Eduardo Luiz Voigt

Partial oxidative protection by enzymatic and non-enzymatic components in cashew leaves under high salinity

The work evaluated the role of enzymatic and non-enzymatic antioxidants in cashew (Anacardium occidentale) leaves under 0, 50, 100, 150 and 200 mM NaCl. Salt stress increased protein oxidation and decreased the lipid peroxidation, indicating that lipids are less susceptible to oxidative damage. The superoxide dismutase (SOD) activity was not changed, ascorbate peroxidase (APX) activity steadily decreased while the catalase (CAT) activity strongly increased with the increasing NaCl concentration. High salinity also induced alterations in the ascorbate (AsA) and glutathione (GSH) redox state. The salt resistance in cashew may be associated with maintaining of SOD activity and upregulation of CAT activity in concert with the AsA and GSH antioxidants.

Salt-induced changes in antioxidative enzyme activities in root tissues do not account for the differential salt tolerance of two cowpea cultivars

The salt stress effect in root growth and antioxidative response were investigated in two cowpea cultivars which differ in salt tolerance in terms of plant growth and leaf oxidative response. Four-day-old seedlings (establishment stage) were exposed to 100 mM NaCl for two days. The roots of the two cultivars presented distinct response in terms of salt-induced changes in elongation and dry weight. Root dry weight was only decreased in Perola (sensitive) cultivar while root elongation was mainly hampered in Pitiuba (tolerant). Root relative water content remained unchanged under salinity, but root Na+ content achieved toxic levels as revealed by the K+/Na+ ratio in both cultivars. Then, root growth inhibition might be due to ionic toxicity rather than by salt-induced water deficit. Although electrolyte leakage markedly increased mainly in the Perola genotype, lipid peroxidation decreased similarly in both salt-stressed cultivars. APX and SOD activities were reduced by salinity in both cultivars reaching similar values despite the decrease in Pitiuba had been higher compared to respective controls. CAT decreased significantly in Pitiuba but did not change in Perola, while POX increased in both cultivars. The salt-induced decrease in the CAT activity of Pitiuba root is, at principle, incompatible to allow a more effective oxidative protection. Our results support the idea that the activities of SOD, APX, CAT and POX and lipid peroxidation in cowpea seedling roots were not associated with differential salt tolerance as previously characterized in terms of growth rate and oxidative response in plant leaves.

Differences in Cowpea Root Growth Triggered by Salinity and Dehydration are Associated with Oxidative Modulation Involving Types I and III Peroxidases and Apoplastic Ascorbate

The aim of this work was to investigate the balance between the activities of ascorbate peroxidase (APX) and phenol peroxidases (POD) and cowpea root growth in response to dehydration and salt stress. Root growth and indicators of oxidative response were markedly changed in response to salinity and dehydration. Salt treatment strongly inhibited root elongation, which was associated with an increase in lignin content and a significant decrease in the concentrations of apoplastic hydrogen peroxide (H2O2) and ascorbate. In conditions of extreme salinity, cytosol–APX activity was significantly decreased. In contrast, cell-wall POD activity was greatly increased, whereas lipid peroxidation was unchanged. These results indicate that POD could be involved in both H2O2 scavenging and the inhibition of root elongation under high salinity. In contrast, dehydration stimulated primary root elongation and increased lipid peroxidation and apoplastic ascorbate content, but it did not change APX and POD activities or H2O2 concentration. When cowpea roots were subjected to salinity followed by dehydration, the water and pressure potentials were decreased, and lipid peroxidation was markedly increased, highlighting the additive nature of the inhibitory effects caused by salt and dehydration. The proline concentration was markedly increased by dehydration alone, as well as by salt followed by dehydration, suggesting a possible role for proline in osmotic adjustment. Salinity and dehydration induce contrasting responses in the growth and morphology of cowpea roots. These effects are associated with different types of oxidative modulation involving cytosolic-APX and cell-wall POD activities and apoplast H2O2 and ascorbate levels.

Atividade de enzimas antioxidantes e inibição do crescimento radicular de feijão caupi sob diferentes níveis de salinidade

Phenol peroxidase (POX) is a dual enzyme that is involved with hydrogen peroxide scavenging and lignin biosynthesis, contributing to growth inhibition by secondary wall thickening. In order to relate growth inhibition to salt-induced oxidative modulation, the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and POX were evaluated in cowpea roots under salinity. Four-day-old seedlings of the Pitiuba and Perola cultivars were exposed to 0, 25, 50, 75 and 100 mM NaCl in germination paper under controlled conditions. After two days of treatment, root length was reduced under 100 mM NaCl by 56 and 26% in Pitiuba and Perola, respectively, which was associated with enhanced electrolyte leakage and cell death in the root apex. NaCl salinity did not trigger lipid peroxidation, indicating that cell death was probably due to membrane damage instead of oxidative stress. Salt stress reduced the activity of SOD, CAT and APX and increased the POX activity, demonstrating that this enzyme plays a role in oxidative protection in cowpea roots exposed to NaCl salinity. In conclusion, salt-induced growth inhibition in cowpea roots could be attributed, at least in part, to a coordinate action involving an increase in POX activity and a drop in CAT and APX activities.

Involvement of cation channels and NH4 +-sensitive K+ transporters in Na+ uptake by cowpea roots under salinity

Na+ accumulation was investigated in the roots of 11-d-old cowpea [Vigna unguiculata (L.) Walp.] plants. The relative contribution of different membrane transporters on Na+ uptake was estimated by applying Ca2+, K+, NH4+, and pharmacological inhibitors. Na+ accumulation into the root symplast was decreased by half in the presence of 1 mM Ca2+ and it was almost abolished by 100 mM K+. The inhibitory effect of external NH4+ on Na+ accumulation was more pronounced in the roots of NH4+-free growing plants. Na+ accumulation was reduced about 73 % by 0.1 mM flufenamate and it was almost blocked by 2 mM quinine. In addition, 20 mM tetraethylammonium and 1.0 mM Cs+ decreased Na+ accumulation by 28 and 30 %, respectively. These results evidenced that low-affinity Na+ uptake by cowpea roots depends on Ca2+-sensitive and Ca2+-insensitive pathways. The Ca2+-sensitive pathway is probably mediated by nonselective cation channels and the Ca2+-insensitive one may involve K+ channels and to a lesser extent NH4+-sensitive K+ transporters.

Salt-induced delay in cotyledonary globulin mobilization is abolished by induction of proteases and leaf growth sink strength at late seedling establishment in cashew.

Seedling establishment in saline conditions is crucial for plant survival and productivity. This study was performed to elucidate the biochemical and physiological mechanisms involved with the recovery and establishment of cashew seedlings subjected to salinity. The changes in the Na+ levels and K/Na ratios, associated with relative water content, indicated that osmotic effects were more important than salt toxicity in the inhibition of seedling growth and cotyledonary protein mobilization. Salinity (50mM NaCl) induced a strong delay in protein breakdown and amino acid accumulation in cotyledons, and this effect was closely related to azocaseinolytic and protease activities. In parallel, proline and free amino acids accumulated in the leaves whereas the protein content decreased. Assays with specific inhibitors indicated that the most important proteases in cotyledons were of serine, cysteine and aspartic types. Proteomic analysis revealed that most of the cashew reserve proteins are 11S globulin-type and that these proteins were similarly degraded under salinity. In the late establishment phase, the salt-treated seedlings displayed an unexpected recovery in terms of leaf growth and N mobilization from cotyledon to leaves. This recovery coordinately involved a great leaf expansion, decreased amino acid content and increased protein synthesis in leaves. This response occurred in parallel with a prominent induction in the cotyledon proteolytic activity. Altogether, these data suggest that a source-sink mechanism involving leaf growth and protein synthesis may have acted as an important sink for reserve mobilization contributing to the seedling establishment under salinity. The amino acids that accumulated in the leaves may have exerted negative feedback to act as a signal for the induction of protease activity in the cotyledon. Overall, these mechanisms employed by cashew seedlings may be part of an adaptive process for the efficient rescue of cotyledonary proteins, as the cashew species originates from an environment with N-poor soil and high salinity.

Modulation of Reserve Mobilization by Sucrose, Glutamine, and Abscisic Acid During Seedling Establishment in Sunflower

We carried out in vitro feeding experiments using sunflower as a model to differentiate the modulatory effects of metabolites (sucrose and glutamine) and hormones (gibberellic acid and abscisic acid) on reserve mobilization, metabolite partitioning, and key enzyme activities. Exogenous sucrose negatively not only modulated the mobilization of carbon reserves (oils and starch), but it also delayed the degradation of nitrogen reserves (storage proteins) in the cotyledons. Similarly, exogenous glutamine negatively not only modulated storage protein hydrolysis, but it also retarded oil and starch degradation. Different from the metabolites, exogenous abscisic acid affected only the mobilization of oils and storage proteins. Sucrose and glutamine caused non-reducing sugar accumulation in the cotyledons and axis, but abscisic acid did not change the content of these compounds in both seedling parts. Curiously, glutamine failed to cause amino acid accumulation in the cotyledons and abscisic acid increased the amino acid content in both cotyledons and axis. Gibberellic acid did not stimulate reserve mobilization and metabolite consumption. Although the mobilization of oils, storage proteins, and starch has been delayed by sucrose and glutamine, these metabolites augmented the activity of isocitrate lyase, acid proteases, and amylases. Only abscisic acid reduced amylase activity and increased glutamine synthetase activity. Accordingly, sucrose and glutamine exert a “crossed effect” on reserve mobilization, that is, sucrose delays storage protein hydrolysis and glutamine retards oil and starch degradation. These effects may be mediated by non-reducing sugars and they are, at least in part, different from those exerted by abscisic acid.