Carolina Prado

Soluble sugars--metabolism, sensing and abiotic stress: a complex network in the life of plants.

Plants are autotrophic and photosynthetic organisms that both produce and consume sugars. Soluble sugars are highly sensitive to environmental stresses, which act on the supply of carbohydrates from source organs to sink ones. Sucrose and hexoses both play dual functions in gene regulation as exemplified by the up-regulation of growth-related genes and down-regulation of stress-related genes. Although coordinately regulated by sugars, these growth- and stress-related genes are up-regulated or down-regulated through HXK-dependent and/or HXK-independent pathways. Sucrose-non-fermenting-1- (SNF1-) related protein pathway, analogue to the protein kinase (SNF-) yeast-signalling pathway, seems also involved in sugar sensing and transduction in plants. However, even if plants share with yeast some elements involved in sugar sensing, several aspects of sugar perception are likely to be peculiar to higher plants. In this paper, we have reviewed recent evidences how plants sense and respond to environmental factors through sugar-sensing mechanisms. However, we think that forward and reverse genetic analysis in combination with expression profiling must be continued to uncover many signalling components, and a full biochemical characterization of the signalling complexes will be required to determine specificity and cross-talk in abiotic stress signalling pathways.

Uptake of chromium by Salvinia minima: Effect on plant growth, leaf respiration and carbohydrate metabolism

Metabolic responses to chromium (Cr) exposure and metal uptake were investigated using Salvinia minima plants. Cr treatment reduced the dry weight of floating and submerged leaves, while photosynthetic pigments were not affected. Measurements of respiratory oxygen uptake with and without inhibitors (KCN and SHAM) demonstrated that total respiration, alternative oxidase capacity and residual respiration were higher in Cr-treated than in Cr-untreated leaves, but the highest values were observed in floating leaves. Cr affected the soluble sugar content. Sucrose concentration was, in general, higher in Cr-treated than in Cr-untreated leaves, while the glucose concentration showed an inverse pattern. Cr also affected soluble acid invertase activity, but affectation trend was different between both leaves. Highest values of invertase activity were observed in Cr-treated floating leaves. According to our data soluble acid invertase and sucrose seem to be related to alternative oxidase capacity and residual respiration in floating and submerged leaves exposed to Cr. Thereby, this study constitutes an important contribution to understand metabolic relationships between mitochondrial respiration, alternative respiratory pathway and soluble carbohydrates in plants exposed to heavy metals.

Seasonal variability of physiological and biochemical aspects of chromium accumulation in outdoor-grown Salvinia minima

Seasonal variations in physiological and biochemical parameters of the aquatic fern Salvinia minima exposed to different Cr(VI) concentrations were studied. Growth, photosynthetic pigments, soluble carbohydrates, sucrose-related enzymes, lipid peroxidation, phenolics, and Cr accumulation in floating and submerged leaves were analyzed. Cr content was lower in winter than in summer, indicating that active metabolic events occurred in metal uptake. Leaf number and metal concentration factor were higher in summer than in winter. Relative growth rate (R(n)) indicated that growth was more affected by Cr in winter than in summer. Biochemical parameters showed great seasonal variations under increasing Cr. Hexose, starch, malondialdehyde and phenolic contents were greatest in winter, but R(n) and protein values were lowest. Sucrose content was highest in summer floating leaves. A great seasonal variability was observed in sucrose-related enzymes with the highest activities occurring in winter lipoxygenase was much higher in winter than in summer, indicating a strong lipid peroxidation. Results indicate that in Salvinia Cr causes seasonal perturbations in carbohydrate metabolism and oxidative stress by altering both sucrose-related enzymes and lipoxygenase activities. Variability in physiological and biochemical parameters seems to indicate that in outdoor conditions different mechanisms, in terms of Cr accumulation and tolerance, may occur in S. minima during summer and winter.

UV-B Radiation, Its Effects and Defense Mechanisms in Terrestrial Plants

The UV-B is an important component of solar radiation to which all terrestrial and aquatic plants were exposed during the early evolutionary phase of the Earth. Hence the plants, principally terrestrial, have evolved different mechanisms to avoid and repair the UV-B damage; therefore, it is not surprising that photomorphogenic responses to the solar UV-B are erroneously assumed to be adaptations to the harmful UV radiation. The responses to UV-B enhancement include changes in the leaf area, leaf thickness, stomatal density, wax deposition, stem elongation, and branching pattern, as well as in the synthesis of secondary metabolites, alterations in plant–pathogen and plant–predator interactions, and in gene expression. However, under field conditions the ambient solar UV-B provides an important signal for the normal plant development and may be perceived by the plants through nondestructive processes involving both UV-B specific and UV-B nonspecific signaling pathways. The specific signaling pathways include the components UVR8 and COP1 which regulate the expression of a set of genes that are essential for the plants’ protection. The nonspecific signaling pathways involve DNA damage, reactive oxygen species (ROS), hormones, and wound/defense signaling molecules. Indeed under the field conditions, the ambient UV-B might more properly be viewed as a photomorphogenic signal than as a stressor. Therefore, it might not be appropriate to evaluate the adaptive roles of plant responses to UV-B cues upon stress tolerance by the simultaneous application of both solar radiation and supplemental UV-B. In this chapter, we analyzed the information regarding physiological and morphogenic responses of the terrestrial plants to the UV-B radiation, as well as the events related to UV-B perception, signal transduction, gene expression, and ROS formation from different studies carried out in greenhouses, growth chambers, and field conditions.

Detoxification of Cr(VI) in Salvinia minima is related to seasonal-induced changes of thiols, phenolics and antioxidative enzymes

In this study, protein- and non-protein-thiol-containing compounds (THCC), soluble phenolics (SP), proline (Pro), proteins and malondialdehyde (MDA) contents, and antioxidative enzyme activities were analyzed in floating and submerged leaves of Salvinia minima to establish their role against Cr-induced oxidative stress. We analyzed relationships among biochemical responses to different Cr(VI) concentrations to explore underlying mechanisms of Cr detoxification in plants growing under field conditions during summer and winter seasons. Significant increases in THCC were observed in submerged leaves from both seasons, while in floating leaves THCC increased only in summer being decreased in winter. Contrarily SP increased in floating leaves and decreased in submerged ones. MDA increased significantly in winter-leaves, but in summer-leaves remained unchanged. Antioxidative enzymes, i.e. guaiacol peroxidase (G-POD), superoxide dismutase (SOD) and catalase (CAT) showed different activity patterns. G-POD significantly increased in Cr-treated leaves from both seasons, while SOD increased in submerged leaves only, remaining practically unchanged in floating ones. CAT activity increased in floating leaves from both seasons, whereas in submerged ones was decreased or increased. Proteins increased in both leaf types during summer whereas decreased or remained unchanged in winter. Pro increased in winter-submerged leaves only. Results show that seasonal-induced changes occur in all measured parameters.

Differential Effects of Cr(VI) on the Ultrastructure of Chloroplast and Plasma Membrane of Salvinia minima Growing in Summer and Winter. Relationships With Lipid Peroxidation, Electrolyte Leakage, Photosynthetic Pigments, and Carbohydrates

Seasonal variations of chloroplast thylakoids and plasma membrane ultrastructure and changes in some biochemical parameters (e.g., metal accumulation, photosynthetic pigments, carbohydrates, lipid peroxidation, and electrolyte leakage) were studied in fronds of Salvinia minima plants exposed to increasing concentrations of Cr(VI) in both winter and summer. Disorganization of stacked (grana) and unstacked (stroma lamellae) thylakoids was greater in winter chloroplasts than in summer chloroplasts. Plasma membrane was less affected than thylakoids. Photosynthetic pigments, lipid peroxidation, soluble sugars, and starch were affected differently in winter and summer. Our results suggest that much greater ultrastructural alterations and changes in metabolite levels occurring in winter fronds are produced by higher oxidative stress resulting from the interactive effect between low temperature, low solar irradiance, and Cr(VI) toxicity, rather than from metal accumulation per se. Seasonal differences occurring in chloroplast ultrastructure and metabolite concentrations were discussed in relation to metabolic implications. Evaluated parameters represent a relevant approach to enhance knowledge on performance and fitness of plants exposed to heavy metals under fluctuating environmental conditions. This work also indicates that selection of suitable macrophytes to remove Cr(VI) requires an additional analyzing focus on structural and metabolic interactions that occur in plants exposed to heavy metals in contrasting seasons.

Chromium and the Plant: A Dangerous Affair?

Abstract Contamination of aquatic and terrestrial environments by toxic chromium (Cr) species is a worldwide increasing problem. Cr occurs in the environment mainly as Cr(III) and Cr(VI) species. Cr is an element whose toxic effects depend on its oxidation state. Cr(VI) is highly mobile and toxic to animals and plants. Conversely, Cr(III) is less mobile and less toxic. Cr does not accumulate in plants naturally, but can be absorbed easily from soils and waters. To face the challenge of adverse effects of Cr species in polluted environments, plants must use different mechanisms either to avoid the entry of toxic Cr species or to block their intracellular toxicity. Although it is always claimed that Cr(VI) is more toxic than Cr(III), when the Cr(VI) enters into cells, it is previously reduced to Cr(III) before its toxic effects occur. From this fact could arise an intriguing question on the unquestioned current dogma about the accumulation of toxic Cr(VI) by plants: that is to say, the conversion of the Cr(VI) form to the “less toxic” Cr(III) form is a key tolerance mechanism. In this chapter, we analyze relationships between Cr(VI), Cr(III), and plants which underlying as “a dangerous affair.”

Phytoremediation: Strategies of Argentinean Plants Against Stress by Heavy Metals

Environmental pollution by organic and inorganic contaminants is an increasing global problem, but mostly affects both poor and developing countries. The effects of environmental pollution on biota and human health are reaching intolerable limits, so urgent actions are required to address their remediation before it is too late to generate a less polluted world and better life quality to next generations. In Argentina the environmental pollution has already been installed in regions once considered non-contaminated sites such as the Andean mountain range and fertile plains (Pampas). For a long time several Argentinean laboratories are working on phytoremediation technologies to clean polluted soils and water bodies. However, most of these studies are carried out at small-scale under laboratory conditions and/or in small-sized confined environments. In fact to date no more than one full-scale experiment has been carried out to decontaminate sewage and industrial effluents from a metallurgical factory. Strikingly, most studies have focused on a few toxic heavy metals, i.e., chromium (Cr), cadmium (Cd), lead (Pb), zinc (Zn), copper (Cu), manganese (Mn), and nickel (Ni), but ongoing investigations have not considered the most widespread of all: arsenic (As). It affects more than half of the Argentine territory, including large areas of the Chaco-Pampean plains and also numerous rivers that provide water for both human and animal consumption, as well as for crop irrigation. Studies on phytoremediation of organic pollutants are less developed.

Differential physiological responses of two Salvinia species to hexavalent chromium at a glance.

In plants of Salvinia rotundifolia and Salvinia minima the effect of two Cr(VI) concentrations (5 and 20mgL(-1)) applied for 7days was assessed by measuring changes in biomass, photosynthetic pigments, Cr accumulation, malondialdehyde (MDA), membrane stability index (MSI), thiols (TT, NPT and PBT), and phenolics (SP and IP). Biomass in S. minima was decreased at highest Cr(VI) concentration, but there were no changes in S. rotundifolia. Metal accumulation was different in both species. S. minima accumulates more metal in fronds, but S. rotundifolia accumulates more metal in lacinias. Results also showed that S. minima translocates more Cr to fronds than S. rotundifolia, but at the whole plant level higher accumulation occurred in this last. Tolerance index (Ti) was higher in S. rotundifolia. Chl b and carotenoids were decreased only upon exposure to high Cr(VI) concentration in both species. Cr(VI) treatment did not enhance MDA accumulation. Cr exposure had no impact on MSI values when comparing with Cr-untreated values. Thiols in fronds and lacinias showed different distribution patterns between species. IP and NPT were higher in S. rotundifolia lacinias that accumulate more Cr than S. minima lacinias. Whilst SP and NPT were higher in S. minima fronds compared with S. rotundifolia ones. This may indicate that these species can cope with Cr(VI) toxicity, either through metal complexation and/or metal reduction or by the scavenging of ROS derived from Cr-induced oxidative stress. Based on Cr accumulation and biomass production, S. rotundifolia seems more suitable to remove Cr(VI) from polluted waters.

Mitochondrial Respiration: Involvement of the Alternative Respiratory Pathway and Residual Respiration in Abiotic Stress Responses

Mitochondrial respiration provides energy necessary to drive the cellular metabolism in all living organisms. From ubiquinone mitochondrial respiratory electron transport chain (ETC) of plants, many fungi and protozoans branch into cyanide-sensitive and cyanide-insensitive pathways. The first one named cytochrome pathway (CP) used cytochrome c oxidase (COX) as terminal oxidase while the second named alternative pathway (AP) utilises an alternative oxidase (AOX) as terminal oxidase. COX reduces O2 to H2O and couples to H+-gradient generation used to make ATP. AOX reduces O2 upon oxidation of ubiquinol (UQH2) without coupling to ATP production. Despite its role as energy-dissipating pathway, AP also plays a key role in coordinated responses of cells to both abiotic and biotic stresses. In this context, when effectiveness of CP becomes limited by stress condition, AP allows a stress-dependent flexibility of respiratory ETC leading to an enhanced fitness of stressed plants. Residual respiration (Rresp), insensitive to cyanide and salicylhydroxamic acid (SHAM), not mediated either by CP or AP, also occurs in living plants. Although nature of this unique respiration is unknown, some new evidences seem to suggest that it plays an important role in abiotic stress responses. Thus, it is becoming clear that both AP, through the AOX activity, and Rresp constitute key components of orchestrated mitochondrial events, occurring in plants exposed to unfavourable environmental conditions.