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The taxonomic position, growth characteristics and antibiotic resistance properties of a slightly yellow-pigmented bacterial strain, designated R26 T , isolated from the midgut of the mosquito Anopheles gambiae, were studied. The isolate produced rod-shaped cells, which stained Gram-negative.

The Alphaproteobacterium Rhizobium radiobacter F4 (RrF4) was originally characterized as an endofungal bacterium in the beneficial endophytic Sebacinalean fungus Piriformospora indica. Although attempts to cure P. indica from RrF4 repeatedly failed, the bacterium can easily be grown in pure culture. Here, we report on RrF4's genome and the beneficial impact the free-living bacterium has on plants. In contrast to other endofungal bacteria, the genome size of RrF4 is not reduced. Instead, it shows a high degree of similarity to the plant pathogenic R. radiobacter (formerly: Agrobacterium tumefaciens) C58, except vibrant differences in both the tumor-inducing (pTi) and the accessor (pAt) plasmids, which can explain the loss of RrF4's pathogenicity. Similar to its fungal host, RrF4 colonizes plant roots without host preference and forms aggregates of attached cells and dense biofilms at the root surface of maturation zones. RrF4-colonized plants show increased biomass and enhanced resistance against bacterial leaf pathogens. Mutational analysis showed that, similar to P. indica, resistance mediated by RrF4 was dependent on the plant's jasmonate-based induced systemic resistance (ISR) pathway. Consistent with this, RrF4-and P. indica-induced pattern of defense gene expression were similar. In clear contrast to P. indica, but similar to plant growth-promoting rhizobacteria, RrF4 colonized not only the root outer cortex but also spread beyond the endodermis into the stele. On the basis of our findings, RrF4 is an efficient plant growth-promoting bacterium.

SummaryThe ultimate goal of taxonomy is to establish a system that mirrors the 'order in nature'. In prokaryote microbiology, almost all taxonomic concepts try to mirror the whole evolutionary order back to the origin of life with the cell as basic unit. The introduction of the 16S rRNA gene as molecular marker allowed for the first time the creation of a hierarchical taxonomic system based on one practical molecular marker. With the development of new and rapid sequencing technologies a wealth of new data can and will be used for critical evaluation of the taxonomic system. Comprehensive analyses of other molecular markers as well as total or partial genome comparisons confirmed the 16S rRNA based hierarchical system as 'backbone of prokaryote taxonomy' at least at the genus level and above. A tendency is visible to classify novel taxa more and more based on the genotype, i.e. comparative analyses of 16S rRNA and/or other gene sequence data (in multilocus sequence analysis, MLSA) at the genus and the species level, sometimes contrary to the indications of other (often phenotypic) data. The understanding of all the information behind these data is lagging far behind their accumulation. Genes and genomes do not function on its own and can only display their potential within the cell as the basic unit of evolution (and hence taxonomy). It is the phenotype and the natural selection that 'drive' evolution in a given environment. In this context, the 'polyphasic taxonomic approach' should be revisited again, taking into account the novel insights into genomes and other 'omic' sciences in a more strict and detailed context with the phenotype. This approach allows a more holistic view and provides a sound basis for describing the diversity of prokaryotes and has the potential to become the foundation of a more stable, in-depth taxonomy of the prokaryotes.

Global warming is currently a much discussed topic with as yet largely unexplored consequences for agro-ecosystems. Little is known about the warming effect on the bacterial microbiota inhabiting the plant surface (phyllosphere), which can have a strong impact on plant growth and health, as well as on plant diseases and colonization by human pathogens. The aim of this study was to investigate the effect of moderate surface warming on the diversity and composition of the bacterial leaf microbiota of the herbaceous plant Galium album. Leaves were collected from four control and four surface warmed (+2°C) plots located at the field site of the Environmental Monitoring and Climate Impact Research Station Linden in Germany over a 6-year period. Warming had no effect on the concentration of total number of cells attached to the leaf surface as counted by Sybr Green I staining after detachment, but changes in the diversity and phylogenetic composition of the bacterial leaf microbiota analyzed by bacterial 16S rRNA gene Illumina amplicon sequencing were observed. The bacterial phyllosphere microbiota were dominated by Proteobacteria, Bacteroidetes, and Actinobacteria. Warming caused a significant higher relative abundance of members of the Gammaproteobacteria, Actinobacteria, and Firmicutes, and a lower relative abundance of members of the Alphaproteobacteria and Bacteroidetes. Plant beneficial bacteria like Sphingomonas spp. and Rhizobium spp. occurred in significantly lower relative abundance in leaf samples of warmed plots. In contrast, several members of the Enterobacteriaceae, especially Enterobacter and Erwinia, and other potential plant or human pathogenic genera such as Acinetobacter and insect-associated Buchnera and Wolbachia spp. occurred in higher relative abundances in the phyllosphere samples from warmed plots. This study showed for the first time the long-term impact of moderate (+2°C) surface warming on the phyllosphere microbiota on plants. A reduction of beneficial bacteria and an enhancement of potential pathogenic bacteria in the phyllosphere of plants may indicate that this aspect of the ecosystem which has been largely neglected up till now, can be a potential risk for pathogen transmission in agro-ecosystems in the near future.