An efficient Agrobacterium-mediated transformation and regeneration system for leaf explants of two elite aspen hybrid clones Populus alba x P. berolinensis and Populus davidiana x P. bolleana

Transgenic technology has been successfully used for gene function analyses and trait improvement in cereal plants. However, its usage is limited in woody plants, especially in the difficult-to-transform but commercially viable hybrid poplar. In this work, an efficient regeneration and transformation system was established for the production of two hybrid aspen clones: Populus alba x P. berolinensis and Populus davidiana x P. bolleana. A plant transformation vector designed to express the reporter gene uidA, encoding beta-glucuronidase (GUS), driven by the cauliflower mosaic virus 35S promoter, was used to detect transformation event at early stages of plant regeneration, and to optimize the parameters that may affect poplar transformation efficiency. Bacterium strain and age of leaf explant are two major factors that affect transformation efficiency. Addition of thidiazuron (TDZ) improved both regeneration and transformation efficiency. The transformation efficiency is approximately 9.3% for P. alba x P. berolinensis and 16.4% for P. davidiana x P. bolleana. Using this system, transgenic plants were usually produced in less than 1 month after co-cultivation. The growth characteristics and morphology of transgenic plants were identical to the untransformed wild type plants, and the transgenes could be inherited by vegetative propagation, as confirmed by PCR, Southern blotting, RT-PCR and beta-glucuronidase staining analyses. The establishment of this system will help to facilitate the studies of gene functions in tree growth and development at a genome level, and as well as the introduction of some valuable traits in aspen breeding.