 d, and when they budded, they had 18-20 leaves and the plant height was about 120 cm on average. Compared with the normal plants, the growth period of early-flowering plants was shortened by 70-90 d, which means that half time of the backcross breeding cycle can be shortened, and the new improved BC₁, BC₂, BC₃ germplasms with TMV resistance were also obtained. 【Conclusion】With the FT gene-mediated early flowering and TMV gene markers, the backcross breeding cycle can be effectively shortened and the tobacco TMV resistance can be improved rapidly. Although it is necessary to use transgenic technology to accelerate the generation process, the transgenic gene can be separated out successfully, and the available innovation germplasms containing no genetically modified elements were obtained.

Key words: tobacco, FT gene, rapid backcross, improvement