Competence of Arabidopsis thaliana genotypes and mutants for Agrobacterium tumefaciens-mediated gene transfer:: role of phytohormones

Many plant species and/or genotypes are highly recalcitrant to Agrobacterium-mediated genetic transformation, and yet little is known about this phenomenon. Using several Arabidopsis genotypes/ecotypes, the results of this study indicated that phytohormone pretreatment could overcome this recalcitrance by increasing the transformation rate in the known recalcitrant genotypes. Transient expression of a T-DNA encoded beta -glucuronidase (GUS) gene and stable kanamycin resistance were obtained for the ten Arabidopsis genotypes tested as well as for the mutant uvh1 (up to 69% of petioles with blue spots and up to 42% resistant calli). Cultivation of Arabidopsis tissues on phytohormones for 2-8 d before co-cultivation with Agrobacterium tumefaciens significantly increased transient GUS gene expression by 2-11-fold and stable T-DNA integration with petiole explants. Different Arabidopsis ecotypes revealed differences in their susceptibility to Agrobacterium-mediated transformation and in their type of reaction to pre-cultivation (three types of reactions were defined by gathering ecotypes into three groups). The Arabidopsis uvh1 mutant described as defective in a DNA repair system showed slightly lower competence to transformation than did its progenitor Colombia, This reduced transformation competence, however, could be overcome by 4-d pre-culture with phytohormones. The importance of pre-cultivation with phytohormones for genetic transformation is discussed.