| Populus, playing an extremely significant role in beautifying environment, ecological defense and industrial application, is one of the most important wood species in artificial forest. China is the country possessing largest artificial wood area in the world while its average woodland area, wood store volume and wood consumption capacity only account for 17.2%, 12% and 17.6% of world average level, respectively. Nowadays, the exploitable resource in natural forest tends to wear out so much that the importance is obvious to develop vigorously artificial populus for alleviating the crisis in forest resource deficiency and the conflict between wood supply and demand in our country, even in the whole world.Conventional cross breeding programmes contributed greatly to populus breeding and obtained many achievements. Owing to its long generation period and big size, however, it is very difficult to breed new species expected by breeding workers through these approaches in a short time. Fortunately, the development of modern biotechnology opens up ways to genetically engineer trees for tree improvement, by which foreign genes can be transferred into target plant genomes. This method, overcoming the biological obstacle of interspecific hybridization, extending its hybridizing scope and resulting in acceleration in variation speed, was proven to be effective for directional improvement of wood plants.Brassinosteroids(BRs), the sixth class plant hormone next to other plant hormones: auxins, gibberelins, cytokinins, abascisic acid and ethylene, were generally accepted as a class new plant growth accelerants. BRs, by regulating many plant physiological responses such as: promoting growth, enhancing plant resistance to environment stress, delaying senescence, promoting cellre-diiferentiation and so on, are essential for plant growth and development. Of these, the most prominent physiological function is to promote plant growth. Recently, the Agricultural Ministry has listed it into " China foison plans" and is spreading its application. So far, nearly 10,000,000 acres of application area have achieved.DET2 is a gene that encodes a key enzyme in the biosynthesis pathway of brassinosteroid. The phenotype of de-etiolation, dwarfism and loss of light regulation function can be observed in det2 mutants, and only BR could rescue the mutant phenotype to the wild type, which provided convincing evidence that BR is indeed an essential plant hormone for normal plant growth and development. This paper is concerned with the transformation of GhDET2 gene cloned from cotton fibers, under the control of CaMV35S promoter, into Poplar. By virtue of over-expressing DET2 gene and increasing endogenous brassinosteroid level in populus, the target to offer new materials for directional high-quality breeding is expected to achieve. The main results are as follows: 1 . The optimization of Agrobacterium-mediated transformation system of poplar stem segmentsThrough comparing the differentiation frequencies of leaves, stems and roots, high frequency regeneration system had been set up, with a 100% regeneration rate and 21.05 shoots per stem explant. The GUS-intron gene was used in experiments to study the factors that influenced transformation efficiency by transient expression, including AS concentration, Agrobacterium concentration, cocultivation time, etc. An agrobacterium-mediated transformation system using stem segments as explants was developed, that is: using 500umol/L AS to activate secondly agrobacteria, OD600=0.3-0.5 of agrobocterium concentration was applied to infect poplar segments for 10-15min, after cocultivating for 3 days at 24C, followed by delayed selection culture for 10 days,50mg/L kanamycin was utilized to screen kanamycin-resistance shoots. The shoots were regenerated on root-inducing medium containing the same concentration of kanamycin.2.Transformation of cotton brassinosteroid biosynthesis gene GhDET2 into poplar.The target gene transformation was conducted via recombinant plasmid pGhDET2. 15...
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