Additional copies of virG from pTiBo542 provide a super-transformation ability to Agrobacterium tumefaciens in citrus

Abstract

Agrobacterium tumefaciens strain A281 carrying pTiBo542 was described as able to induce large and early-appearing tumours on a wider range of plants than other Agrobacterium strains. Here we show a strict correlation between the supervirulence of A281 and super-transformation ability of its disarmed derivative EHA105/p35SGUSINT. This strain produced much higher transformation than the disarmed derivative of the common strain C58, called C58(pMP90)/p35SGUSINT. Transformation frequency increased from six to 22-fold, depending on the citrus genotype. This increase permitted to regenerate for the first time, whole transgenic plants from lemon, alemow and Cleopatra mandarin. Since the chromosomal background from C58 as well as the T-DNA from p35SGUSINT are identical for both bacterial strains, the super-transformation ability of EHA105/p35SGUSINT in citrus may be attributed to the vir region of pTiBo542. In addition, introduction of extra copies of virG from pTiBo542 into either EHA105/p35SGUSINT or C58(pMP90)/p35SGUSINT further increased transformation, demonstrating the importance of vir induction, and mainly virG activation, in the super-transformation ability of this Ti plasmid in citrus.