Grapevine stem segments were cocultivated with
three different Agrobacterium tumefaciens and three
different A. vitis strains. A. tumefaciens strains induced
tumors at variable frequencies, while A. vitis-infected
stem segments never formed crown galls. The tumorous
nature of tissues grown on hormone free medium was confirmed by opine assays. Bioinformatic and PCR
analysis of the virulence regions of various A. tumefaciens and A. vitis Ti plasmids showed that virH2 and
virK genes are common in A. tumefaciens but they are
lacking from A. vitis. Thus virH2 and virK genes may be
essential for grapevine stem segment transformation,
but expression of certain T-DNA genes of A. vitis may
also prevent the growth of transformed cells. Our data
indicate that the tumorigenic ability of A. vitis is different
on intact plant and on their explants, and that
the specific host association of A. vitis on grapevine is
probably determined by physiological and biochemical
factors (e. g., better colonizing ability) rather than
by its increased tumorigenic ability. Therefore it is not
reasonable to develop „helper” plasmids for grapevine
transformation from A. vitis pTis, unless their
avirulence on in vitro explants is determined by T-DNA
gene(s). Due to the inability of A. vitis to induce tumors
on grapevine stem segments, the use of in vitro explant
assays cannot be reliably used to select A. vitis resistant
grapevine genotypes or transgenic lines
