Rhizobacteria Impact Colonization of Listeria monocytogenes on Arabidopsis thaliana Roots

In spite of its relevance as a foodborne pathogen, we have limited knowledge about Listeria monocytogenes in the environment. L. monocytogenes outbreaks have been linked to fruits and vegetables; thus, a better understanding of the factors influencing its ability to colonize plants is important. We tested how environmental factors and other soil- and plant-associated bacteria influenced L. monocytogenes\u27 ability to colonize plant roots using Arabidopsis thaliana seedlings in a hydroponic growth system. We determined that the successful root colonization of L. monocytogenes 10403S was modestly but significantly enhanced by the bacterium being pregrown at higher temperatures, and this effect was independent of the biofilm and virulence regulator PrfA. We tested 14 rhizosphere-derived bacteria for their impact on L. monocytogenes 10403S, identifying one that enhanced and 10 that inhibited the association of 10403S with plant roots. We also characterized the outcomes of these interactions under both coinoculation and invasion conditions. We characterized the physical requirements of five of these rhizobacteria to impact the association of L. monocytogenes 10403S with roots, visualizing one of these interactions by microscopy. Furthermore, we determined that two rhizobacteria (one an inhibitor, the other an enhancer of 10403S root association) were able to similarly impact 10 different L. monocytogenes strains, indicating that the effects of these rhizobacteria on L. monocytogenes are not strain specific. Taken together, our results advance our understanding of the parameters that affect L. monocytogenes plant root colonization, knowledge that may enable us to deter its association with and, thus, downstream contamination of, food crops. IMPORTANCE Listeria monocytogenes is ubiquitous in the environment, being found in or on soil, water, plants, and wildlife. However, little is known about the requirements for L. monocytogenes\u27 existence in these settings. Recent L. monocytogenes outbreaks have been associated with contaminated produce; thus, we used a plant colonization model to investigate factors that alter L. monocytogenes\u27 ability to colonize plant roots. We show that L. monocytogenes colonization of roots was enhanced when grown at higher temperatures prior to inoculation but did not require a known regulator of virulence and biofilm formation. Additionally, we identified several rhizobacteria that altered the ability of 11 different strains of L. monocytogenes to colonize plant roots. Understanding the factors that impact L. monocytogenes physiology and growth will be crucial for finding mechanisms (whether chemical or microbial) that enable its removal from plant surfaces to reduce L. monocytogenes contamination of produce and eliminate foodborne illness

Natural variation in a short region of the Acidovorax citrulli type III‐secreted effector AopW1 is associated with differences in cytotoxicity and host adaptation

Bacterial fruit blotch, caused by Acidovorax citrulli, is a serious disease of melon and watermelon. The strains of the pathogen belong to two major genetic groups: group I strains are strongly associated with melon, while group II strains are more aggressive on watermelon. A. citrulli secretes many protein effectors to the host cell via the type III secretion system. Here we characterized AopW1, an effector that shares similarity to the actin cytoskeleton-disrupting effector HopW1 of Pseudomonas syringae and with effectors from other plant-pathogenic bacterial species. AopW1 has a highly variable region (HVR) within amino acid positions 147 to 192, showing 14 amino acid differences between group I and II variants. We show that group I AopW1 is more toxic to yeast and Nicotiana benthamiana cells than group II AopW1, having stronger actin filament disruption activity, and increased ability to induce cell death and reduce callose deposition. We further demonstrated the importance of some amino acid positions within the HVR for AopW1 cytotoxicity. Cellular analyses revealed that AopW1 also localizes to the endoplasmic reticulum, chloroplasts, and plant endosomes. We also show that overexpression of the endosome-associated protein EHD1 attenuates AopW1-induced cell death and increases defense responses. Finally, we show that sequence variation in AopW1 plays a significant role in the adaptation of group I and II strains to their preferred hosts, melon and watermelon, respectively. This study provides new insights into the HopW1 family of bacterial effectors and provides first evidence on the involvement of EHD1 in response to biotic stress.United States-Israel Binational Agriculture Research and Development (BARD) Fund S-5023-17

Tuning the locus of oxidation in Cu-diamido-diphenoxo complexes: From Cu(III) to Cu(II)-phenoxyl radical

While N2O2 tetraanionic ligands containing a strong N-amidate sigma-donor are generally assumed to stabilise metal high valence states, we herein have shown that, in dianionic Cu(II)-diamido-diphenoxo complexes, H-bonding and electronic effects on the phenolate groups may modulate the electronic structure of their oxidised species from Cu(III) to Cu(II)-phenoxyl radical complexes; and so in the negative potential range. We observe that electron-poor phenolate complexes 2(2) and 3(2) oxidise to Cu(III) species, whereas electron rich phenolate complex 1(2) oxidises to a Cu(II)-phenoxyl radical. Our DFT results suggest that pi-electron-rich phenolate rings in 1(2) are responsible for an increase of the HOMO orbital energy, bringing the HOMO-SOMO gap small enough to favour a ligand-based oxidation process. Further DFT-calculations have also shown that upon changing the o,p-phenol substituent from electron-widthdrawing groups (NO2) to electron-donating ones (OMe), the favoured oxidised state switches from Cu(III) to Cu(II)-radical. These results emphasize the use of the versatile diamido-diphenoxo backbone as a promising way to novel GO-chemical models, as well as molecular switches. (C) 2017 Elsevier B.V. All rights reserved

Phenotypic variation in the plant pathogenic bacterium Acidovorax citrulli.

Acidovorax citrulli causes bacterial fruit blotch (BFB) of cucurbits, a disease that threatens the cucurbit industry worldwide. Despite the economic importance of BFB, little is known about pathogenicity and fitness strategies of the bacterium. We have observed the phenomenon of phenotypic variation in A. citrulli. Here we report the characterization of phenotypic variants (PVs) of two strains, M6 and 7a1, isolated from melon and watermelon, respectively. Phenotypic variation was observed following growth in rich medium, as well as upon isolation of bacteria from inoculated plants or exposure to several stresses, including heat, salt and acidic conditions. When grown on nutrient agar, all PV colonies possessed a translucent appearance, in contrast to parental strain colonies that were opaque. After 72 h, PV colonies were bigger than parental colonies, and had a fuzzy appearance relative to parental strain colonies that are relatively smooth. A. citrulli colonies are generally surrounded by haloes detectable by the naked eye. These haloes are formed by type IV pilus (T4P)-mediated twitching motility that occurs at the edge of the colony. No twitching haloes could be detected around colonies of both M6 and 7a1 PVs, and microscopy observations confirmed that indeed the PVs did not perform twitching motility. In agreement with these results, transmission electron microscopy revealed that M6 and 7a1 PVs do not produce T4P under tested conditions. PVs also differed from their parental strain in swimming motility and biofilm formation, and interestingly, all assessed variants were less virulent than their corresponding parental strains in seed transmission assays. Slight alterations could be detected in some DNA fingerprinting profiles of 7a1 variants relative to the parental strain, while no differences at all could be seen among M6 variants and parental strain, suggesting that, at least in the latter, phenotypic variation is mediated by slight genetic and/or epigenetic alterations

Os(VI)O₂/K Metal–Organic Frameworks: Infinite Chain, Grid, and Porous Networks

The design of multianionic chelating ligands as new organic linker for producing metal–organic frameworks (MOFs) is discussed. Three potentially polyanionic pro-ligands, 3,5-di-<i>tert</i>-butyl-2-hydroxy-<i>N</i>-(2-hydroxyethyl)­benzamide (¹LH₃), bis­(2-aminoethyl)-5-(<i>tert</i>-butyl)-2-hydroxyisophthalate (^2aLH₃), and 5-(<i>tert</i>-butyl)-2-hydroxy-<i>N</i>1,<i>N</i>3-bis­(1-hydroxy-2-methylpropan-2-yl)­isophthalamide (^2bLH₃), were synthesized and found to coordinate the osmyl ion in trianionic NO₂ fashion through the N-amidate, O-phenolate, and O-alcoholate donor atoms. The X-ray crystal structures of three dioxo-Os­(VI) complexes: [Os₂O₄(¹L)₂(OH)­K₂(H₂O)­(C₃H₆O)] (<b>Os</b>^<b>1</b>), [OsO₂(^2aL^3–)­(MeOH)₄(MeO)­K₂] (<b>Os</b>^<b>2a</b>), and [OsO₂(^2bL)­(H₂O)­(C₃H₆O)­K] (<b>Os</b>^<b>2b</b>) reveal that the osmyl moiety and the ligand establish distinctive interactions with the potassium ions, yielding unprecedented infinite network from stepladder chain (in <b>Os</b>^<b>2b</b>) and 2D-grid (in <b>Os</b>^<b>1</b>) to 3D-porous H-bonding network (in <b>Os</b>^<b>2a</b>)