Stella Castro

Physiological responses of a halophytic shrub to salt stress by Na2SO4 and NaCl: oxidative damage and the role of polyphenols in antioxidant protection

We studied the halophytic shrub Prosopis strombulifera to investigate whether the differential ability of this species to grow under increasing salt concentrations and mixtures was related to the synthesis of polyphenolic compounds and to the maintenance of leaf pigment contents for an adequate photosynthetic activity. The significant accumulation of flavonoids in tissues under Na2SO4 treatment and their powerful antioxidant activity indicates a role for these compounds in counteracting the oxidative damage induced by severe salt stress, particularly, ionic stress. We demonstrate that ionic interactions between different salts in salinized soils modify the biochemical and morpho-physiological responses of Prosopis strombulifera plants to salinity.

Physiological and Biochemical Responses to Drought Stress and Subsequent Rehydration in the Symbiotic Association Peanut-Bradyrhizobium sp.

Drought stress is one of the most important environmental factors that regulate plant growth and development and limit its production. Peanut (Arachis hypogaea L.) is an agriculturally valuable plant with widespread distribution in the world serving as a subsistence food crop as well as a source of various food products. The aims of this work were to evaluate growth and nodulation as well as some physiological and biochemical stress indicators in response to drought stress and subsequent rehydration in the symbiotic association peanut-Bradyrhizobium sp. SEMIA6144. Drought stress affected peanut growth reducing shoot dry weight, nodule number, and dry weight as well as nitrogen content, but root dry weight increased reaching a major exploratory surface. Besides, this severe water stress induced hydrogen peroxide production associated with lipid and protein damage; however, the plant was able to increase soluble sugar and abscisic acid contents as avoidance strategies to cope with drought stress. These physiological and biochemical parameters were completely reversed upon rehydration, in a short period of time, in the symbiotic association peanut-Bradyrhizobium sp. Thus, the results provided in this work constitute the initial steps of physiological and biochemical responses to drought stress and rehydration in this nodulated legume.

Involvement of glutathione and enzymatic defense system against cadmium toxicity in Bradyrhizobium sp. strains (peanut symbionts)

In this study, the effects of cadmium (Cd) on cell morphology and antioxidant enzyme activities as well as the distribution of the metal in different cell compartments in Bradyrhizobium sp. strains were investigated. These strains were previously classified as sensitive (Bradyrhizobium sp. SEMIA 6144) and tolerant (Bradyrhizobium sp. NLH25) to Cd. Transmission electron micrographs showed large electron-translucent inclusions in the sensitive strain and electron-dense bodies in the tolerant strain, when exposed to Cd. Analysis of Cd distribution revealed that it was mainly bounded to cell wall in both strains. Antioxidant enzyme activities were significantly different in each strain. Only the tolerant strain was able to maintain a glutathione/oxidized glutathione (GSH/GSSG) ratio by an increase of GSH reductase (GR) and GSH peroxidase (GPX) enzyme activities. GSH S-transferase (GST) and catalase (CAT) activities were drastically inhibited in both strains while superoxide dismutase (SOD) showed a significant decrease only in the sensitive strain. In conclusion, our findings suggest that GSH content and its related enzymes are involved in the Bradyrhizobium sp. tolerance to Cd contributing to the cellular redox balance.

Glutamate Is Involved in Acid Stress Response in Bradyrhizobium sp. SEMIA 6144 (Arachis hypogaea L.) Microsymbiont

In the present study, the effect of acid stress on ammonium assimilation in Bradyrhizobium sp. SEMIA 6144 (Arachis hypogaea L.) microsymbiont was analyzed. The bacterial growth rate was decreased by 50%, and a significant increase in intracellular glutamate concentration was detected when the strain grew at acid pH (5.5). Assays of the enzymes involved in glutamate synthesis showed increased activities of glutamine synthetase (GS) and glutamate synthase (NADPH-GOGAT) under acid stress condition. This would support the contention that the GS/NADPH-GOGAT pathway contributes to the increase of glutamate synthesis as a compatible solute in response to acid stress.

Antioxidant defense system responses and role of nitrate reductase in the redox balance maintenance in Bradyrhizobium japonicum strains exposed to cadmium

In this work, we evaluated the effects of cadmium (Cd) on the antioxidant defense system responses and the role of nitrate reductase (NR) in the redox balance maintenance in Bradyrhizobium japonicum strains. For that, B. japonicum USDA110 and its NR defective mutant strain (GRPA1) were used. Results showed that the addition of 10μM Cd did not modify the aerobic growth of the wild type strain while the mutant strain was strongly affected. Anaerobic growth revealed that only the parental strain was able to grow under this condition. Cd reduced drastically the NR activity in B. japonicum USDA110 and increased lipid peroxide content in both strains. Cd decreased reduced glutathione (GSH)/oxidized glutathione (GSSG) ratio in B. japonicum USDA110 although, a significant increased was observed in the mutant GRPA1. GSH-related enzymes were induced by Cd, being more evident the increase in the mutant strain. This different behavior observed between strains suggests that NR enzyme plays an important role in the redox balance maintenance in B. japonicum USDA 110 exposed to Cd.

Growth of Bradyrhizobium sp. SEMIA 6144 in Response to Methylglyoxal: Role of Glutathione

We previously showed the important role of glutathione (GSH) in the protection mechanism against different stresses, such as acid pH, saline, and oxidative stress, using a GSH-deficient mutant of Bradyrhizobium sp. (peanut microsymbiont). In this work, we studied the role of GSH in the protection mechanism against methylglyoxal (MG) toxicity. MG is a naturally occurring toxic electrophilic compound, and it has been shown that GSH is involved in the detoxification of MG in Escherichia coli. One recognized component of this detoxification process is the formation of a GSH adduct, which in turn transports potassium (K+) out of bacterial cells. Our results showed that growth of wild-type strain Bradyrhizobium sp. SEMIA 6144 was not affected at a MG concentration of 0.5 mM in the yeast extract–mannitol culture medium. However, a reduction of growth, at concentrations of 1.5 and 2.5 mM MG and reaching complete growth inhibition at 3.0 mM MG, was observed. In wild-type strain, intracellular GSH content decreased, and intracellular K+ content was unchanged, whereas GSH-deficient mutant SEMIA 6144-S7Z was unable to grow at 1.5 mM MG. The addition of external GSH to the incubation medium did not restore the growth rate either in wild-type or mutant strains. Our findings showed that GSH has not proven to be protective against the cell-growth inhibiting activity of MG. Therefore, the response of Bradyrhizobium sp. growth to MG is different from that reported in E. coli and other Gram-negative bacteria.

Dynamic responses of photosynthesis and the antioxidant system during a drought and rehydration cycle in peanut plants

Drought stress is one of the most important environmental factors that adversely affect the productivity and quality of crops. Most studies focus on elucidating plant responses to this stress but the reversibility of these effects is less known. The aim of this work was to evaluate whether drought-stressed peanut (Arachis hypogaea L.) plants were capable of recovering their metabolism upon rehydration, with a focus on their antioxidant system. Peanut plants in the flowering phase (30 days after sowing) were exposed to drought stress by withholding irrigation during 14 days and subsequent rehydration during 3 days. Under these conditions, physiological status indicators, reactive oxygen species production and antioxidant system activity were evaluated. Under drought stress, the stomatal conductance, photosynthetic quantum yield and 13C:12C ratio of the peanut plants were negatively affected, and also they accumulated reactive oxygen species. The antioxidant system of peanut plants showed increases in superoxide dismutase-, ascorbate peroxidase- and glutathione reductase-specific activities, as well as the total ascorbate content. All of these responses were reversed upon rehydration at 3 days. The efficient and dynamic regulation of variables related to photosynthesis and the antioxidant system during a drought and rehydration cycle in peanut plants was demonstrated. It is suggested that the activation of the antioxidant system could mediate the signalling of drought stress responses that enable the plant to survive and recover completely within 3 days of rehydration.

Substrate selection as a possible strategy for amelioration of acid pH by Rhizobium leguminosarum biovar viciae.

A possible mechanism for bacteria to ameliorate an unfavorably acidic external pH would be to select for catabolism of those substrates in a mixture that would alkalinize the medium. This hypothesis was tested with cultures of Rhizobium leguminosarum biovar viciae WSM710 grown at pH 7.0 or 5.5 on binary mixtures of glucose plus fumarate or glucose plus histidine. The results showed no significant increase in the absolute or relative utilization of the alkalinizing substrates (fumarate or histidine) at pH 5.5.

Importance of Glutathione in the Legume-Rhizobia Symbiosis

Glutathione (GSH) is essential for the proper development of root nodules during the symbiotic association of legume and rhizobia. It is involved in the antioxidant defense, the detoxification of xenobiotics, and the tolerance to abiotic and biotic stresses. The high level of GSH in root nodules and the presence of an active ascorbate-glutathione (AsA-GSH) cycle suggest that GSH participates in the protection of the nitrogen-fixing process against reactive oxygen species (ROS) resulting from the active nodule metabolism. Glutathione-related enzymes also play a critical role in defense against ROS: (a) glutathione peroxidase (GPX) is a H2O2 scavenger that uses GSH as a reductant, (b) glutathione reductase (GR) reduces GSSG using NADPH as a source of reducing power and maintaining the GSH/GSSG ratio in cells, (c) glutathione-S-transferase (GST) catalyzes the nucleophilic conjugation of GSH with several electrophilic substrates, and (d) glutaredoxins (GRXs), small redox proteins from the thioredoxin (TRX) superfamily, use GSH as electron donor. In this chapter, the role of GSH and its related enzymes was analyzed in free-living rhizobia and in the symbiosis with the legumes as well as the responses to different abiotic stresses (acid pH, saline, drought, and heavy metals/metalloids).

Signaling Role of ROS in Modulating Drought Stress Tolerance

Tolerance of plants to drought stress is a consequence of the activation of multicomponent signaling pathways to achieve cellular homeostasis and promote survival. Evidence of regulatory systems that link sensing and signaling of environmental conditions and the intracellular redox status have shed light on reactive oxygen species (ROS) as key components of transduction pathways. Thus, ROS play a dual role in the response of plants to abiotic stresses functioning as toxic by-products of stress metabolism, as well as important signal transduction molecules. The signaling pathway is accomplished by the balance between the intensity, duration, and subcellular localization of ROS generation. In this sense, the subcellular compartmentalization of oxidants and the associated changes in redox state are important aspects to be considered beyond the overall ROS cellular content. Hydrogen peroxide (H2O2), the main signaling molecule studied so far, is the most likely ROS to act as messenger because of its relative stability, and it can cross membranes through aquaporins. Nowadays, signaling mechanisms involve not only toxic molecules but also detoxification systems. Effective ROS signaling may require increased flux through antioxidant components, notably those that are thiol dependent. For signal transduction, ROS can interact with other signaling pathways such as activation of NADPH oxidase dependent on monomeric G protein, lipid-derived signals, induction of MAPK, redox-sensitive transcription factors, regulation of Ca2+, and hormones. In this chapter, aspects of reactive oxygen species as signaling molecules modulating drought stress tolerance are reported.