The authors describe the construction of new helper Ti plasmids for Agrobacterium-mediated plant transformation. These plasmids, derived from three different Agrobacterium tumefaciens Ti plasmids (pTiB6, pTC58, and pTiBo542), have their T-DNA regions deleted using site-directed mutagenesis to yield replicons carrying the vir genes that can complement binary vectors in trans. The study demonstrates the strain utility and discusses the advantages of using these 'disarmed' Ti plasmids for plant transformation via Agrobacterium. The introduction highlights the limitations of Agrobacterium-mediated gene transfer, particularly the host range of A. tumefaciens, and the potential of developing non-oncogenic or disarmed strains for broader application in plant genetic engineering. The paper also provides examples of plants transformed by various octopine, nopaline, and L,L-succinamopine strains of A. tumefaciens.The authors describe the construction of new helper Ti plasmids for Agrobacterium-mediated plant transformation. These plasmids, derived from three different Agrobacterium tumefaciens Ti plasmids (pTiB6, pTC58, and pTiBo542), have their T-DNA regions deleted using site-directed mutagenesis to yield replicons carrying the vir genes that can complement binary vectors in trans. The study demonstrates the strain utility and discusses the advantages of using these 'disarmed' Ti plasmids for plant transformation via Agrobacterium. The introduction highlights the limitations of Agrobacterium-mediated gene transfer, particularly the host range of A. tumefaciens, and the potential of developing non-oncogenic or disarmed strains for broader application in plant genetic engineering. The paper also provides examples of plants transformed by various octopine, nopaline, and L,L-succinamopine strains of A. tumefaciens.