The Agrobacterium Ti Plasmids

Abstract

Agrobacterium tumefaciens is a plant pathogen with the capacity to deliver a segment of oncogenic DNA carried on a large plasmid called the tumor-inducing or Ti plasmid to susceptible plant cells. A. tumefaciens belongs to the class Alphaproteobacteria , whose members include other plant pathogens ( Agrobacterium rhizogenes ), plant and insect symbionts ( Rhizobium spp. and Wolbachia spp., respectively), human pathogens ( Brucella spp., Bartonella spp., Rickettsia spp.), and nonpathogens ( Caulobacter crescentus, Rhodobacter sphaeroides ). Many species of Alphaproteobacteria carry large plasmids ranging in size from ∼100 kb to nearly 2 Mb. These large replicons typically code for functions essential for cell physiology, pathogenesis, or symbiosis. Most of these elements rely on a conserved gene cassette termed repABC for replication and partitioning, and maintenance at only one or a few copies per cell ( 1 ). The subject of this review is the ∼200-kb Ti plasmids carried by infectious strains of A. tumefaciens . We will summarize the features of this plasmid as a representative of the repABC family of megaplasmids. We will also describe novel features of this plasmid that enable A. tumefaciens cells to incite tumor formation in plants, sense and respond to an array of plant host and bacterial signal molecules, and maintain and disseminate the plasmid among populations of agrobacteria. At the end of this review, we will describe how this natural genetic engineer has been adapted to spawn an entire industry of plant biotechnology and review its potential for use in future therapeutic applications of plant and nonplant species.

Plant–phytopathogen interactions: bacterial responses to environmental and plant stimuli

“…This involves the second T4SS encoded by the Ti plasmid, the Tra/TrB T4SS (Table 1). In fact, several Ti plasmids exist and they can be distinguished by the type of opines they produce (Subramoni et al, 2014;Gordon and Christie, 2014). Octopine and nopaline-type Ti plasmids are the most studied plasmids.…”

Cyclic β-glucans at the bacteria-host cells interphase: One sugar ring to rule them all

“…The Ti plasmid integrates into the plant genome directing the synthesis of different factors necessary for tumourigenesis, crown gall induction, and maintenance (Gordon & Christie, 2014). tumefaciens is a plant pathogen that provokes neoplastic transformation of dicotyledonous plants called crown gall tumours.…”

“…Subsequently, bacteria inject the tumour induction (Ti) plasmid into the host cells by means of the VirB Type IV secretion system (T4SS). The Ti plasmid integrates into the plant genome directing the synthesis of different factors necessary for tumourigenesis, crown gall induction, and maintenance (Gordon & Christie, 2014). Comparable with that observed in symbiotic bacteria, A. tumefaciens chvB and chvA mutants have a pleiotropic phenotype and are avirulent.…”

Annotation of plasmid genes

“…Good examples of this are RP4 whose two transfer regions contain genes traABCDE etc and trbABCDEFGHIJK etc (Pansegrau et al, 1994). Perhaps the most influential plasmids in this category are the Ti plasmids (Christie and Gordon, 2014), such as pTiC58 that carries operons involved in tumorigenesis in plants named virA,B,C,D etc with the genes in each operon named virA, virB1-11, virC1-2, virD1-4 etc. The virB operon defines the type IV secretion system (T4SS) involved in transfer of the tumorigenic DNA (tDNA) to the target plant cell (a process related to plasmid conjugative transfer) whereas the virD operon defines the gene products involved in DNA processing (VirD2 is the relaxase, VirD4 is the coupling protein).…”