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Biomedical subjects

Ron Mittler

Publications and source records attributed to Ron Mittler.

At least 19 recordsLinked to original sources
PubMed2006

Gain- and loss-of-function mutations in Zat10 enhance the tolerance of plants to abiotic stress.

C(2)H(2)-zinc finger proteins that contain the EAR repressor domain are thought to play a key role in modulating the defense response of plants to abiotic stress. Constitutive expression of the C(2)H(2)-EAR zinc finger protein Zat10 in Arabidopsis was found to elevate the expression of reactive oxygen-defense transcripts and to enhance the tolerance of plants to salinity, heat and osmotic stress. Surprisingly, knockout and RNAi mutants of Zat10 were also more tolerant to osmotic and salinity stress. Our results suggest that Zat10 plays a key role as both a positive and a negative regulator of plant defenses.

Abscisic Acid↗
PubMed2006

Could heat shock transcription factors function as hydrogen peroxide sensors in plants?

BACKGROUND: Heat shock transcription factors (Hsfs) are modular transcription factors encoded by a large gene family in plants. They bind to the consensus sequence 'nGAAnnTCCn' found in the promoters of many defence genes, and are thought to function as a highly redundant and flexible gene network that controls the response of plants to different environmental stress conditions, including biotic and abiotic stresses. Hsf proteins encoded by different genes exhibit a high degree of complexity in their interactions. They can potentially bind and activate their own promoters, as well as the promoters of other members of their gene family, and they can form homo- or heterotrimers resulting in altered nuclear localization, as well as enhanced or suppressed transcription. SCOPE: In this review, we summarize recent studies on Hsf function in Arabidopsis and tomato and present evidence obtained from microarray expression studies in Arabidopsis that the Hsf gene network is highly flexible and specialized, with specific members and/or member combinations controlling the response of plants to particular stress conditions. In addition, we describe recent studies that support the hypothesis that certain Hsfs function as molecular sensors that directly sense reactive oxygen species (ROS) and control the expression of oxidative stress response genes during oxidative stress.

Arabidopsis↗
PubMed2006

Transcriptomic footprints disclose specificity of reactive oxygen species signaling in Arabidopsis.

Reactive oxygen species (ROS) are key players in the regulation of plant development, stress responses, and programmed cell death. Previous studies indicated that depending on the type of ROS (hydrogen peroxide, superoxide, or singlet oxygen) or its subcellular production site (plastidic, cytosolic, peroxisomal, or apoplastic), a different physiological, biochemical, and molecular response is provoked. We used transcriptome data generated from ROS-related microarray experiments to assess the specificity of ROS-driven transcript expression. Data sets obtained by exogenous application of oxidative stress-causing agents (methyl viologen, Alternaria alternata toxin, 3-aminotriazole, and ozone) and from a mutant (fluorescent) and transgenic plants, in which the activity of an individual antioxidant enzyme was perturbed (catalase, cytosolic ascorbate peroxidase, and copper/zinc superoxide dismutase), were compared. In total, the abundance of nearly 26,000 transcripts of Arabidopsis (Arabidopsis thaliana) was monitored in response to different ROS. Overall, 8,056, 5,312, and 3,925 transcripts showed at least a 3-, 4-, or 5-fold change in expression, respectively. In addition to marker transcripts that were specifically regulated by hydrogen peroxide, superoxide, or singlet oxygen, several transcripts were identified as general oxidative stress response markers because their steady-state levels were at least 5-fold elevated in most experiments. We also assessed the expression characteristics of all annotated transcription factors and inferred new candidate regulatory transcripts that could be responsible for orchestrating the specific transcriptomic signatures triggered by different ROS. Our analysis provides a framework that will assist future efforts to address the impact of ROS signals within environmental stress conditions and elucidate the molecular mechanisms of the oxidative stress response in plants.

Arabidopsis↗
PubMed2006

What makes species unique? The contribution of proteins with obscure features.

BACKGROUND: Proteins with obscure features (POFs), which lack currently defined motifs or domains, represent between 18% and 38% of a typical eukaryotic proteome. To evaluate the contribution of this class of proteins to the diversity of eukaryotes, we performed a comparative analysis of the predicted proteomes derived from 10 different sequenced genomes, including budding and fission yeast, worm, fly, mosquito, Arabidopsis, rice, mouse, rat, and human. RESULTS: Only 1,650 protein groups were found to be conserved among these proteomes (BLAST E-value threshold of 10(-6)). Of these, only three were designated as POFs. Surprisingly, we found that, on average, 60% of the POFs identified in these 10 proteomes (44,236 in total) were species specific. In contrast, only 7.5% of the proteins with defined features (PDFs) were species specific (17,554 in total). As a group, POFs appear similar to PDFs in their relative contribution to biological functions, as indicated by their expression, participation in protein-protein interactions and association with mutant phenotypes. However, POF have more predicted disordered structure than PDFs, implying that they may exhibit preferential involvement in species-specific regulatory and signaling networks. CONCLUSION: Because the majority of eukaryotic POFs are not well conserved, and by definition do not have defined domains or motifs upon which to formulate a functional working hypothesis, understanding their biochemical and biological functions will require species-specific investigations.

Animals↗
PubMed2005

Abiotic stress, the field environment and stress combination.

Farmers and breeders have long known that often it is the simultaneous occurrence of several abiotic stresses, rather than a particular stress condition, that is most lethal to crops. Surprisingly, the co-occurrence of different stresses is rarely addressed by molecular biologists that study plant acclimation. Recent studies have revealed that the response of plants to a combination of two different abiotic stresses is unique and cannot be directly extrapolated from the response of plants to each of the different stresses applied individually. Tolerance to a combination of different stress conditions, particularly those that mimic the field environment, should be the focus of future research programs aimed at developing transgenic crops and plants with enhanced tolerance to naturally occurring environmental conditions.

Crops, Agricultural↗
PubMed2005

Enhanced tolerance to environmental stress in transgenic plants expressing the transcriptional coactivator multiprotein bridging factor 1c.

Abiotic stresses cause extensive losses to agricultural production worldwide. Acclimation of plants to abiotic conditions such as drought, salinity, or heat is mediated by a complex network of transcription factors and other regulatory genes that control multiple defense enzymes, proteins, and pathways. Associated with the activity of different transcription factors are transcriptional coactivators that enhance their binding to the basal transcription machinery. Although the importance of stress-response transcription factors was demonstrated in transgenic plants, little is known about the function of transcriptional coactivators associated with abiotic stresses. Here, we report that constitutive expression of the stress-response transcriptional coactivator multiprotein bridging factor 1c (MBF1c) in Arabidopsis (Arabidopsis thaliana) enhances the tolerance of transgenic plants to bacterial infection, heat, and osmotic stress. Moreover, the enhanced tolerance of transgenic plants to osmotic and heat stress was maintained even when these two stresses were combined. The expression of MBF1c in transgenic plants augmented the accumulation of a number of defense transcripts in response to heat stress. Transcriptome profiling and inhibitor studies suggest that MBF1c expression enhances the tolerance of transgenic plants to heat and osmotic stress by partially activating, or perturbing, the ethylene-response signal transduction pathway. Present findings suggest that MBF1 proteins could be used to enhance the tolerance of plants to different abiotic stresses.

Acclimatization↗
PubMed2005

The zinc-finger protein Zat12 plays a central role in reactive oxygen and abiotic stress signaling in Arabidopsis.

Plant acclimation to environmental stress is controlled by a complex network of regulatory genes that compose distinct stress-response regulons. In contrast to many signaling and regulatory genes that are stress specific, the zinc-finger protein Zat12 responds to a large number of biotic and abiotic stresses. Zat12 is thought to be involved in cold and oxidative stress signaling in Arabidopsis (Arabidopsis thaliana); however, its mode of action and regulation are largely unknown. Using a fusion between the Zat12 promoter and the reporter gene luciferase, we demonstrate that Zat12 expression is activated at the transcriptional level during different abiotic stresses and in response to a wound-induced systemic signal. Using Zat12 gain- and loss-of-function lines, we assign a function for Zat12 during oxidative, osmotic, salinity, high light, and heat stresses. Transcriptional profiling of Zat12-overexpressing plants and wild-type plants subjected to H(2)O(2) stress revealed that constitutive expression of Zat12 in Arabidopsis results in the enhanced expression of oxidative- and light stress-response transcripts. Under specific growth conditions, Zat12 may therefore regulate a collection of transcripts involved in the response of Arabidopsis to high light and oxidative stress. Our results suggest that Zat12 plays a central role in reactive oxygen and abiotic stress signaling in Arabidopsis.

Arabidopsis↗
PubMed2004

Cytosolic ascorbate peroxidase 1 is a central component of the reactive oxygen gene network of Arabidopsis.

Reactive oxygen species (ROS), such as O2- and H2O2, play a key role in plant metabolism, cellular signaling, and defense. In leaf cells, the chloroplast is considered to be a focal point of ROS metabolism. It is a major producer of O2- and H2O2 during photosynthesis, and it contains a large array of ROS-scavenging mechanisms that have been extensively studied. By contrast, the function of the cytosolic ROS-scavenging mechanisms of leaf cells is largely unknown. In this study, we demonstrate that in the absence of the cytosolic H2O2-scavenging enzyme ascorbate peroxidase 1 (APX1), the entire chloroplastic H2O2-scavenging system of Arabidopsis thaliana collapses, H2O2 levels increase, and protein oxidation occurs. We further identify specific proteins oxidized in APX1-deficient plants and characterize the signaling events that ensue in knockout-Apx1 plants in response to a moderate level of light stress. Using a dominant-negative approach, we demonstrate that heat shock transcription factors play a central role in the early sensing of H2O2 stress in plants. Using knockout plants for the NADPH oxidase D protein (knockout-RbohD), we demonstrate that RbohD might be required for ROS signal amplification during light stress. Our study points to a key role for the cytosol in protecting the chloroplast during light stress and provides evidence for cross-compartment protection of thylakoid and stromal/mitochondrial APXs by cytosolic APX1.

Arabidopsis↗
PubMed2004

When defense pathways collide. The response of Arabidopsis to a combination of drought and heat stress.

Within their natural habitat, plants are subjected to a combination of abiotic conditions that include stresses such as drought and heat. Drought and heat stress have been extensively studied; however, little is known about how their combination impacts plants. The response of Arabidopsis plants to a combination of drought and heat stress was found to be distinct from that of plants subjected to drought or heat stress. Transcriptome analysis of Arabidopsis plants subjected to a combination of drought and heat stress revealed a new pattern of defense response in plants that includes a partial combination of two multigene defense pathways (i.e. drought and heat stress), as well as 454 transcripts that are specifically expressed in plants during a combination of drought and heat stress. Metabolic profiling of plants subjected to drought, heat stress, or a combination of drought and heat stress revealed that plants subject to a combination of drought and heat stress accumulated sucrose and other sugars such as maltose and glucose. In contrast, Pro that accumulated in plants subjected to drought did not accumulate in plants during a combination of drought and heat stress. Heat stress was found to ameliorate the toxicity of Pro to cells, suggesting that during a combination of drought and heat stress sucrose replaces Pro in plants as the major osmoprotectant. Our results highlight the plasticity of the plant genome and demonstrate its ability to respond to complex environmental conditions that occur in the field.

Acclimatization↗
PubMed2004

The zinc finger protein Zat12 is required for cytosolic ascorbate peroxidase 1 expression during oxidative stress in Arabidopsis.

Cytosolic ascorbate peroxidase 1 (Apx1) is a key H(2)O(2) removal enzyme in plants. Microarray analysis of Apx1-deficient Arabidopsis plants revealed that the expression of two zinc finger proteins (Zat12 and Zat7) and a WRKY transcription factor (WRKY25) is elevated in knock-out Apx1 plants grown under controlled conditions. Because mutants lacking Apx1 accumulate H(2)O(2), we examined the correlation between H(2)O(2) and the expression of Zat12, Zat7, WRKY25, and Apx1. The expression of Zat12, Zat7, and WRKY25 was simultaneously elevated in cells in response to oxidative stress (i.e. H(2)O(2) or paraquat application), heat shock, or wounding. In contrast, light or osmotic stress did not enhance the expression of these putative transcription factors. All stresses tested enhanced the expression of Apx1. Transgenic plants expressing Zat12 or Zat7 could tolerate oxidative stress. In contrast, transgenic plants expressing WRKY25 could not. Although the expression of Zat12, Zat7, or WRKY25 in transgenic plants did not enhance the expression of Apx1 under controlled conditions, Zat12-deficient plants were unable to enhance the expression of Apx1, Zat7, or WRKY25 in response to H(2)O(2) or paraquat application. Zat12-deficient plants were also more sensitive than wild type plants to H(2)O(2) application as revealed by a higher level of H(2)O(2)-induced protein oxidation detected in these plants by protein blots. Our results suggest that Zat12 is an important component of the oxidative stress response signal transduction network of Arabidopsis required for Zat7, WRKY25, and Apx1 expression during oxidative stress.

Arabidopsis↗
PubMed2004

Measuring programmed cell death in plants.

Methods for the detection of programmed cell death (PCD) in plants are reviewed with references for different biochemical, microscopic, and molecular assays. A detailed description of three different methods for the detection of biotic or abiotic PCD in plant tissues is included. The reader is encouraged to use all three methods in parallel to obtain a reliable measure of PCD. Critical considerations are highlighted.

Apoptosis↗
PubMed2003

The water-water cycle is essential for chloroplast protection in the absence of stress.

Maintaining electron flow through the photosynthetic apparatus, even in the absence of a sufficient amount of NADP+ as an electron acceptor, is essential for chloroplast protection from photooxidative stress. At least two different pathways are thought to participate in this process, i.e. cyclic electron flow and the water-water cycle. Although the function of the water-water cycle was inferred from a number of biochemical and physiological studies, genetic evidence for the function of this cycle is very limited. Here we show that knockdown Arabidopsis plants with suppressed expression of the key water-water cycle enzyme, thylakoid-attached copper/zinc superoxide dismutase (KD-SOD), are suppressed in their growth and development. Chloroplast size, chlorophyll content, and photosynthetic activity were also reduced in KD-SOD plants. Microarray analysis of KD-SOD plants, grown under controlled conditions, revealed changes in transcript expression consistent with an acclimation response to light stress. Although a number of transcripts involved in the defense of plants from oxidative stress were induced in KD-SOD plants, and seedlings of KD-SOD plants were more tolerant to oxidative stress, these mechanisms were unable to compensate for the suppression of the water-water cycle in mature leaves. Thus, the localization of copper/zinc superoxide dismutase at the vicinity of photosystem I may be essential for its function. Our studies provide genetic evidence for the importance of the water-water cycle in protecting the photosynthetic apparatus of higher plants from photooxidative damage.

Arabidopsis↗
PubMed2003

Growth suppression, altered stomatal responses, and augmented induction of heat shock proteins in cytosolic ascorbate peroxidase (Apx1)-deficient Arabidopsis plants.

The accumulation of hydrogen peroxide (H2O2) in plants is typically associated with biotic or abiotic stresses. However, H2O2 is continuously produced in cells during normal metabolism. Yet, little is known about how H2O2 accumulation will affect plant metabolism in the absence of pathogens or abiotic stress. Here, we report that a deficiency in the H2O2-scavenging enzyme, cytosolic ascorbate peroxidase (APX1), results in the accumulation of H2O2 in Arabidopsis plants grown under optimal conditions. Knockout-Apx1 plants were characterized by suppressed growth and development, altered stomatal responses, and augmented induction of heat shock proteins during light stress. The inactivation of Apx1 resulted in the induction of several transcripts encoding signal transduction proteins. These were not previously linked to H2O2 signaling during stress and may belong to a signal transduction pathway specifically involved in H2O2 sensing during normal metabolism. Surprisingly, the expression of transcripts encoding H2O2 scavenging enzymes, such as catalase or glutathione peroxidase, was not elevated in knockout-Apx1 plants. The expression of catalase, two typical plant peroxidases, and several different heat shock proteins was however elevated in knockout-Apx1 plants during light stress. Our results demonstrate that in planta accumulation of H2O2 can suppress plant growth and development, interfere with different physiological processes, and enhance the response of plants to abiotic stress conditions. Our findings also suggest that at least part of the induction of heat shock proteins during light stress in Arabidopsis is mediated by H2O2 that is scavenged by APX1.

Arabidopsis↗
PubMed2003

Enhanced photosynthesis and growth of transgenic plants that express ictB, a gene involved in HCO3- accumulation in cyanobacteria.

Transgenic Arabidopsis thaliana and Nicotiana tabacum plants that express ictB, a gene involved in HCO3- accumulation within the cyanobacterium Synechococcus sp. PCC 7942, exhibited significantly faster photosynthetic rates than the wild-types under limiting but not under saturating CO2 concentrations. Under conditions of low relative humidity, growth of the transgenic A. thaliana plants was considerably faster than the wild-type. This enhancement of growth was not observed under humid conditions. There was no difference in the amount of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) detected in the wild-types and their respective transgenic plants. Following activation in vitro, the activities of RubisCO from either low- or high-humidity-grown transgenic plants were similar to those observed in the wild-types. In contrast, the in vivo RubisCO activity, i.e. without prior activation, in plants grown under low humidity was considerably higher in ictB-expressing plants than in their wild-types. The CO2 compensation point in the transgenic plants that express ictB was lower than in the wild-types, suggesting that the concentration of CO2 in close proximity to RubisCO was higher. This may explain the higher activation level of RubisCO and enhanced photosynthetic activities and growth in the transgenic plants. These data indicated a potential use of ictB for the stimulation of crop yield.

Journal Article↗
PubMed2002

Oxidative stress, antioxidants and stress tolerance.

Traditionally, reactive oxygen intermediates (ROIs) were considered to be toxic by-products of aerobic metabolism, which were disposed of using antioxidants. However, in recent years, it has become apparent that plants actively produce ROIs as signaling molecules to control processes such as programmed cell death, abiotic stress responses, pathogen defense and systemic signaling. Recent advances including microarray studies and the development of mutants with altered ROI-scavenging mechanisms provide new insights into how the steady-state level of ROIs are controlled in cells. In addition, key steps of the signal transduction pathway that senses ROIs in plants have been identified. These raise several intriguing questions about the relationships between ROI signaling, ROI stress and the production and scavenging of ROIs in the different cellular compartments.

Adaptation, Physiological↗
PubMed2002

Reactive oxygen intermediates and glutathione regulate the expression of cytosolic ascorbate peroxidase during iron-mediated oxidative stress in bean.

Excess of free iron is thought to harm plant cells by enhancing the intracellular production of reactive oxygen intermediates (ROI). Cytosolic ascorbate peroxidase (cAPX) is an iron-containing, ROI-detoxifying enzyme induced in response to iron overload or oxidative stress. We studied the expression of cAPX in leaves of de-rooted bean plants in response to iron overload. cAPX expression, i.e., mRNA and protein, was rapidly induced in response to iron overload. This induction correlated with the increase in iron content in leaves and occurred in the light as well as in the dark. Reduced glutathione (GSH), which plays an important role in activating the ROI signal transduction pathway as well as in ROI detoxification, was found to enhance the induction of APX mRNA by iron. To determine whether cAPX induction during iron overload was due to an increase in the amount of free iron, which serves as a co-factor for cAPX synthesis, or due to iron-mediated increase in ROI production, we tested the expression of APX in leaves under low oxygen pressure. This treatment, which suppresses the formation of ROI, completely abolished the induction of cAPX mRNA during iron overload, without affecting the rate of iron uptake by plants. Taken together, our results suggest that high intracellular levels of free iron in plants lead to the enhanced production of ROI, which in turn induces the expression of cAPX, possibly using GSH as an intermediate signal. We further show, using cAPX-antisense transgenic plants, that cAPX expression is essential to prevent iron-mediated tissue damage in tobacco.

Ascorbate Peroxidases↗