Plant dehydrins and stress tolerance: Versatile proteins for complex mechanisms

Dehydrins (DHNs), or group 2 LEA (Late Embryogenesis Abundant) proteins, play a fundamental role in plant response and adaptation to abiotic stresses. They accumulate typically in maturing seeds or are induced in vegetative tissues following salinity, dehydration, cold and freezing stress. The generally accepted classification of dehydrins is based on their structural features, such as the presence of conserved sequences, designated as Y, S and K segments. The K segment representing a highly conserved 15 amino acid motif forming amphiphilic a-helix is especially important since it has been found in all dehydrins. Since more than 20 y, they are thought to play an important protective role during cellular dehydration but their precise function remains unclear. This review outlines the current status of the progress made toward the structural, physico-chemical and functional characterization of plant dehydrins and how these features could be exploited in improving stress tolerance in plants.Key words: abiotic stress, dehydration stress, drought, cold acclimation, freezing tolerance, LEA proteins, dehydrins     

Polyploidy and high environmental tolerance increase the invasive success of plants

Journal of Plant Research - Ploidy level and genome size (GS) could affect the invasive capacity of plants, although these parameters can be contradictory. While small GS seems to favor dispersion,...     

Aquatic pollution increases the relative success of invasive species

Although individual ecosystems vary greatly in the degree to which they have been invaded by exotic species, it has remained difficult to isolate mechanisms influencing invader success. One largely anecdotal observation is that polluted or degraded areas will accumulate more invaders than less-impacted sites. However, the role of abiotic factors alone in influencing invisibility has been difficult to isolate, often because the supply of potential invaders is confounded with conditions thought to increase vulnerability to invasion. Here, we conducted a field experiment to test how the assemblages of exotic versus native marine invertebrates changed during community assembly under different exposure levels of a common pollutant, copper. The experiment was conducted by deploying fouling panels in a Randomized Block Design in San Francisco Bay. Panels were periodically removed, placed into buckets with differing copper concentrations, and returned to the field after 3 days. This design allowed propagule availability to the plates to be statistically independent of short-term copper exposure. The results demonstrate that copper caused significant differences in community structure. Average native species richness was significantly affected by copper exposure, but average exotic richness was not. The total native species pool within treatments exhibited a greater than 40% decline within increasing copper, while the exotic species pool did not change significantly. These results confirm that anthropogenic alteration of abiotic factors influences invader success, indicating that management strategies to reduce invader impacts should include both efforts to improve environmental conditions as well as reduce invader supply.     

Cold tolerance of the invasive larch casebearer and implications for invasion success

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Evolution and mechanisms of plant tolerance to flooding stress

Background

In recognition of the 200th anniversary of Charles Darwin''s birth, this short article on flooding stress acknowledges not only Darwin''s great contribution to the concept of evolution but also to the study of plant physiology. In modern biology, Darwin-inspired reductionist physiology continues to shed light on mechanisms that confer competitive advantage in many varied and challenging environments, including those where flooding is prevalent.

Scope

Mild flooding is experienced by most land plants but as its severity increases, fewer species are able to grow and survive. At the extreme, a highly exclusive aquatic lifestyle appears to have evolved numerous times over the past 120 million years. Although only 1–2% of angiosperms are aquatics, some of their adaptive characteristics are also seen in those adopting an amphibious lifestyle where flooding is less frequent. Lowland rice, the staple cereal for much of tropical Asia falls into this category. But, even amongst dry-land dwellers, or certain of their sub-populations, modest tolerance to occasional flooding is to be found, for example in wheat. The collection of papers summarized in this article describes advances to the understanding of mechanisms that explain flooding tolerance in aquatic, amphibious and dry-land plants. Work to develop more tolerant crops or manage flood-prone environments more effectively is also included. The experimental approaches range from molecular analyses, through biochemistry and metabolomics to whole-plant physiology, plant breeding and ecology.Key words: Abiotic stress, adaptation, anoxia, Charles Darwin, environmental stress, evolution, flooding, hypoxia, rice, submergence, wetlands     

Heat-shock proteins and stress tolerance in microorganisms.

Heat-shock proteins help microorganisms cope with the toxic effects of a wide variety of stresses. Some help the organism grow under moderately stressful conditions, others help it to survive more extreme conditions. Surprisingly, the relative importance of individual proteins differs between organisms.     

Geographical range,heat tolerance and invasion success in aquatic species

Species with broader geographical ranges are expected to be ecological generalists, while species with higher heat tolerances may be relatively competitive at more extreme and increasing temperatures. Thus, both traits are expected to relate to increased survival during transport to new regions of the globe, and once there, establishment and spread. Here, we explore these expectations using datasets of latitudinal range breadth and heat tolerance in freshwater and marine invertebrates and fishes. After accounting for the latitude and hemisphere of each species’ native range, we find that species introduced to freshwater systems have broader geographical ranges in comparison to native species. Moreover, introduced species are more heat tolerant than related native species collected from the same habitats. We further test for differences in range breadth and heat tolerance in relation to invasion success by comparing species that have established geographically restricted versus extensive introduced distributions. We find that geographical range size is positively related to invasion success in freshwater species only. However, heat tolerance is implicated as a trait correlated to widespread occurrence of introduced populations in both freshwater and marine systems. Our results emphasize the importance of formal risk assessments before moving heat tolerant species to novel locations.     

Tropical seagrass species tolerance to hypersalinity stress

The long-term sustainability of seagrasses in the subtropics and tropics depends on their ability to adapt to shifts in salinity regimes, particularly in light of present increases in coastal freshwater extractions and future climate change scenarios. Although there are major concerns world-wide on increased salinity in coastal estuaries, there is little quantitative information on the specific upper salinity tolerance of tropical and subtropical seagrass species. We examined seagrass hypersalinity tolerance under two scenarios: (1) when salinity is raised rapidly simulating a pulsed event, such as exposure to brine effluent, and (2) when salinity is raised slowly, characteristic of field conditions in shallow evaporative basins; the first in hydroponics (Experiments I and II) and the second in large mesocosms using intact sediment cores from the field (Experiment III). The three tropical seagrass species investigated in this study were highly tolerant of hypersaline conditions with a slow rate of salinity increase (1 psu d⁻¹). None of the three species elicited total shoot mortality across the range of salinities examined (35–70 psu over 30 days exposures); representing in situ exposure ranges in Florida Bay, a shallow semi-enclosed subtropical lagoon with restricted circulation. Based on stress indicators, shoot decline, growth rates, and PAM florescence, all three species were able to tolerate salinities up to 55 psu, with Thalassia testudinum (60 psu) and Halodule wrightii (65 psu) eliciting a slightly higher salinity threshold than Ruppia maritima (55 psu). However, when salinity was pulsed, without a slow osmotic adjustment period, threshold levels dropped 20 psu to approximately 45 psu for T. testudinum. While we found these three seagrass species to be highly tolerant of high salinity, and conclude that hypersalinity probably does not solely cause seagrass dieoff events in Florida Bay, high salinity can modify carbon and O₂ balance in the plant, potentially affecting the long-term health of the seagrass community.     

Competition,legacy, and priority and the success of three invasive species

Competitive ability, the ability to generate legacy effects, and the potential to benefit from priority, individually or interactively, are traits that may increase the invasive potential of plants. In this project we examine these three traits in three invasive species (Agropyron cristatum, Bromus tectorum, and Taeniatherum caput-medusae). Specifically in this study, we examine competitive effects of these invasive species, the ability of these invasive species to generate legacy effects (as plant–soil feedback), and the potential of these three species benefit from priority (being sown concurrently, 30 days before, and 30 days after the restoration species) in a greenhouse study using field collected soil. Our results suggest that all three invasive species can benefit from priority and all three have high competitive ability. However, only A. cristatum benefited from legacy effects of plant–soil feedback.     

Hydrogen peroxide- and glutathione-associated mechanisms of acclimatory stress tolerance and signalling

Plants adapt to environmental stresses through specific genetic responses. The molecular mechanisms associated with signal transduction, leading to changes in gene expression early in the stress response, are largely unknown. It is clear, however, that gene expression associated with acclimatory responses is sensitive to the redox state of the cell. Of the many components which contribute to the redox balance of the cell, two factors have been shown to be crucial in mediating stress responses. Thiol/disulphide exchange reactions, particularly involving the glutathione pool and the generation of the oxidant H₂O₂, are central components of signal transduction in both environmental and biotic stresses. These molecules are multifunctional triggers, modulating metabolism and gene expression. Both are able to cross biological membranes and diffuse or be transported long distances from their sites of origin. Glutathione and H₂O₂ may act alone or in unison, in intracellular and systemic signalling systems, to achieve acclimation and tolerance to biotic and abiotic stresses.     

Biomass allocation, shade tolerance and seedling survival of the invasive species

Berberis darwinii (Berberidaceae) is a serious environmental weed in New Zealand, capable of invading a range of different light environments from grazed pasture to intact forest. According to optimal partitioning models, some plants optimise growth under different environmental conditions by shifting biomass allocation among tissue types (e.g. roots, shoots) to maximise the capture of limiting resources (e.g. water, light). We examined patterns of growth, biomass allocation, and seedling survival in Berberis darwinii to determine whether any of these factors might be contributing to invasion success. Growth and biomass allocation parameters were measured on seedlings grown for 7 months in five natural light environments in the field. Survival was high in the sunniest sites, and low in the shadiest sites. Seedlings grown in full sun were an order of magnitude taller and heavier, had five times as many leaves, and proportionally more biomass allocated to leaves than seedlings grown in other light environments. In the shade, leaves were bigger and thinner, and leaf area as a proportion of total plant biomass increased, but the proportion of above- to below-ground biomass was similar across all light and soil moisture environments. In summary, although leaf traits were plastic, patterns of biomass allocation did not vary according to optimal partitioning models, and were not correlated with patterns of seedling survival. Implications for the management of this invasive species are discussed.     

Competitive ability,not tolerance,may explain success of invasive plants over natives

When entering a new community, introduced species leave behind members of their native community while simultaneously forming novel biotic interactions. Escape from enemies during the process of introduction has long been hypothesized to drive the increased performance of invasive species. However, recent studies and quantitative syntheses find that invaders often receive similar, or even more, damage from enemies than do native species. Therefore, invasives may be those more tolerant to enemy damage, or those able to maintain competitive ability in light of enemy damage. Here, we investigate whether tolerance and competitive ability could contribute to invasive plant success. We determined whether invasive plants were more competitive than native or noninvasive exotic species in both the presence and absence of simulated herbivory. We found competition and herbivory additively reduced individual performance, and affected the performance of native, invasive, and noninvasive exotic species’ to the same degree. However, invasives exerted stronger competitive effects on an abundant native species (Elymus canadensis) in both the presence and absence of herbivory. Therefore, while invasive species responded similarly to competition and simulated herbivory, their competitive effects on natives may contribute to their success in their introduced range.     

Short-term salt tolerance mechanisms in differentially salt tolerant tomato species

The physiological changes induced by a daily increase of NaCl level, over a period of 4 d, were studied in leaves of the salt-sensitive cultivated tomato species Lycopersicon esculentum and its wild salt-tolerant relative Lycopersicon pennellii. A higher solute contribution to the osmotic adjustment was observed in NaCl-treated leaves of L. pennellii than in those of L. esculentum. This response together with the higher accumulation of inorganic solutes in the wild species and of organic solutes in the cultivated species verified the different salt tolerance mechanisms operating in the two species in the short-term. With regard to the changes induced by salt stress on the free polyamine levels, the putrescine and spermine levels increased with salinity, whereas the spermine levels decreased in both tomato species; nevertheless, the main difference between the two species lays in an earlier and greater accumulation of putrescine induced by salinity in L. pennellii than in L. esculentum. The changes in putrescine levels were associated to changes in amino acids related to its synthesis, and the changes were different in both species. In L. esculentum, the high concentrations of some intermediate compounds (glutamate and arginine) were related to the low accumulation rate of both proline and putrescine. In contrast, in L. pennellii, important reductions in glutamate and arginine levels were found at the end of the salinization period. Moreover, in this last situation, a decline in the putrescine level ran parallel to a high proline accumulation, which suggests that the higher the stress level, the higher the deviation of glutamate to proline occurring in the salt tolerant species. It could be concluded that an early accumulation of the diamine putrescine seems to be associated with salt tolerance in the short-term.     

盐胁迫下树种幼苗生长及其耐盐性

采用盆栽方法,以11个树种实生幼苗为材料,用不同浓度(0、3、5、8 g·kg-1和10 g·kg-1)NaCl溶液进行1次性浇灌处理,对盐胁迫下各树种的形态表现、生长及耐盐性进行了研究,结果表明:(1)当盐含量达到8 g·kg-1时,欧洲荚蒾、甜桦和光叶漆植株死亡,当含量增加到10 g·kg-1时,沃氏金链花植株死亡,其它各存活树种也均出现不同程度的盐害症状;(2)盐胁迫后,各树种的苗高生长量下降、生物量累积减少,且随着处理浓度的增加均呈下降趋势,其中榆桔、甜桦和光叶漆的降幅最大;(3)盐处理后,各树种的根冠比值增大,其中盐胁迫对光叶漆、银水牛果和沃氏金链花有显著影响(p<0.05);(4)综合分析各树种的生长和形态表现,认为日本丁香、银水牛果、三裂叶漆和豆梨具有高度耐盐性,沃氏金链花、金雀儿、鹰爪豆和榆桔具有中高度耐盐性,而欧洲荚蒾、甜桦和光叶漆具有中度耐盐性.