High-efficiency Agrobacterium-mediated transformation in Quercus robur: selection by use of a temporary immersion system and assessment by quantitative PCR

An efficient protocol for genetic transformation of somatic embryos of Quercus robur by selection in a temporary immersion system is reported. The transformation frequency was 5 times higher than achieved by conventional culture on semi-solid medium, ranging between 6 and 26 % for the four genotypes evaluated. Clumps of globular or torpedo somatic embryos were precultured for 7–10 days, inoculated with Agrobacterium tumefaciens strain EHA105:p35SGUSINT and cocultivated for 4 days before being cultured for 4 weeks on semi-solid selection medium supplemented with 25 mg L^?1 kanamycin. Explants were transferred to RITA^® bioreactors and subjected to a two-step selection protocol involving immersion in liquid medium supplemented with 25 mg L^?1 kanamycin, for 18 weeks, and then with 75 mg L^?1 kanamycin. Putatively transformed explants appeared after serial transfer to selection medium over 12–16 weeks. The presence of neomycin phosphotransferase II and β-glucuronidase genes in the plant genome was confirmed by histochemical and molecular analysis, and the copy number was determined by Southern blotting and real-time quantitative polymerase chain reaction. Transformed somatic embryos were germinated and transferred to soil for acclimatization, approximately 8 months after inoculation of the original tissue with bacteria. As the limiting factor for recovery of plants from oak embryogenic lines is the low embryo conversion rate, axillary shoot lines were established from transformed germinated embryos. Transformed embryos and shoots were cultured in medium with or without kanamycin and the responses to several morphogenetic processes (recovery after cryopreservation, germination, shoot proliferation, and rooting) were evaluated.