The Agrobacterium VirE2 effector interacts with multiple members of the Arabidopsis VIP1 protein family

T-DNA transfer from Agrobacterium to its host plant genome relies on multiple interactions between plant proteins and bacterial effectors. One such plant protein is the Arabidopsis VirE2 interacting protein (AtVIP1), a transcription factor that binds Agrobacterium tumefaciens C58 VirE2, potentially acting as an adaptor between VirE2 and several other host factors. It remains unknown, however, whether the same VirE2 protein has evolved to interact with multiple VIP1 homologues in the same host, and whether VirE2 homologues encoded by different bacterial strains/species recognize AtVIP1 or its homologues. Here, we addressed these questions by systematic analysis (using the yeast two-hybrid and co-immunoprecipitation approaches) of interactions between VirE2 proteins encoded by four major representatives of known bacterial species/strains with functional T-DNA transfer machineries and eight VIP1 homologues from Arabidopsis and tobacco. We also analysed the determinants of the VirE2 sequence involved in these interactions. These experiments showed that the VirE2 interaction is degenerate: the same VirE2 protein has evolved to interact with multiple VIP1 homologues in the same host, and different and mutually independent VirE2 domains are involved in interactions with different VIP1 homologues. Furthermore, the VIP1 functionality related to the interaction with VirE2 is independent of its function as a transcriptional regulator. These observations suggest that the ability of VirE2 to interact with VIP1 homologues is deeply ingrained into the process of Agrobacterium infection. Indeed, mutations that abolished VirE2 interaction with AtVIP1 produced no statistically significant effects on interactions with VIP1 homologues or on the efficiency of genetic transformation.

Direct fluorescence detection of VirE2 secretion by Agrobacterium tumefaciens

VirE2 is a ssDNA binding protein essential for virulence in Agrobacterium tumefaciens. A tetracysteine mutant (VirE2-TC) was prepared for in vitro and in vivo fluorescence imaging based on the ReAsH reagent. VirE2-TC was found to be biochemically active as it binds both ssDNA and the acidic secretion chaperone VirE1. It was also biologically functional in complementing virE2 null strains transforming Arabidopsis thaliana roots and Nicotiana tabacum leaves. In vitro experiments demonstrated a two-color fluorescent complex using VirE2-TC/ReAsH and Alexa Fluor 488 labeled ssDNA. In vivo, fluorescent VirE2-TC/ReAsH was detected in bacteria and in plant cells at time frames relevant to transformation.

Agrobacterium tumefaciens Gene Transfer: How a Plant Pathogen Hacks the Nuclei of Plant and Nonplant Organisms

Agrobacterium species are soilborne gram-negative bacteria exhibiting predominantly a saprophytic lifestyle. Only a few of these species are capable of parasitic growth on plants, causing either hairy root or crown gall diseases. The core of the infection strategy of pathogenic Agrobacteria is a genetic transformation of the host cell, via stable integration into the host genome of a DNA fragment called T-DNA. This genetic transformation results in oncogenic reprogramming of the host to the benefit of the pathogen. This unique ability of interkingdom DNA transfer was largely used as a tool for genetic engineering. Thus, the artificial host range of Agrobacterium is continuously expanding and includes plant and nonplant organisms. The increasing availability of genomic tools encouraged genome-wide surveys of T-DNA tagged libraries, and the pattern of T-DNA integration in eukaryotic genomes was studied. Therefore, data have been collected in numerous laboratories to attain a better understanding of T-DNA integration mechanisms and potential biases. This review focuses on the intranuclear mechanisms necessary for proper targeting and stable expression of Agrobacterium oncogenic T-DNA in the host cell. More specifically, the role of genome features and the putative involvement of host's transcriptional machinery in relation to the T-DNA integration and effects on gene expression are discussed. Also, the mechanisms underlying T-DNA integration into specific genome compartments is reviewed, and a theoretical model for T-DNA intranuclear targeting is presented.

Unfoldome variation upon plant-pathogen interactions: strawberry infection by Colletotrichum acutatum

Intrinsically disordered proteins (IDPs) are proteins that lack secondary and/or tertiary structure under physiological conditions. These proteins are very abundant in eukaryotic proteomes and play crucial roles in all molecular mechanisms underlying the response to environmental challenges. In plants, different IDPs involved in stress response have been identified and characterized. Nevertheless, a comprehensive evaluation of protein disorder in plant proteomes under abiotic or biotic stresses is not available so far. In the present work the transcriptome dataset of strawberry (Fragaria X ananassa) fruits interacting with the fungal pathogen Colletotrichum acutatum was actualized onto the woodland strawberry (Fragaria vesca) genome. The obtained cDNA sequences were translated into protein sequences, which were subsequently subjected to disorder analysis. The results, providing the first estimation of disorder abundance associated to plant infection, showed that the proteome activated in the strawberry red fruit during the active fungal propagation is remarkably depleted in disorder. On the other hand, in the resistant white fruit, no significant disorder reduction is observed in the proteins expressed in response to fungal infection. Four representative proteins, FvSMP, FvPRKRIP, FvPCD-4 and FvFAM32A-like, predicted as mainly disordered and never experimentally characterized before, were isolated, and the absence of structure was validated at the secondary and tertiary level using circular dichroism and differential scanning fluorimetry. Their quaternary structure was also established using light scattering. The results are discussed considering the role of protein disorder in plant defense.