Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of aerobic metabolism. Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Whereas plants are surfeited with mechanisms to combat increased ROS levels during abiotic stress conditions, in other circumstances plants appear to purposefully generate ROS as signaling molecules to control various processes including pathogen defense, programmed cell death, and stomatal behavior. This review describes the mechanisms of ROS generation and removal in plants during development and under biotic and abiotic stress conditions. New insights into the complexity and roles that ROS play in plants have come from genetic analyses of ROS detoxifying and signaling mutants. Considering recent ROS-induced genome-wide expression analyses, the possible functions and mechanisms for ROS sensing and signaling in plants are compared with those in animals and yeast.

REACTIVE OXYGEN SPECIES: Metabolism, Oxidative Stress, and Signal Transduction

When considering new sensory technologies one should look to nature for guidance. Indeed, living organisms have developed the ultimate chemical sensors. Many insects can detect chemical signals with perfect specificity and incredible sensitivity. Mammalian olfaction is based on an array of less discriminating sensors and a memorized response pattern to identify a unique odor. It is important to recognize that the extraordinary sensory performance of biological systems does not originate from a single element. In actuality, their performance is derived from a completely interactive system wherein the receptor is served by analyte delivery and removal mechanisms, selectivity is derived from receptors, and sensitivity is the result of analyte-triggered biochemical cascades. Clearly, optimal artificial sensory sys-

Conjugated polymer-based chemical sensors.

In 1998 we updated earlier descriptions of the largest family of secondary transport carriers found in living organisms, the major facilitator superfamily (MFS). Seventeen families of transport proteins were shown to comprise this superfamily. We here report expansion of the MFS to include 29 established families as well as five probable families. Structural, functional, and mechanistic features of the constituent permeases are described, and each newly identified family is shown to exhibit specificity for a single class of substrates. Phylogenetic analyses define the evolutionary relationships of the members of each family to each other, and multiple alignments allow definition of family-specific signature sequences as well as all wellconserved sequence motifs. The work described serves to update previous publications and allows extrapolation of structural, functional and mechanistic information obtained with any one member of the superfamily to other members with limitations determined by the degrees of sequence divergence.

The major facilitator superfamily.

Recent advancements in morphology control and surface functionalization of mesoporous silica nanoparticles (MSNs) have enhanced the biocompatibility of these materials with high surface areas and pore volumes. Several recent reports have demonstrated that the MSNs can be efficiently internalized by animal and plant cells. The functionalization of MSNs with organic moieties or other nanostructures brings controlled release and molecular recognition capabilities to these mesoporous materials for drug/gene delivery and sensing applications, respectively. Herein, we review recent research progress on the design of functional MSN materials with various mechanisms of controlled release, along with the ability to achieve zero release in the absence of stimuli, and the introduction of new characteristics to enable the use of nonselective molecules as screens for the construction of highly selective sensor systems.

Mesoporous silica nanoparticles for drug delivery and biosensing applications

Summary: High-affinity iron acquisition is mediated by siderophore-dependent pathways in the majority of pathogenic and nonpathogenic bacteria and fungi. Considerable progress has been made in characterizing and understanding mechanisms of siderophore synthesis, secretion, iron scavenging, and siderophore-delivered iron uptake and its release. The regulation of siderophore pathways reveals multilayer networks at the transcriptional and posttranscriptional levels. Due to the key role of many siderophores during virulence, coevolution led to sophisticated strategies of siderophore neutralization by mammals and (re)utilization by bacterial pathogens. Surprisingly, hosts also developed essential siderophore-based iron delivery and cell conversion pathways, which are of interest for diagnostic and therapeutic studies. In the last decades, natural and synthetic compounds have gained attention as potential therapeutics for iron-dependent treatment of infections and further diseases. Promising results for pathogen inhibition were obtained with various siderophore-antibiotic conjugates acting as “Trojan horse” toxins and siderophore pathway inhibitors. In this article, general aspects of siderophore-mediated iron acquisition, recent findings regarding iron-related pathogen-host interactions, and current strategies for iron-dependent pathogen control will be reviewed. Further concepts including the inhibition of novel siderophore pathway targets are discussed.

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Siderophore-Based Iron Acquisition and Pathogen Control

Diphenyl ether herbicides induce an accumulation of protoporphyrin IX in plant tissues. By analogy to human porphyria, the accumulation could be attributed to decreased (Mg or Fe)-chelatase or protoporphyrinogen oxidase activities. Possible effects of acifluorfen-methyl on these enzymes were investigated in isolated corn (maize, Zea mays) etioplasts, potato (Solanum tuberosum) and mouse mitochondria, and yeast mitochondrial membranes. Acifluorfen-methyl was strongly inhibitory to protoporphyrinogen oxidase activities whatever their origins [concn. causing 50% inhibition (IC50) = 4 nM for the corn etioplast enzyme]. By contrast, it was roughly 100,000 times less active on (Mg or Fe)-chelatase activities (IC50 = 80-100 microM). Our results lead us to propose protoporphyrinogen oxidase as a cellular target for diphenyl ether herbicides.

Protoporphyrinogen oxidase as a molecular target for diphenyl ether herbicides.

Publisher Summary This chapter describes two methods: mechanical cell rupture and cell wall digestion by snail gut juice. It provides an account of new modifications of the two methods, the advantages—greater simplicity and shorter manipulation time. Both, the mechanical and the enzymatic methods have specific advantages and drawbacks. The main advantage of the mechanical method is speed; this is of critical importance in certain cases, for example, when a long physiological study of freshly prepared mitochondria is planned. The preparation of mitochondria by the mechanical methods takes only two hours as compared to about six hours for methods involving enzymatic cell wall digestion. The main advantage of the latter over the mechanical method is that mitochondria of higher integrity are obtained. The respiratory control ratios are higher for mitochondria obtained by the enzymatic method, although the mechanical method yields similar phosphate/oxygen (P/O) ratios and properly oriented mitochondrial membranes. Representative electron micrographs for the enzymatic and mechanical methods are illustrated in the chapter.

Preparation of yeast mitochondria (Saccharomyces cerevisiae) with good P/O and respiratory control ratios.

Summary: Uptake of iron from various siderophores by a Δfet3Δfet4 strain of Saccharomyces cerevisiae was investigated. The catecholate enterobactin and the hydroxamate coprogen were taken up by the cells by passive diffusion, whereas the hydroxamates ferrioxamine B (FOB) and ferricrocin (FC) were taken up via a high-affinity energy-dependent mechanism. The kinetics of FOB and FC uptake showed reciprocal competitive inhibition. The transport was regulated by iron availability, but was independent of the Aft1p and Mac1p transcriptional activators. Mutants affected in the transport of FOB were isolated. The transport of FC was not impaired in these mutants. Functional complementation of one mutant allowed the identification of the SIT1 gene (Siderophore iron Transport) encoding a putative permease belonging to the major facilitator superfamily. The Sit1 protein is probably a permease specific for the transport of ferrioxamine-type siderophores. The evidence suggests that the uptake of ferrichrome-type siderophores like FC involves other specific permease(s), although there seems to be a common handling of FOB and FC following their internalization by the cell.

Siderophore-mediated iron uptake in Saccharomyces cerevisiae: the SIT1 gene encodes a ferrioxamine B permease that belongs to the major facilitator superfamily.

Pierre Labbe

Papers

Protoporphyrinogen oxidase as a molecular target for diphenyl ether herbicides.

Preparation of yeast mitochondria (Saccharomyces cerevisiae) with good P/O and respiratory control ratios.

Siderophore-mediated iron uptake in Saccharomyces cerevisiae: the SIT1 gene encodes a ferrioxamine B permease that belongs to the major facilitator superfamily.

Localization within chloroplasts of protoporphyrinogen oxidase, the target enzyme for diphenylether-like herbicides.

Evidence for the Saccharomyces cerevisiae Ferrireductase System Being a Multicomponent Electron Transport Chain