Evolutionary Relatedness and Classification of Tumor-Inducing and Opine-Catabolic Plasmids in Three Rhizobium rhizogenes Strains Isolated from the Same Crown Gall Tumor

Defensive alteration of root exudate composition by grafting Prunus sp. onto resistant rootstock contributes to reducing crown gall disease

Grafting is a traditional and significant strategy to suppress soil-borne diseases, such as the crown gall disease caused by tumorigenic Agrobacterium and Rhizobium. Root exudates and the rhizosphere microbiome play critical roles in controlling crown gall disease, but their roles in suppressing crown gall disease in grafted plants remain unclear. Here, disease-susceptible cherry rootstock 'Gisela 6' and disease-resistant cherry rootstock 'Haiying 1' were grafted onto each other or self-grafted. The effect of their root exudates on the soil microbiome composition and the abundance of pathogenic Agrobacterium were studied. Grafting onto the disease-resistant rootstock helped to reduce the abundance of pathogenic Agrobacterium, accompanied by altering root exudation, enriching potential beneficial bacteria, and changing soil function. Then, the composition of the root exudates from grafted plants was analyzed and the potential compounds responsible for decreasing pathogenic Agrobacterium abundance were identified. Based on quantitative measurement of the concentrations of the compounds and testing the impacts of supplied pure chemicals on abundance and chemotaxis of pathogenic Agrobacterium and potential beneficial bacteria, the decreased valine in root exudates of the plant grafted onto resistant rootstock was found to contribute to decreasing Agrobacterium abundance, enriching some potential beneficial bacteria and suppressing crown gall disease. This study provides insights into the mechanism whereby grafted plants suppress soil-borne disease.

Virulence and Ecology of Agrobacteria in the Context of Evolutionary Genomics

Among plant-associated bacteria, agrobacteria occupy a special place. These bacteria are feared in the field as agricultural pathogens. They cause abnormal growth deformations and significant economic damage to a broad range of plant species. However, these bacteria are revered in the laboratory as models and tools. They are studied to discover and understand basic biological phenomena and used in fundamental plant research and biotechnology. Agrobacterial pathogenicity and capability for transformation are one and the same and rely on functions encoded largely on their oncogenic plasmids. Here, we synthesize a substantial body of elegant work that elucidated agrobacterial virulence mechanisms and described their ecology. We review findings in the context of the natural diversity that has been recently unveiled for agrobacteria and emphasize their genomics and plasmids. We also identify areas of research that can capitalize on recent findings to further transform our understanding of agrobacterial virulence and ecology.

Diversity and Evolutionary History of Ti Plasmids of "tumorigenes" Clade of Rhizobium spp. and Their Differentiation from Other Ti and Ri Plasmids

Agrobacteria are important plant pathogens responsible for crown/cane gall and hairy root diseases. Crown/cane gall disease is associated with strains carrying tumor-inducing (Ti) plasmids, while hairy root disease is caused by strains harboring root-inducing (Ri) plasmids. In this study, we analyzed the sequences of Ti plasmids of the novel "tumorigenes" clade of the family Rhizobiaceae ("tumorigenes" Ti plasmids), which includes two species, Rhizobium tumorigenes and Rhizobium rhododendri. The sequences of reference Ti/Ri plasmids were also included, which was followed by a comparative analysis of their backbone and accessory regions. The "tumorigenes" Ti plasmids have novel opine signatures compared with other Ti/Ri plasmids characterized so far. The first group exemplified by pTi1078 is associated with production of agrocinopine, nopaline, and ridéopine in plant tumors, while the second group comprising pTi6.2 is responsible for synthesis of leucinopine. Bioinformatic and chemical analyses, including opine utilization assays, indicated that leucinopine associated with pTi6.2 most likely has D,L stereochemistry, unlike the L,L-leucinopine produced in tumors induced by reference strains Chry5 and Bo542. Most of the "tumorigenes" Ti plasmids have conjugative transfer system genes that are unusual for Ti plasmids, composed of avhD4/avhB and traA/mobC/parA regions. Next, our results suggested that "tumorigenes" Ti plasmids have a common origin, but they diverged through large-scale recombination events, through recombination with single or multiple distinct Ti/Ri plasmids. Lastly, we showed that Ti/Ri plasmids could be differentiated based on pairwise Mash or average amino-acid identity distance clustering, and we supply a script to facilitate application of the former approach by other researchers.

The Ti Plasmid, Driver of Agrobacterium Pathogenesis

The phytopathogenic bacterium Agrobacterium tumefaciens causes crown gall disease in plants, characterized by the formation of tumor-like galls where wounds were present. Nowadays, however, the bacterium and its Ti (tumor-inducing) plasmid is better known as an effective vector for the genetic manipulation of plants and fungi. In this review, I will briefly summarize some of the major discoveries that have led to this bacterium now playing such a prominent role worldwide in plant and fungal research at universities and research institutes and in agricultural biotechnology for the production of genetically modified crops. I will then delve a little deeper into some aspects of Agrobacterium biology and discuss the diversity among agrobacteria and the taxonomic position of these bacteria, the diversity in Ti plasmids, the molecular mechanism used by the bacteria to transform plants, and the discovery of protein translocation from the bacteria to host cells as an essential feature of Agrobacterium-mediated transformation.

Complete Sequence of Succinamopine Ti-Plasmid pTiEU6 Reveals Its Evolutionary Relatedness with Nopaline-Type Ti-Plasmids

Agrobacterium tumefaciens is the etiological agent of plant crown gall disease, which is induced by the delivery of a set of oncogenic genes into plant cells from its tumor-inducing (Ti) plasmid. Here we present the first complete sequence of a succinamopine-type Ti-plasmid. Plasmid pTiEU6 is comprised of 176,375 bp with an overall GC content of 56.1% and 195 putative protein-coding sequences could be identified. This Ti-plasmid is most closely related to nopaline-type Ti-plasmids. It contains a single T-region which is somewhat smaller than that of the nopaline-type Ti-plasmids and in which the gene for nopaline synthesis is replaced by a gene (sus) for succinamopine synthesis. Also in pTiEU6 the nopaline catabolic genes are replaced by genes for succinamopine catabolism. In order to trace the evolutionary origin of pTiEU6, we sequenced six nopaline Ti-plasmids to enlarge the scope for comparison to this class of plasmids. Average nucleotide identity analysis revealed that pTiEU6 was most closely related to nopaline Ti-plasmids pTiT37 and pTiSAKURA. In line with this traces of several transposable elements were present in all the nopaline Ti plasmids and in pTiEU6, but one specific transposable element insertion, that of a copy of IS1182, was present at the same site only in pTiEU6, pTiT37, and pTiSAKURA, but not in the other Ti plasmids. This suggests that pTiEU6 evolved after diversification of nopaline Ti-plasmids by DNA recombination between a pTiT37-like nopaline Ti-plasmid and another plasmid, thus introducing amongst others new catabolic genes matching a new opine synthase gene for succinamopine synthesis.

Isolation and Characterization of Avirulent and Virulent Strains of Agrobacterium tumefaciens from Rose Crown Gall in Selected Regions of South Korea

Agrobacterium tumefaciens is a plant pathogen that causes crown gall disease in various hosts across kingdoms. In the present study, five regions (Wonju, Jincheon, Taean, Suncheon, and Kimhae) of South Korea were chosen to isolate A. tumefaciens strains on roses and assess their opine metabolism (agrocinopine, nopaline, and octopine) genes based on PCR amplification. These isolated strains were confirmed as Agrobacterium using morphological, biochemical, and 16S rDNA analyses; and pathogenicity tests, including the growth characteristics of the white colony appearance on ammonium sulfate glucose minimal media, enzyme activities, 16S rDNA sequence alignment, and pathogenicity on tomato (Solanum lycopersicum). Carbon utilization, biofilm formation, tumorigenicity, and motility assays were performed to demarcate opine metabolism genes. Of 87 isolates, 18 pathogenic isolates were affirmative for having opine plasmid genes. Most of these isolates showed the presence of an agrocinopine type of carbon utilization. Two isolates showed nopaline types. However, none of these isolates showed octopine metabolic genes. The objectives of the present study were to isolate and confirm virulent strains from rose crown galls grown in the different regions of Korea and characterize their physiology and opine types. This is the first report to describe the absence of the octopine type inciting the crown gall disease of rose in South Korea.