Conjugal transfer but not quorum-dependent tra gene induction of pTiC58 requires a solid surface

Donors of Agrobacterium, tumefaciens harboring a, transfer-constitutive der ivative of the nopaline-type Ti plasmid pTiC58 transferred this element at frequencies 3 to 4 orders of magnitude higher in matings conducted on solid surfaces than in those conducted in liquid medium. However, as measured wi th a lacZ reporter fusion, the ka genes of the wild-type Ti plasmid were in ducible by opines to indistinguishable levels on solid and in liquid medium . Donors induced in liquid transferred the Ti plasmid at high frequency whe n mated with recipients on solid medium. We conclude that while formation o f stable mating pairs and subsequent transfer of the Ti plasmid is dependen t on a solid stratum, the regulatory system can activate tra gene expressio n to equivalent levels in liquid and on solid surfaces.