The monocotyledonous plant Zea mays does not develop tumors after inoculation with Agrobacterium tumefaciens and is thus defined as nonhost. Agroinfection, Agrobacterium-mediated delivery of maize streak virus, demonstrates that transferred DNA (T-DNA) transfer to the plant does occur. Nopaline-type Agrobacterium strains such as C58 are efficient in the transfer process whereas the octopine-type strain A6 is unable to transfer T-DNA to maize. This phenotypic difference maps to the tumor-inducing (Ti) plasmid but not to the T-DNA. Steps preceding T-DNA transfer, such as attachment and induction of the virulence genes, were shown to take place in the octopine strain. The nopaline-plasmidspecific locus tzs and the octopine-plasmid-specific locus pinF (virH) are not involved in the strain specificity. However, mutations in the virF locus rendered the octopine strain agroinfectious on maize, whereas such virF-defective octopine strains, when complemented by virF on a plasmid, completely lost their agroinfectivity. We propose that VirF, known to increase the host range of the bacteria in other systems, acts as an inhibitor of T-DNA transfer to maize.Agrobacterium tumefaciens is able to incite tumors on a wide range of dicotyledonous plants, on a number of gymnosperms, and on a few monocotyledonous plants (1)(2)(3).Oncogenic strains of Agrobacterium harbor a large tumorinducing plasmid, the Ti plasmid. During infection, part of this plasmid, the transferred DNA (T-DNA), is transferred to the host plant cell, where it becomes stably integrated in the nuclear genome. The T-DNA contains genes necessary for expression of the tumorous phenotype and genes encoding opines, such as nopaline and octopine, according to which the bacteria are classified. The Ti-plasmid-located virulence region (vir region) is responsible for the transformation process and acts in trans (for reviews, see refs. 4-11).The vir region consists of essential loci (virA, virG, virD, and virB) (4)(5)(6)(7)(8)(9)(10)(11).Additional vir loci are found to be specific for the various types of Ti plasmids. The trans-zeatin secretion (tzs) locus is specific to nopaline-type Ti plasmids; tzs expression leads to the production and secretion by the bacterium of the plant hormone trans-zeatin (12-14). Octopine-type Ti plasmids do not carry the tzs locus but contain the pinF (virH) (15, 16) and virF loci (17-19), both absent from the nopaline-type Ti plasmids. Their mode of action is not understood yet, although both loci have an effect on host range (16)(17)(18)(19).Even though the graminaceous plant maize does not form tumors when inoculated with Agrobacterium strains (20), T-DNA transfer to maize and other gramineae has been proven (21-23). This was first shown using the sensitive technique of agroinfection, the delivery of maize streak virus (MSV) sequences to the plant by A. tumefaciens (21). T-DNA transfer to maize (as judged by MSV symptom production) requires the same vir operons necessary for T-DNA transfer to host plants (as judged by tumor for...

Maize streak virus as well as other geminiviruses contain a potential hairpin structure with the conserved sequence TAATATTAC in the loop. We assessed the possible involvement of this structure in replication and symptom induction of the virus. A series of insertion and deletion mutants were analyzed by agroinfection. Deletion of the hairpin or insertions in the conserved sequence abolished symptom development. Viral DNA could not be detected in the infected tissue. However, a mutant with a point mutation in the 'conserved' sequence, isolated after inoculation of maize plants with an insertion mutant, was able to replicate and to induce symptoms.

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