Barbara Karpinska

Photosynthetic electron transport regulates the expression of cytosolic ascorbate peroxidase genes in Arabidopsis during excess light stress.

Exposure of Arabidopsis plants that were maintained under low light (200 mumol of photons m-2 sec-1) to excess light (2000 mumol of photons m-2 sec-1) for 1 hr caused reversible photoinhibition of photosynthesis. Measurements of photosynthetic parameters and the use of electron transport inhibitors indicated that a novel signal transduction pathway was initiated at plastoquinone and regulated, at least in part, by the redox status of the plastoquinone pool. This signal, which preceded the photooxidative burst of hydrogen peroxide (H2O2) associated with photoinhibition of photosynthesis, resulted in a rapid increase (within 15 min) in mRNA levels of two cytosolic ascorbate peroxidase genes (APX1 and APX2). Treatment of leaves with exogenous reduced glutathione abolished this signal, suggesting that glutathione or the redox status of the glutathione pool has a regulatory impact on this signaling pathway. During recovery from photooxidative stress, transcripts for cytosolic glutathione reductase (GOR2) increased, emphasizing the role of glutathione in this stress.

Chloroplast Signaling and LESION SIMULATING DISEASE1 Regulate Crosstalk between Light Acclimation and Immunity in Arabidopsis

Plants are simultaneously exposed to abiotic and biotic hazards. Here, we show that local and systemic acclimation in Arabidopsis thaliana leaves in response to excess excitation energy (EEE) is associated with cell death and is regulated by specific redox changes of the plastoquinone (PQ) pool. These redox changes cause a rapid decrease of stomatal conductance, global induction of ASCORBATE PEROXIDASE2 and PATHOGEN RESISTANCE1, and increased production of reactive oxygen species (ROS) and ethylene that signals through ETHYLENE INSENSITIVE2 (EIN2). We provide evidence that multiple hormonal/ROS signaling pathways regulate the plants response to EEE and that EEE stimulates systemic acquired resistance and basal defenses to virulent biotrophic bacteria. In the Arabidopsis LESION SIMULATING DISEASE1 (lsd1) null mutant that is deregulated for EEE acclimation responses, propagation of EEE-induced programmed cell death depends on the plant defense regulators ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and PHYTOALEXIN DEFICIENT4 (PAD4). We find that EDS1 and PAD4 operate upstream of ethylene and ROS production in the EEE response. The data suggest that the balanced activities of LSD1, EDS1, PAD4, and EIN2 regulate signaling of programmed cell death, light acclimation, and holistic defense responses that are initiated, at least in part, by redox changes of the PQ pool.

Light perception in plant disease defence signalling

Light is a predominant factor in the control of plant growth, development and stress responses. Many biotic stress responses in plants are therefore specifically adjusted by the prevailing light conditions. The plant cell is equipped with sophisticated light-sensing mechanisms that are localised inside and outside of the chloroplast and the nucleus. Recent progress has provided models of how the signalling pathways that are involved in light perception and in defence could operate and interact to form a plant defence network. Such a signalling network includes systems to sense light and regulate gene expression. Photo-produced H(2)O(2) and other reactive oxygen species in the cell also play an essential role in this regulatory network, controlling biotic and abiotic stress responses.

Transcriptional regulation of the CRK/DUF26 group of Receptor-like protein kinases by ozone and plant hormones in Arabidopsis

BackgroundPlant Receptor-like/Pelle kinases (RLK) are a group of conserved signalling components that regulate developmental programs and responses to biotic and abiotic stresses. One of the largest RLK groups is formed by the Domain of Unknown Function 26 (DUF26) RLKs, also called Cysteine-rich Receptor-like Kinases (CRKs), which have been suggested to play important roles in the regulation of pathogen defence and programmed cell death. Despite the vast number of RLKs present in plants, however, only a few of them have been functionally characterized.ResultsWe examined the transcriptional regulation of all Arabidopsis CRKs by ozone (O3), high light and pathogen/elicitor treatment - conditions known to induce the production of reactive oxygen species (ROS) in various subcellular compartments. Several CRKs were transcriptionally induced by exposure to O3 but not by light stress. O3 induces an extracellular oxidative burst, whilst light stress leads to ROS production in chloroplasts. Analysis of publicly available microarray data revealed that the transcriptional responses of the CRKs to O3 were very similar to responses to microbes or pathogen-associated molecular patterns (PAMPs). Several mutants altered in hormone biosynthesis or signalling showed changes in basal and O3-induced transcriptional responses.ConclusionsCombining expression analysis from multiple treatments with mutants altered in hormone biosynthesis or signalling suggest a model in which O3 and salicylic acid (SA) activate separate signaling pathways that exhibit negative crosstalk. Although O3 is classified as an abiotic stress to plants, transcriptional profiling of CRKs showed strong similarities between the O3 and biotic stress responses.

MYB transcription factors are differentially expressed and regulated during secondary vascular tissue development in hybrid aspen

More than 120,000 poplar ESTs have been sequenced from 20 different cDNA libraries by the Swedish Centre for Tree Functional Genomics. We screened this EST collection for MYB transcription factors involved in secondary vascular tissue formation, and genes assigned as PttMYB3Ra, PttMYB4a and PttMYB21a were selected for further characterisation. Three MYB genes showed different expression patterns in various organs, tissues and stem sub-sections representing different developmental stages of vascular tissue formation. Furthermore, the analysis showed that PttMYB21a expression was much higher in secondary cell wall formation zone of xylem and phloem fibers than in other developmental zones. Transgenic hybrid aspen plants, expressing the 3′-part of the PttMYB21a gene in antisense orientation were generated to assess the function of PttMYB21a gene in vascular tissue formation and lignification. All transgenic lines showed reduced growth and had fewer internodes compared to the wild-type. The analysis of selected lines showed that acid soluble lignin present in the bark was higher in transgenic lines as compared to wild-type plants. Moreover a higher transcript level of caffeoyl-CoA 3-O-methyltransferase [CCoAOMT]; EC 2.1.1.104) was found in the phloem of the transgenic plants, suggesting that PttMYB21a might function as a transcriptional repressor.

Antagonistic Effects of Hydrogen Peroxide and Glutathione on Acclimation to Excess Excitation Energy in Arabidopsis

The redox status of the quinone B (QB) and plastoquinone (PQ) pools plays a key role in the cellular and systemic signalling processes that control acclimatory responses in plants. In this study, we demonstrate the effects of hydrogen peroxide and glutathione on acclimatory responses controlled by redox events in the proximity of the QB‐PQ pools. Our results suggest that the chloroplast is a sink for H2O2 and that, paradoxically, high concentrations of H2O2 in the chloroplast protect the photosynthetic apparatus and the plant cell from photoinhibition and photooxidative damage. Excess glutathione, however, caused an effect antagonistic to that observed for high H2O2. An explanation of this apparent paradox and a hypothetical redox‐signalling model are suggested.

Redox control of oxidative stress responses in the C3-CAM intermediate plant Mesembryanthemum crystallinum

Abstract Crassulacean acid metabolism (CAM) is named after the Crassulaceae family of succulent plants, in which this type of metabolism was first discovered at the beginning of the 19th century. In recent years, Mesembryanthemum crystallinum, a facultative halophyte and C3–CAM intermediate plant, has become a favoured plant for studying stress response mechanisms during C3–CAM shifts. Recent studies in this and related areas can provide a new model of how such mechanisms could operate for acclimation to high salinity or excess excitation energy. These include roles for photosynthetic electron transport chain components and reactive oxygen species. The diurnal rhythms of catalase, superoxide dismutase and some CAM-related enzyme activities are discussed in relation to the protective role of photorespiration during C3–CAM transition. The role of excess excitation energy and redox events in the proximity of photosystem II (PSII) in regulation of ascorbate peroxidase (APX), superoxide dismutase (SOD): copper/zinc SOD (Cu/ZnSOD), iron SOD (FeSOD), and NAD(P)-malic enzyme gene expression are also discussed. We suggest a model in which the chloroplast plays a major role in regulation of acclimation to high salinity and/or excess exitation energy.

Molecular Responses to Photooxidative Stress in Pinus sylvestris (L.) (II. Differential Expression of CuZn-Superoxide Dismutases and Glutathione Reductase

The influence of photooxidative stress on genes expressing superoxide dismutase (Sod) and glutathione reductase (Gor) was analyzed in needles of top and side shoots of 3-year-old Pinus sylvestris (L.) seedlings. The study was carried out in the field during spring recovery. From mid-April the top shoots of seedlings protruded above the snow and thus were exposed to sunlight, whereas the side shoots were covered with snow until May 4. Needles were sampled from top and side shoots on five different occasions. At the beginning of May the mRNA levels for cytosolic CuZn-Sod were significantly higher in top-shoot needles than in side-shoot needles. Similar results were obtained for chloroplastic CuZn-Sod mRNA. After May 6 we could not detect any significant differences between top- and side-shoot needles for either CuZn-Sod mRNA level. Transcript accumulation for the chloroplastic CuZn-Sod was up to 4-fold higher than for cytosolic CuZn-Sod in both types of shoots. On June 1 minimum transcript levels were observed for both CuZn-SOD isoforms. Protein activity analysis for CuZn-SOD isozymes did not reveal any significant differences between top- and side-shoot needles during the whole period of measurements. The mRNA level for chloroplastic Gor was similar in both types of shoots. However, the total GR activity was significantly higher in top-shoot needles than in side-shoot needles at the beginning of May. The analysis of mRNA accumulation for chloroplastic CuZn-Sod and Gor indicates that transcript levels were at least 5- to 20-fold higher for CuZn-Sod than for chloroplastic Gor. The differential expressions of Sod and Gor genes are discussed in relation to regulation of the enzymic scavenging system during photooxidative stress conditions.

Aphid Acceptance of Barley Exposed to Volatile Phytochemicals Differs Between Plants Exposed in Daylight and Darkness

It is well known that volatile cues from damaged plants may induce resistance in neighbouring plants. Much less is known about the effects of volatile interaction between undamaged plants. In this study, barley plants, Hordeum vulgare cv. Kara, were exposed to volatiles from undamaged plants of barley cv. Alva or thistle Cirsium vulgare, and to the volatile phytochemicals, and to methyl salicylate or methyl jasmonate. Exposures were made either during natural daylight or darkness. Acceptance of exposed plants by the aphid Rhopalosiphum padi was assessed, as well as the expression of putative marker genes for the different treatments. Aphid acceptance of plants exposed to either barley or C. vulgare was significantly reduced, and an effect of the volatiles from undamaged plants was confirmed by the induction of pathogenesis-related protein, PR1a in exposed plants. However the effect on aphid acceptance was seen only when plants were exposed during darkness, whereas PR1a was induced only after treatment during daylight. Aphid acceptance of plants exposed to either methyl salicylate or methyl jasmonate was significantly reduced, but only when plants were exposed to the chemicals during daylight. AOS2 (allene oxide synthase) was induced by methyl jasmonate and BCI-4 (barley chemical inducible gene-4) by methyl salicylate in both daylight and darkness. It is concluded that (a) the effects on aphids of exposing barley to volatile phytochemicals was influenced by the presence or absence of light and (b) the response of barley to methyl salicylate/methyl jasmonate and to volatiles from undamaged plants differed at the gene and herbivore level.

The chlB gene encoding a subunit of light-independent protochlorophyllide reductase is edited in chloroplasts of conifers

ChlB is one of three chloroplast genes shown so far to be required for light-independent chlorophyll synthesis. It occurs in some algae, lower plants, and gymnosperms, but not in angiosperms. We have demonstrated, for the first time in conifer chloroplasts, the presence of two internal C to U editing sites in this transcript. In the chlB transcript of Pinus sylvestris, the editing of the second position in a CCG codon leads to an amino-acid substitution from proline to leucine. Editing of a nearby CGG codon, resulting in an arginine to tryptophan substitution, has also been observed. The nucleotide sequence of this region has been compared with other species of gymnosperms. Out of seven species analysed, editing at both sites has only been detected in spruce, while in Larix only the editing which results in the Arg to Trp substitution was found. In other cases, both leucine and tryptophan are encoded by cpDNA, suggesting that conservation of these amino acids, through encoding by DNA or by editing of the RNA, is critical for the protein function. Transcripts are partially edited at the CGG codon and the relative abundance of cDNA molecules with the edited C is species-specific. The possible involvement of RNA editing in the regulation of gene expression in different organs of pine seedlings is discussed.