VirA, the plant-signal receptor, is responsible for the Ti plasmid-specific transfer of DNA to maize by Agrobacterium.

Abstract

Agrobacteria exhibit marked Ti (tumorinducing)/Ri (root-inducing) plasmid specificity in their interaction with the Gramineae. In this study, we have used the technique of "agroinfection," in which Agrobacterium-mediated delivery of viral genomes into plants is detected by the development of viral disease symptoms, to identify the region of the Ti plasmid which is responsible for the major differences seen in the ability of nopaline-vs. plants (4, 5). However, more recent work reported that certain monocotyledonous plants (monocots) are also susceptible (6-9). Furthermore, although maize and other graminaceous monocots have remained refractory to tumor formation, the agroinfection technique used to study the transfer of viral DNA to dicots (10, 11) has confirmed that Agrobacterium is able to transfer DNA to cereal plants (12)(13)(14)(15)(16)(17). Agroinfection is mediated through the same vir gene pathway as is required for tumor formation in dicots (18).This technique provides a very sensitive assay for Agrobacterium-mediated DNA transfer, as the viral sequences escape from the T-DNA and are amplified as the virus replicates and spreads systemically. DNA transfer is thus detected by the formation of disease symptoms.The ability to agroinfect maize was found to be Ti plasmidspecific (13, 16): octopine-type strains either were unable, or only weakly able (<5%), to transfer maize streak virus (MSV) DNA, while nopaline-type strains infected 80-100%o of the inoculated plants. Agropine-and mannopine-type strains of A. rhizogenes also efficiently agroinfected maize. With appropriate viral DNAs as markers, similar results were obtained for other members of the Gramineae (14,15,17). In contrast, Ti plasmid specificity is not typical of agroinfection-dicot interactions (10, 11).A "disarmed" (T-DNA-less) nopaline strain agroinfected maize as efficiently as the wild type (13). Therefore, Ti plasmid specificity cannot be due to differences in phytohormone production specified by the T-DNA. One explanation for the inability of octopine-type strains to agroinfect maize and other graminaceous monocots is that octopine-type Ti plasmids lack an essential function present in nopaline-, agropine-, and mannopine-type Ti/Ri plasmids. Alternatively, octopine-type Ti plasmids might express a counteractive function.We have used the agroinfection system to characterize the Ti plasmid requirements for transfer of MSV DNA to maize. We observed that a DNA fragment containing the nopalinetype virA locus complemented octopine-type Ti plasmids to high levels of agroinfection. Furthermore, whereas preinduction of octopine-type strains does not promote maize agroinfection, octopine-type virA mutant strains, which express vir genes at high levels in the absence of the inducing compound acetosyringone, efficiently transferred MSV DNA to maize. We discuss here the implications of these observations and possible causes. MATERIALS AND METHODSBacterial Strains, Plasmids, and Media. The MSV-containing binary vectors and other plasmids used in...

Mutational analysis of the input domain of the VirA protein of Agrobacterium tumefaciens

“…pJD102W was constructed as follows: a PstI site was introduced into the C58 virA gene of pDR169 (38) by changing thymine to cytosine at position 717 in the coding sequence and guanine to cytosine at position 720 by means of PCR amplification. The mutations were confirmed by DNA sequencing using the Sequenase version 2.0 kit according to the instructions provided by the manufacturer (United States Biochemical).…”

Protein secretion and the pathogenesis of bacterial infections

“…11;Leroux et al 1987;Porter et al 1987;Christie et al 1988;Jin et al 1990c;Kuldau et al 1990;Shirasu et al 1990). virA and virG encode a histidine protein kinase and the cognate response regulator, two components of a signal transduction device that al-lows Agrobacteria to respond to specific plant signals by expressing virulence genes (Jin et al 1990a,b,c;Raineri et al 1993). VirD1-3 fulfill an analogous function as TraHIJ with VirD2 being linked to the cleaved T-DNA (Yanofsky et al 1986;Herrera-Estrella et al 1988;Ward and Barnes 1988;Young and Nester 1988;Howard et al 1989).…”

Variable efficiency of a Ti plasmid-encoded VirA protein in different agrobacterial hosts

“…DNA sequencing reactions were initiated on both strands by using M13 forward and reverse primers. The C58 virA sequence, used in comparative analyses, was as reported by Rogowski et al (39) and, when useful, confirmed by examination of the C58 virA sequence of pDR169 (37).…”