Charlotte E. Seal

A Central Role for Thiols in Plant Tolerance to Abiotic Stress

Abiotic stress poses major problems to agriculture and increasing efforts are being made to understand plant stress response and tolerance mechanisms and to develop new tools that underpin successful agriculture. However, the molecular mechanisms of plant stress tolerance are not fully understood, and the data available is incomplete and sometimes contradictory. Here, we review the significance of protein and non-protein thiol compounds in relation to plant tolerance of abiotic stress. First, the roles of the amino acids cysteine and methionine, are discussed, followed by an extensive discussion of the low-molecular-weight tripeptide, thiol glutathione, which plays a central part in plant stress response and oxidative signalling and of glutathione-related enzymes, including those involved in the biosynthesis of non-protein thiol compounds. Special attention is given to the glutathione redox state, to phytochelatins and to the role of glutathione in the regulation of the cell cycle. The protein thiol section focuses on glutaredoxins and thioredoxins, proteins with oxidoreductase activity, which are involved in protein glutathionylation. The review concludes with a brief overview of and future perspectives for the involvement of plant thiols in abiotic stress tolerance.

Genome-wide association mapping and biochemical markers reveal that seed ageing and longevity are intricately affected by genetic background and developmental and environmental conditions in barley

Globally, over 7.4 million accessions of crop seeds are stored in gene banks, and conservation of genotypic variation is pivotal for breeding. We combined genetic and biochemical approaches to obtain a broad overview of factors that influence seed storability and ageing in barley (Hordeum vulgare). Seeds from a germplasm collection of 175 genotypes from four continents grown in field plots with different nutrient supply were subjected to two artificial ageing regimes. Genome-wide association mapping revealed 107 marker trait associations, and hence, genotypic effects on seed ageing. Abiotic and biotic stresses were found to affect seed longevity. To address aspects of abiotic, including oxidative, stress, two major antioxidant groups were analysed. No correlation was found between seed deterioration and the lipid-soluble tocochromanols, nor with oil, starch and protein contents. Conversely, the water-soluble glutathione and related thiols were converted to disulphides, indicating a strong shift towards more oxidizing intracellular conditions, in seeds subjected to long-term dry storage at two temperatures or to two artificial ageing treatments. The data suggest that intracellular pH and (bio)chemical processes leading to seed deterioration were influenced by the type of ageing or storage. Moreover, seed response to ageing or storage treatment appears to be significantly influenced by both maternal environment and genetic background.

Back to the future with the AGP–Ca2+ flux capacitor

BACKGROUND Arabinogalactan proteins (AGPs) are ubiquitous in green plants. AGPs comprise a widely varied group of hydroxyproline (Hyp)-rich cell surface glycoproteins (HRGPs). However, the more narrowly defined classical AGPs massively predominate and cover the plasma membrane. Extensive glycosylation by pendant polysaccharides O-linked to numerous Hyp residues like beads of a necklace creates a unique ionic compartment essential to a wide range of physiological processes including germination, cell extension and fertilization. The vital clue to a precise molecular function remained elusive until the recent isolation of small Hyp-arabinogalactan polysaccharide subunits; their structural elucidation by nuclear magentic resonance imaging, molecular simulations and direct experiment identified a 15-residue consensus subunit as a β-1,3-linked galactose trisaccharide with two short branched sidechains each with a single glucuronic acid residue that binds Ca(2+) when paired with its adjacent sidechain. SCOPE AGPs bind Ca(2+) (Kd ∼ 6 μm) at the plasma membrane (PM) at pH ∼5·5 but release it when auxin-dependent PM H(+)-ATPase generates a low periplasmic pH that dissociates AGP-Ca(2+) carboxylates (pka ∼3); the consequential large increase in free Ca(2+) drives entry into the cytosol via Ca(2+) channels that may be voltage gated. AGPs are thus arguably the primary source of cytosolic oscillatory Ca(2+) waves. This differs markedly from animals, in which cytosolic Ca(2+) originates mostly from internal stores such as the sarcoplasmic reticulum. In contrast, we propose that external dynamic Ca(2+) storage by a periplasmic AGP capacitor co-ordinates plant growth, typically involving exocytosis of AGPs and recycled Ca(2+), hence an AGP-Ca(2+) oscillator. CONCLUSIONS The novel concept of dynamic Ca(2+) recycling by an AGP-Ca(2+) oscillator solves the long-standing problem of a molecular-level function for classical AGPs and thus integrates three fields: AGPs, Ca(2+) signalling and auxin. This accounts for the involvement of AGPs in plant morphogenesis, including tropic and nastic movements.

Glutathione half-cell reduction potential and α-tocopherol as viability markers during the prolonged storage of Suaeda maritima seeds.

Antioxidants protect seeds from oxidative damage during storage, supporting the maintenance of seed viability and the ability to germinate post-storage. No data on antioxidants during long-term storage are available for the seeds of the halophyte Suaeda maritima. Therefore, changes in lipid-soluble antioxidants in the tocopherol family (α-, γ-, δ-tocopherol), were investigated in seeds stored for up to 16 years at 4°C at a seed moisture content of 10–13%, as well as changes in the water-soluble antioxidant glutathione (GSH) and electrolyte leakage. Seed oil content was also measured and determined to be 22%. During the first 3 years of storage, seed viability remained high and the concentration of total tocopherol was stable. Thereafter, both seed viability and α-tocopherol concentration rapidly decreased and electrolyte leakage increased, while γ-tocopherol and δ-tocopherol concentrations did not correlate with seed viability. Although the concentrations of neither GSH nor glutathione disulphide (GSSG) alone were correlated with seed viability, the glutathione half-cell reduction potential (EGSSG/2GSH) was strongly correlated with viability throughout storage and increased before the onset of viability loss. Hence, in this species EGSSG/2GSH appeared to be an ‘early warning’ system preceding viability loss while α-tocopherol concentration changed concomitantly with viability.

Salt stress, signalling and redox control in seeds

Abiotic stresses, including salt stress, can impair electron transport chains, thereby increasing the production of reactive oxygen species (ROS). An excess of ROS can damage macromolecular and cellular structure, but ROS are also key components of signalling networks, through which they regulate developmental processes. Surprisingly little is known about the effects of salt stress upon seeds given their pivotal role in plant reproduction and dispersal. This review provides information on tolerance mechanisms and redox control in relation to seed metabolism and performance. First, the effects of salt stress throughout the seed life cycle are discussed, comprising salt effects on the mother plant and its implications on seed development, salt uptake upon seed imbibition and effects on seed germination. Then, responses to elevated salt concentrations are discussed according to a recently proposed triphasic seed stress model comprising the phases alarm, resistance and exhaustion. Implications of redox control in seeds on the physiological, biochemical and molecular level are considered and the review concludes with a perspective on future research in relation to salt stress and seed biology.

Redox state of low-molecular-weight thiols and disulphides during somatic embryogenesis of salt-treated suspension cultures of Dactylis glomerata L.

Abstract The tripeptide antioxidant γ-L-glutamyl-L-cysteinyl-glycine, or glutathione (GSH), serves a central role in ROS scavenging and oxidative signalling. Here, GSH, glutathione disulphide (GSSG), and other low-molecular-weight (LMW) thiols and their corresponding disulphides were studied in embryogenic suspension cultures of Dactylis glomerata L. subjected to moderate (0.085 M NaCl) or severe (0.17 M NaCl) salt stress. Total glutathione (GSH + GSSG) concentrations and redox state were associated with growth and development in control cultures and in moderately salt-stressed cultures and were affected by severe salt stress. The redox state of the cystine (CySS)/2 cysteine (Cys) redox couple was also affected by developmental stage and salt stress. The glutathione half-cell reduction potential (EGSSG/2 GSH) increased with the duration of culturing and peaked when somatic embryos were formed, as did the half-cell reduction potential of the CySS/2 Cys redox couple (ECySS/2 Cys). The most noticeable relationship between cellular redox state and developmental state was found when all LMW thiols and disulphides present were mathematically combined into a ‘thiol–disulphide redox environment’ (Ethiol–disulphide), whereby reducing conditions accompanied proliferation, resulting in the formation of pro-embryogenic masses (PEMs), and oxidizing conditions accompanied differentiation, resulting in the formation of somatic embryos. The comparatively high contribution of ECySS/2 Cys to Ethiol–disulphide in cultures exposed to severe salt stress suggests that Cys and CySS may be important intracellular redox regulators with a potential role in stress signalling.

Simulating the germination response to diurnally alternating temperatures under climate change scenarios: comparative studies on Carex diandra seeds

BACKGROUND AND AIMS Environmental temperature regulates plant regeneration via seed in several superimposed ways, and this complex regulation will be disrupted by climate change. The role of diurnally alternating temperatures (ΔT) in terminating dormancy will be a major factor in this disruption, as its effects on seed germination are immediate. METHODS The effect of ΔT on seed germination was modelled using two populations of the wetland sedge Carex diandra, one from a montane site and one from a subalpine site. A cardinal-temperature model was fitted to germination results obtained from a thermal gradient plate, and the model was used to simulate changes in germination under two possible future climate scenarios (RCP2·6 and RCP8·5, for representative concentration pathways) as defined by the Intergovernmental Panel on Climate Change. KEY RESULTS Scenario RCP2·6 projected moderate increases in average temperatures and ΔT, whereas RCP8·5 projected greater warming and higher ΔT. Increasing ΔT decreased the base temperature for seed germination and the thermal time required for germination. The effect of higher ΔT together with the higher temperatures increased germination under both climate scenarios. CONCLUSIONS Carex diandra germination is highly responsive to potential changes in ΔT, and thus this study highlights the role of ΔT in seed responses to climate change. Comprehensive cardinal-temperature models, encompassing the different effects of temperature on seed germination, are needed to understand how climate change will affect plant regeneration.

Manipulating the antioxidant capacity of halophytes to increase their cultural and economic value through saline cultivation

Halophytes, salt-tolerant plants, are a source of valuable secondary metabolites with potential as functional foods or nutraceuticals. We are interested in finding the optimal cultivation conditions for increasing the contents of these valuable compounds. Growth conditions away from the optimum can induce stress resulting in changes in secondary metabolites. We analyzed metabolites with antioxidant capacity in seedlings and plants from different families and habitats grown under different salt concentrations. We show that it is possible to manipulate the antioxidant capacity of plants and seedlings by altering the saline growing environment, the length of time under saline cultivation and the developmental stage.

Quantification of seed oil from species with varying oil content using supercritical fluid extraction

INTRODUCTION The quantity and composition of seed oil affects seed viability and storability and hence the value of a species as a resource for nutrition and plant conservation. Supercritical fluid extraction with carbon dioxide (SFE-CO2) offers a rapid, environmentally friendly alternative to traditional solvent extraction. OBJECTIVE To develop a method using SFE-CO2 to quantify the seed oil content in a broad range of species with high to low oil contents. METHODOLOGY Seed oil was extracted using SFE-CO2 from four crop species representing high, medium and low oil content: Helianthus annuus, Asteraceae, with ca. 55% oil; Brassica napus, Brassicaceae, with ca. 50% oil; Glycine max, Fabaceae, with ca. 20% oil; and Pisum sativum, Fabaceae, with ca. 2% oil. Extraction pressures of 5000, 6000 and 7500 psi and temperatures of 40, 60 and 80 degrees C were examined and a second step using 15% ethanol as a modifier included. Oil yields were compared with that achieved from Smalley Butt extraction. The optimised SFE-CO2 method was validated on six species from taxonomically distant families and with varying oil contents: Swietenia humilis (Meliaceae), Stenocereus thurberi (Cactaceae), Sinapis alba (Brassicaceae), Robinia pseudoacacia (Fabaceae), Poa pratensis (Poaceae) and Trachycarpus fortunei (Arecaceae). RESULTS The two-step extraction at 6000 psi and 80 degrees C produced oil yields equivalent to or higher than Smalley Butt extraction for all species, including challenging species from the Brassicaceae family. CONCLUSION SFE-CO2 enables the rapid analysis of seed oils across a broad range of seed oil contents.

Salicornia as a crop plant in temperate regions: selection of genetically characterized ecotypes and optimization of their cultivation conditions.

Salinization of groundwater results in fast dwindling sources of freshwater. Our aim was to develop genetically characterized lines of the salt-tolerant Salicornia (marsh samphire) and Sarcocornia (shrubby Swampfire) as new crop plants. To obtain a large genetic pool, seeds were collected from different countries and ecological conditions. The application of a genetic marker showed a clear distinction between the two genera and between 57 Salicornia taxa. For the determination of optimal cultivation conditions, experiments on germination, seedling establishment and growth to a harvestable size were performed using different Salicornia accessions. Further optimization of cultivation conditions is necessary for commercial use.