The respiratory burst and electrolyte leakage induced by sulfhydryl blockers in egeria densa leaves are associated with H2O2 production and are dependent on Ca2+ influx

The defence elicitor AsES causes a rapid and transient membrane depolarization, a triphasic oxidative burst and the accumulation of nitric oxide

The newly characterized elicitor AsES obtained from Acremonium strictum induces a strong defence response in strawberry plants and confers plants resistance against the fungal pathogen Colletotricum acutatum the casual agent of anthracnose disease. Previous studies showed that AsES causes the accumulation of reactive oxygen species (ROS) that peaked 4 h post treatment (hpt), but due to the experimental approach used it was not clear whether the accumulation of ROS observed was intracellular or extracellular or took place as a single peak. By using a different experimental setup, a more complex early events associated to the activation of the innate immunity were observed. In this paper we report that strawberry plant cells treated with AsES exhibits a triphasic production of H2O2 and a rapid intracellular accumulation of NO. The first phase consists in a progressive extracellular accumulation of H2O2 that starts immediately after the treatment with AsES and is preceded by a rapid and transient cell membrane depolarization. During this phase takes place also a rapid intracellular accumulation of NO. Microscopic observations of mesophyll cells treated with AsES reveals that NO accumulates at the chloroplast. After the first extracellular H2O2 production phase, two intracellular H2O2 accumulation events occur, the first 2 hpt, and the second 7 hpt. Cells treated with AsES also show a transient increase of ion leakage, and a progressive alkalinization of the extracellular medium.

Physiological effects of ozone exposure on De Colgar and Rechaiga II tomato (Solanum lycopersicum L.) cultivars

The sensitivity of two tomato (Solanum lycopersicum L.) cultivars, Rechaiga II and De Colgar, to 50, 80, and 100 ppb ozone (O3) exposures was assessed in fumigation chamber, during 4 h per day over a period of 7 days. The Rechaiga II variety was shown to be sensitive to the dose of 50 ppb, showing chlorotic spots on the adaxial leaf surface and alterations of some physiological parameters. During 1-week fumigation, ozone caused a decrease in stomatal conductance, chlorophylls a and b, total chlorophylls, and carotenoids, although soluble sugars and membrane integrity were significantly increased in fumigated plants compared to controls. This trend was similar for the three pollutant doses used in fumigation. The De Colgar tomato remained asymptomatic.

Tropospheric ozone and plants: absorption, responses, and consequences

Ozone is now considered to be the second most important gaseous pollutant in our environment. The phytotoxic potential of O₃ was first observed on grape foliage by B.L. Richards and coworkers in 1958 (Richards et al. 1958). To date, unsustainable resource utilization has turned this secondary pollutant into a major component of global climate change and a prime threat to agricultural production. The projected levels to which O₃ will increase are critically alarming and have become a major issue of concern for agriculturalists, biologists, environmentalists and others plants are soft targets for O₃. Ozone enters plants through stomata, where it disolves in the apoplastic fluid. O₃ has several potential effects on plants: direct reaction with cell membranes; conversion into ROS and H₂O₂ (which alters cellular function by causing cell death); induction of premature senescence; and induction of and up- or down-regulation of responsive components such as genes , proteins and metabolites. In this review we attempt to present an overview picture of plant O₃ interactions. We summarize the vast number of available reports on plant responses to O₃ at the morphological, physiological, cellular, biochemical levels, and address effects on crop yield, and on genes, proteins and metabolites. it is now clear that the machinery of photosynthesis, thereby decreasing the economic yield of most plants and inducing a common morphological symptom, called the "foliar injury". The "foliar injury" symptoms can be authentically utilized for biomonitoring of O₃ under natural conditions. Elevated O₃ stress has been convincingly demonstrated to trigger an antioxidative defense system in plants. The past several years have seen the development and application of high-throughput omics technologies (transcriptomics, proteomics, and metabolomics) that are capable of identifying and prolifiling the O₃-responsive components in model and nonmodel plants. Such studies have been carried out ans have generated an inventory of O₃-Responsive components--a great resource to the scientific community. Recently, it has been shown that certain organic chemicals ans elevated CO₂ levels are effective in ameliorating O₃-generated stress. Both targeted and highthroughput approaches have advanced our knowledge concerning what O₃-triggerred signaling and metabolic pathways exist in plants. Moreover, recently generated information, and several biomarkers for O₃, may, in the future, be exploited to better screen and develop O₃-tolerant plants.

Modification of the biochemical pathways of plants induced by ozone: what are the varied routes to change?

When plants are observed under a low dose of ozone, some physiological and metabolic shifts occur. Barring extreme injury such as tissue damage or stomata closure, most of these disruptive changes are likely to have been initiated at the level of gene expression. The belief is oxidative products formed in ozone exposed leaves, e.g. hydrogen peroxide, are responsible for much of the biochemical adjustments. The first line of defense is a range of antioxidants, such as ascorbate and glutathione, but if this defense is overwhelmed, subsequent actions occur, similar to systemic acquired resistance or general wounding. Yet there are seemingly unrelated metabolic responses which are also triggered, such as early senescence. We discuss here the current understanding of gene control and signal transduction/control in order to increase our comprehension of how ozone alters the basic metabolism of plants and how plants counteract or cope with ozone.

The role of aquaporins and membrane damage in chilling and hydrogen peroxide induced changes in the hydraulic conductance of maize roots

When chilling-sensitive plants are chilled, root hydraulic conductance (L(o)) declines precipitously; L(o) also declines in chilling-tolerant plants, but it subsequently recovers, whereas in chilling-sensitive plants it does not. As a result, the chilling-sensitive plants dry out and may die. Using a chilling-sensitive and a chilling-tolerant maize genotype we investigated the effect of chilling on L(o), and its relationship to osmotic water permeability of isolated root cortex protoplasts, aquaporin gene expression, aquaporin abundance, and aquaporin phosphorylation, hydrogen peroxide (H(2)O(2)) accumulation in the roots and electrolyte leakage from the roots. Because chilling can cause H(2)O(2) accumulation we also determined the effects of a short H(2)O(2) treatment of the roots and examined the same parameters. We conclude from these studies that the recovery of L(o) during chilling in the chilling-tolerant genotype is made possible by avoiding or repairing membrane damage and by a greater abundance and/or activity of aquaporins. The same changes in aquaporins take place in the chilling-sensitive genotype, but we postulate that membrane damage prevents the L(o) recovery. It appears that the aquaporin response is necessary but not sufficient to respond to chilling injury. The plant must also be able to avoid the oxidative damage that accompanies chilling.

Calcium signaling during abiotic stress in plants

Plants experience a wide array of environmental stimuli, not all of which are favorable, and, unlike animals, are unable to move away from stressful environments. They therefore require a mechanism with which to recognize and respond to abiotic stresses of many different types. Frequently this mechanism involves intracellular calcium. Stress-induced changes in the cytosolic concentration of Ca2+ ([Ca2+]cyt) occur as a result of influx of Ca2+ from outside the cell, or release of Ca2+ from intracellular stores. These alterations in [Ca2+]cyt constitute a signal that is transduced via calmodulin, calcium-dependent protein kinases, and other Ca(2+)-controlled proteins to effect a wide array of downstream responses involved in the protection of the plant and adjustment to the new environmental conditions. Ca2+ signaling has been implicated in plant responses to a number of abiotic stresses including low temperature, osmotic stress, heat, oxidative stress, anoxia, and mechanical perturbation, which are reviewed in this article.

In vivo characterization of a thioredoxin h target protein defines a new peroxiredoxin family

Disruption of the two thioredoxin genes in yeast dramatically affects cell viability and growth. Expression of Arabidopsis thioredoxin AtTRX3 in the Saccharomyces thioredoxin Delta strain EMY63 restores a wild-type cell cycle, the ability to grow on methionine sulfoxide, and H2O2 tolerance. In order to isolate thioredoxin targets related to these phenotypes, we prepared a C35S (Escherichia coli numbering) thioredoxin mutant to stabilize the intermediate disulfide bridged complex and we added a polyhistidine N-terminal extension in order to purify the complex rapidly. Expression of this mutant thioredoxin in the wild-type yeast induces a reduced tolerance to H2O2, but only limited change in the cell cycle and no change in methionine sulfoxide utilization. Expression in the Delta thioredoxin strain EMY63 allowed us to isolate a complex of the thioredoxin with YLR109, an abundant yeast protein related to PMP20, a peroxisomal protein of Candida. No function has so far been attributed to this protein or to the other numerous homologues described in plants, animals, fungi, and prokaryotes. On the basis of the complementation and of low similarity with peroxiredoxins, we produced YLR109 and one of its Arabidopsis homologues in E. coli to test their peroxiredoxins activity. We demonstrate that both recombinant proteins present a thioredoxin-dependent peroxidase activity in vitro. The possible functions of this new peroxiredoxin family are discussed.