Leaf Age-Dependent Photoprotective and Antioxidative Response Mechanisms to Paraquat-Induced Oxidative Stress in Arabidopsis thaliana

Exposure of Arabidopsis thaliana young and mature leaves to the herbicide paraquat (Pq) resulted in a localized increase of hydrogen peroxide (H₂O₂) in the leaf veins and the neighboring mesophyll cells, but this increase was not similar in the two leaf types. Increased H₂O₂ production was concomitant with closed reaction centers (q_P). Thirty min after Pq exposure despite the induction of the photoprotective mechanism of non-photochemical quenching (NPQ) in mature leaves, H₂O₂ production was lower in young leaves mainly due to the higher increase activity of ascorbate peroxidase (APX). Later, 60 min after Pq exposure, the total antioxidant capacity of young leaves was not sufficient to scavenge the excess reactive oxygen species (ROS) that were formed, and thus, a higher H₂O₂ accumulation in young leaves occurred. The energy allocation of absorbed light in photosystem II (PSII) suggests the existence of a differential photoprotective regulatory mechanism in the two leaf types to the time-course Pq exposure accompanied by differential antioxidant protection mechanisms. It is concluded that tolerance to Pq-induced oxidative stress is related to the redox state of quinone A (Q_A).     

Interactions of Escherichia coli O157:H7, Salmonella typhimurium and Listeria monocytogenes plants cultivated in a gnotobiotic system

The growth and persistence of Escherichia coli O157:H7, Salmonella typhimurium and Listeria monocytogenes on a diverse range of plant types over extended cultivation periods was studied. When introduced on the seed of carrot, cress, lettuce, radish, spinach and tomato all the pathogens became rapidly established shortly after germination, attaining cell densities of the order of 5.5-6.5 log cfu/g. In general, Es. coli O157:H7 and L. monocytogenes became established and persisted at significantly higher levels on seedlings (9 days post-germination) than Salmonella. Es. coli O157:H7 became internalized in cress, lettuce, radish and spinach seedlings but was not recovered within the tissues of mature plants. Internalization of Salmonella was also observed in lettuce and radish but not cress or spinach seedlings. In contrast, L. monocytogenes did not internalize within seedlings but did persist on the surface of plants throughout the cultivation period. Co-inoculation of isolates recovered from the rhizosphere of plants did not significantly affect the numbers or persistence of human pathogens. The only exception was with Enterobacter cloacae, which reduced Es. coli O157:H7 Ph1 and L. monocytogenes levels by ca. 1 log cfu/g on lettuce. With the bioluminescent phenotype of Es. coli O157:H7 Ph1, it was demonstrated that the human pathogen became established on the roots of growing plants. Scanning electron micrographs of root seedlings suggested that Es. coli O157:H7 Ph1 preferentially colonized the root junctions of seedlings. It is proposed that such colonization sites enhanced the persistence of Es. coli O157:H7 on plants and facilitated internalization within developing seedlings. The results suggest that the risk associated with internalized human pathogens in salad vegetables at harvest is low. Nevertheless, the introduction of human pathogens at an early stage of plant development could enhance their persistence in the rhizosphere. The implications of the study with regards to on-farm food safety initiatives are discussed.     

Effect of cold temperature on the composition of different lipid classes of the foodborne pathogen Listeria monocytogenes: focus on neutral lipids

In this work a thorough consideration of the membrane lipid composition of Listeria monocytogenes together with DSC analysis is described in order to estimate the biological importance of lipid changes during low-temperature adaptation. Furthermore, these studies provide comparative data for fatty acid changes for neutral, NL and polar lipids, PL separately. The cold adaptation (5 degrees C) response of L. monocytogenes showed (i) an increase in the level of NL content (30%) among the total lipids, TL and (ii) that the increase (7-fold) in the anteiso-15:0/anteiso-17:0 fatty acid ratio, FAr, for cold NL was at variance with the ratio for TL and PL (about 10-fold). We correlated our findings with DSC studies on phase transition temperature (Tc), enthalpy difference (DeltaH) and peak range of the transition for TL, PL, NL (from cultures at 30 and 5 degrees C); The decrease of Tc (10.5 degrees C) and DeltaH (51%) for TL is a reflection of the decrease of Tc (11.5 degrees C) and DeltaH (56%) for PL. This large decrease is interpreted by the high (10-fold) increase of a-15:0/a-17:0 FAr of PL5 degrees C. In NL the decrease of Tc (3 degrees C) and of DeltaH (42%) is interpreted by both adaptation mechanisms: the (lower) 7-fold increase of anteiso-15:0/anteiso-17:0 FAr and the NL percentage calculated from increased mass values. The peak range of TL5 degrees C (from -15 to 25 degrees C) is a reflection of the peak range of NL5 degrees C, which is unchanged, as is the peak range of NL30 degrees C.     

Superhard tungsten tetraboride films prepared by pulsed laser deposition method

Attempts to synthesize and/or theoretically predict new superhard materials are the subject of an intense research activity. The trials to deposit them in the form of films have just began. WB(2) (77 wt % WB(2) and 23 wt % WB(4)) and WB(4) (65 wt % WB(4) and 35 wt % WB(2)) polycrystalline bulk samples were obtained in this work via electron beam synthesis technique and, subsequently, used as targets for films preparation by the pulsed laser deposition method. The targets were irradiated by a frequency-doubled Nd:glass laser with a pulse duration of 250 fs. The films grown on SiO(2) substrates at 600 °C were characterized by X-ray diffraction, scanning electron and atomic force microscopies, and Vickers microhardness technique. The deposited films are composed of WB(4). The intrinsic film hardness, calculated according to the "law-of-mixtures" model, lies in the superhardness region 42-50 GPa.     

A research note: the potential for transfer of Salmonella from irrigation water to tomatoes

The transmission of Salmonella Enteritidis from soil to fruit by contaminated irrigated water was studied using 20 patio tomato plants. In order to track the presence of Salmonella in the soil and plants a luminescent strain transformed with the full luxCDABE gene cassette from Photorhabdus luminescens was used. The tomato plants were irrigated every other day by direct application of water containing Salmonella Enteritidis (10⁵ CFU ml^?1) to the soil. Samples of soil, stem, leaf and fruit were taken weekly and assayed for Salmonella by plating onto Luria Bertani agar containing 50 µg ml^?1 ampicillin. There was a significant difference (P < 0.05) in Salmonella counts from soils sampled during the course of the study. No Salmonella were recovered from the leaf, stem, and fruit samples taken from the tomato plants. This indicates that, under these test conditions, watering with contaminated water directly into the soil does not result in the transmission of Salmonella, and possibly other pathogens, to tomatoes. Copyright © 2004 Society of Chemical Industry     

Harnessing the Role of Foliar Applied Salicylic Acid in Decreasing Chlorophyll Content to Reassess Photosystem II Photoprotection in Crop Plants

Salicylic acid (SA), an essential plant hormone, has received much attention due to its role in modulating the adverse effects of biotic and abiotic stresses, acting as an antioxidant and plant growth regulator. However, its role in photosynthesis under non stress conditions is controversial. By chlorophyll fluorescence imaging analysis, we evaluated the consequences of foliar applied 1 mM SA on photosystem II (PSII) efficiency of tomato (Solanum lycopersicum L.) plants and estimated the reactive oxygen species (ROS) generation. Tomato leaves sprayed with 1 mM SA displayed lower chlorophyll content, but the absorbed light energy was preferentially converted into photochemical energy rather than dissipated as thermal energy by non-photochemical quenching (NPQ), indicating photoprotective effects provided by the foliar applied SA. This decreased NPQ, after 72 h treatment by 1 mM SA, resulted in an increased electron transport rate (ETR). The molecular mechanism by which the absorbed light energy was more efficiently directed to photochemistry in the SA treated leaves was the increased fraction of the open PSII reaction centers (qp), and the increased efficiency of open reaction centers (Fv’/Fm’). SA induced a decrease in chlorophyll content, resulting in a decrease in non-regulated energy dissipated in PSII (Φ_NO) under high light (HL) treatment, suggesting a lower amount of triplet excited state chlorophyll (³Chl*) molecules available to produce singlet oxygen (¹O₂). Yet, the increased efficiency, compared to the control, of the oxygen evolving complex (OEC) on the donor side of PSII, associated with lower formation of hydrogen peroxide (H₂O₂), also contributed to less creation of ROS. We conclude that under non stress conditions, foliar applied SA decreased chlorophyll content and suppressed phototoxicity, offering PSII photoprotection; thus, it can be regarded as a mechanism that reduces photoinhibition and photodamage, improving PSII efficiency in crop plants.     

High-Fat Diet Impairs Mouse Median Eminence: A Study by Transmission and Scanning Electron Microscopy Coupled with Raman Spectroscopy

Hypothalamic dysfunction is an initial event following diet-induced obesity, primarily involving areas regulating energy balance such as arcuate nucleus (Arc) and median eminence (ME). To gain insights into the early hypothalamic diet-induced alterations, adult CD1 mice fed a high-fat diet (HFD) for 6 weeks were studied and compared with normo-fed controls. Transmission and scanning electron microscopy and histological staining were employed for morphological studies of the ME, while Raman spectroscopy was applied for the biochemical analysis of the Arc-ME complex. In HFD mice, ME β2-tanycytes, glial cells dedicated to blood-liquor crosstalk, exhibited remarkable ultrastructural anomalies, including altered alignment, reduced junctions, degenerating organelles, and higher content of lipid droplets, lysosomes, and autophagosomes. Degenerating tanycytes also displayed an electron transparent cytoplasm filled with numerous vesicles, and they were surrounded by dilated extracellular spaces extending up to the subependymal layer. Consistently, Raman spectroscopy analysis of the Arc-ME complex revealed higher glycogen, collagen, and lipid bands in HFD mice compared with controls, and there was also a higher band corresponding to the cyanide group in the former compared to the last. Collectively, these data show that ME β₂-tanycytes exhibit early structural and chemical alterations due to HFD and reveal for the first-time hypothalamic cyanide presence following high dietary lipids consumption, which is a novel aspect with potential implications in the field of obesity.