Growth control of woody plants by genetic engineering with genes for gibberellin-biosynthetic enzymes.

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Gibberellins (GAs) are essential endogenous regulators of growth and developmental processes throughout the plant life cycle. Lombardy poplar (Populus nigra) has GA_1, GA_3 and GA_4 as the active form of GA. We examined the shoot elongation, the dry-matter production, the wood anatomy, the cell wall components, the net photosynthetic rate and the levels of pigments of lombardy poplar after treatment with GA_3, and found that the GA_3 treatments have positive effects on the biomass production of terrestrial parts. We also isolated the two genes for GA20-oxidase and GA3β-hydroxylase of GA-biosynthetic enzymes, and the seven genes for GA2β-hydroxylase which is one of GA-deactivation enzymes. During the treatment with GA_3, the expression of both genes for GA20-oxidase and GA3β-hydroxylase was down-regulated while that of the genes for GA2β-hydroxylase was up-regulated. In fact, the internal level of GA_3 increased markedly after the treatments, and the levels of GA_1, GA_4, GA_5, GA_9 and GA_<20> significantly decreased after the GA_3 treatments. Thus, the levels of expression of these genes were highly related to the changes of the levels of endogenous GAs. Furthermore, we established the simple and reliable procedure for the regeneration of transgenic poplar using selection of herbicide 'bialaphos' and antibiotic ‘hygromycin B'. With these transformation systems, we introduced the gene for GA20-oxidase into lombardy poplar, and the gene for the α-subunit of the rice heterotrimeric G proteins which participate in signaling of gibberellin at low concentrations, under control of the cauliflower mosaic virus 35S promoter. We have examined the growth characteristics of obtained transformants in a phytotron under artificial-light. In conclusion, it may be possible to control the growth by up-regulation of the genes for GA20-oxidase and GA3β-hydroxylase or down-regulation of the genes for GA2β-hydroxylase.