Transformation and regeneration of transgenic aspen plants via shoot formation from stem explants

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Abstract
An Agrobacterium‐mediated transformation procedure for aspen (Populus tremula L.), involving the direct regeneration of shoot‐buds from stem explants, is described. Disarmed Agrobacterium tumefaciens strain EHA101 harboring the binary plasmid pKIW1105 (which carries the uidA and nptII genes, coding for β‐glucuronidase [GUS] and neomycin phosphotransferase II, respectively) was used for the transformation of stem explants. An incubation period of 48 to 72 h was found to be most effective in terms of transient GUS expression on the cut surface of the stem explants. Adventitious shoots regenerated after 2–3 weeks of culture in a woody plant medium (WPM) supplemented with TDZ (1‐phenyl‐3‐[1,2,3‐thiadiazol‐5‐yl]‐urea, Thidiazuron) and carbenicillin. Three different kanamycin‐based selection schemes were evaluated for optimization of transformation efficiency: (1) Kanamycin was added only to the rooting medium (5 to 6 weeks post‐inoculation), or (2) to the regeneration medium 10–14 days after inoculation, or (3) after 2 days of co‐cultivation. The third selection scheme was found to be optimal for adventitious shoots with regard to both the time required and the transformation efficiency, the latter being much higher than with the other schemes. Leaf samples from kanamycin‐resistant shoots and plantlets were tested for GUS expression, and subjected to polymerase chain reaction (PCR) analysis of uidA and nptII genes. A Southern blot of the corresponding PCR‐amplified fragments confirmed their authenticity and Southern blots of total plant DNA confirmed integration of the nptII gene into the plant genome.