Salt acclimation in sorghum plants by exogenous proline: physiological and biochemical changes and regulation of proline metabolism.

KEY MESSAGE: Mitigation of deleterious effects of salinity promoted by exogenous proline can be partially explained by changes in proline enzymatic metabolism and expression of specific proline-related genes. Proline accumulation is a usual response to salinity. We studied the ability of exogenous proline to mitigate the salt harmful effects in sorghum (Sorghum bicolor) leaves. Ten-day-old plants were cultivated in Hoagland's nutrient solution in either the absence or presence of salinity (NaCl at 75 mM) and sprayed with distilled water or 30 mM proline solution. Salinity deleterious effects were alleviated by exogenous proline 14 days after treatment, with a return in growth and recovery of leaf area and photosynthetic parameters. Part of the salinity response reflected an improvement in ionic homeostasis, provided by reduction in Na+ and Cl- ions and increases in K+ and Ca2+ ions as well as increases of compatible solutes. In addition, the application of proline decreased membrane damage and did not increase relative water content. Proline-treated salt-stressed plants displayed increase in proline content, a response counterbalanced by punctual modulation in proline synthesis (down-regulation of Δ1-pyrroline-5-carboxylate synthetase activity) and degradation (up-regulation of proline dehydrogenase activity) enzymes. These responses were correlated with expression of specific proline-related genes (p5cs1 and prodh). Our findings clearly show that proline treatment results in favorable changes, reducing salt-induced damage and improving salt acclimation in sorghum plants.

Putative role of glutamine in the activation of CBL/CIPK signalling pathways during salt stress in sorghum.

The salt overly sensitive (SOS) pathway is the only mechanism known for Na+ extrusion in plant cells. SOS pathway activation involves Ca2+-sensing proteins, such as calcineurin B-like (CBL) proteins, and CBL-interacting protein kinases (CIPKs). In this signalling mechanism, a transit increase in cytosolic Ca2+ concentration triggered by Na+ accumulation is perceived by CBL (also known as SOS3). Afterward, SOS3 physically interacts with a CIPK (also known as SOS2), forming the SOS2/SOS3 complex, which can regulate the number downstream targets, controlling ionic homeostasis. For instance, the SOS2/SOS3 complex phosphorylates and activates the SOS1 plasmalemma protein, which is a Na+/H+ antiporter that extrudes Na+ out of the cell. The CBL-CIPK networking system displays specificity, complexity and diversity, constituting a critical response against salt stress and other abiotic stresses. In a study reported in the journal Plant and Cell Physiology, we showed that NH4+ induces the robust activation of transporters for Na+ homeostasis in root cells, especially the SOS1 antiporter and plasma membrane H+-ATPase, differently than does NO3-. Despite some studies having shown that external NH4+ ameliorates salt-induced effects on ionic homeostasis, there is no evidence that NH4+ per se or some product of its assimilation is responsible for these responses. Here, we speculate about the signalling role behind glutamine in CBL-CIPK modulation, which could effectively activate the SOS pathway in NH4+-fed stressed plants.

Integrative Control Between Proton Pumps and SOS1 Antiporters in Roots is Crucial for Maintaining Low Na+ Accumulation and Salt Tolerance in Ammonium-Supplied Sorghum bicolor.

An effective strategy for re-establishing K+ and Na+ homeostasis is a challenge for the improvement of plant performance in saline soil. Specifically, attempts to understand the mechanisms of Na+ extrusion from plant cells, the control of Na+ loading in the xylem and the partitioning of the accumulated Na+ between different plant organs are ongoing. Our goal was to provide insight into how an external nitrogen source affects Na+ accumulation in Sorghum bicolor under saline conditions. The NH4+ supply improved the salt tolerance of the plant by restricting Na+ accumulation and improving the K+/Na+ homeostasis in shoots, which was consistent with the high activity and expression of Na+/H+ antiporters and proton pumps in the plasma membrane and vacuoles in the roots, resulting in low Na+ loading in the xylem. Conversely, although NO3--grown plants had exclusion and sequestration mechanisms for Na+, these responses were not sufficient to reduce Na+ accumulation. In conclusion, NH4+ acts as an efficient signal to activate co-ordinately responses involved in the regulation of Na+ homeostasis in sorghum plants under salt stress, which leads to salt tolerance.

Proteomic analysis of salt stress and recovery in leaves of Vigna unguiculata cultivars differing in salt tolerance.

KEY MESSAGE: Cowpea cultivars differing in salt tolerance reveal differences in protein profiles and adopt different strategies to overcome salt stress. Salt-tolerant cultivar shows induction of proteins related to photosynthesis and energy metabolism. Salinity is a major abiotic stress affecting plant cultivation and productivity. The objective of this study was to examine differential proteomic responses to salt stress in leaves of the cowpea cultivars Pitiúba (salt tolerant) and TVu 2331 (salt sensitive). Plants of both cultivars were subjected to salt stress (75 mM NaCl) followed by a recovery period of 5 days. Proteins extracted from leaves of both cultivars were analyzed by two-dimensional electrophoresis (2-DE) under salt stress and after recovery. In total, 22 proteins differentially regulated by both salt and recovery were identified by LC-ESI-MS/MS. Our current proteome data revealed that cowpea cultivars adopted different strategies to overcome salt stress. For the salt-tolerant cultivar (Pitiúba), increase in abundance of proteins involved in photosynthesis and energy metabolism, such as rubisco activase, ribulose-5-phosphate kinase (Ru5PK) (EC 2.7.1.19), glycine decarboxylase (EC 1.4.4.2) and oxygen-evolving enhancer (OEE) protein 2, was observed. However, these vital metabolic processes were more profoundly affected in salt-sensitive cultivar (TVu), as indicated by the down-regulation of OEE protein 1, Mn-stabilizing protein-II, carbonic anhydrase (EC 4.2.1.1) and Rubisco (EC 4.1.1.39), leading to energy reduction and a decline in plant growth. Other proteins differentially regulated in both cultivars corresponded to different physiological responses. Overall, our results provide information that could lead to a better understanding of the molecular basis of salt tolerance and sensitivity in cowpea plants.

Catalase plays a key role in salt stress acclimation induced by hydrogen peroxide pretreatment in maize.

Pretreatment in plants is recognized as a valuable strategy to stimulate plant defenses, leading to better plant development. This study evaluated the effects of H2O2 leaf spraying pretreatment on plant growth and investigated the antioxidative mechanisms involved in the response of maize plants to salt stress. It was found that salinity reduced maize seedling growth when compared to control conditions, and H2O2 foliar spraying was effective in minimizing this effect. Analysis of the antioxidative enzymes catalase (EC 1.11.1.6), guaiacol peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.1) and superoxide dismutase (EC 1.15.1.1) revealed that H2O2 spraying increased antioxidant enzyme activities. Catalase (CAT) was the most responsive of these enzymes to H2O2, with higher activity early (48 h) in the treatment, while guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) were responsive only at later stages (240 h) of treatment. Increased CAT activity appears linked to gene expression regulation. Lower malondialdehyde levels were detected in plants with higher CAT activity, which may result from the protective function of this enzyme. Overall, we can conclude that pretreatment with H2O2 leaf spraying was able to reduce the deleterious effects of salinity on seedling growth and lipid peroxidation. These responses could be attributed to the ability of H2O2 to induce antioxidant defenses, especially CAT activity.

NH(4)(+)-stimulated low-K(+) uptake is associated with the induction of H(+) extrusion by the plasma membrane H(+)-ATPase in sorghum roots under K(+) deficiency.

The effect of external inorganic nitrogen and K(+) content on K(+) uptake from low-K(+) solutions and plasma membrane (PM) H(+)-ATPase activity of sorghum roots was studied. Plants were grown for 15 days in full-nutrient solutions containing 0.2 or 1.4mM K(+) and inorganic nitrogen as NO(3)(-), NO(3)(-)/NH(4)(+) or NH(4)(+) and then starved of K(+) for 24, 48 and 72 h. NH(4)(+) in full nutrient solution significantly affected the uptake efficiency and accumulation of K(+), and this effect was less pronounced at the high K(+) concentration. In contrast, the translocation rate of K(+) to the shoot was not altered. Depletion assays showed that plants grown with NH(4)(+) more efficiently depleted the external K(+) and reached higher initial rates of low-K(+) uptake than plants grown with NO(3)(-). One possible influence of K(+) content of shoot, but not of roots, on K(+) uptake was evidenced. Enhanced K(+)-uptake capacity was correlated with the induction of H(+) extrusion by PM H(+)-ATPase. In plants grown in high K(+) solutions, the increase in the active H(+) gradient was associated with an increase of the PM H(+)-ATPase protein concentration. In contrast, in plants grown in solutions containing 0.2mM K(+), only the initial rate of H(+)-pumping and ATP hydrolysis were affected. Under these conditions, two specific isoforms of PM H(+)-ATPase were detected, independent of the nitrogen source and deficiency period. No change in enzyme activity was observed in NO(3)(-)-grown plants. The results suggest that K(+) homeostasis in NH(4)(+)-grown sorghum plants may be regulated by a high capacity for K(+) uptake, which is dependent upon the H(+)-pumping activity of PM H(+)-ATPase.

Acumulação de biomassa e extração de nutrientes por plantas de feijão-de-corda irrigadas com água salina em diferentes estádios de desenvolvimento / Biomass accumulation and nutrient extraction by cowpea plants irrigated with saline water at different growth stage

A utilização de águas de moderada e alta salinidade na agricultura é uma realidade cada vez mais próxima, tendo em vista a limitada disponibilidade de águas de baixa salinidade para expansão da agricultura irrigada. O objetivo deste trabalho foi investigar os efeitos da aplicação de água salina sobre o crescimento e a extração de nutrientes pela cultura nos diferentes estádios de desenvolvimento de plantas de feijão-de-corda. O experimento foi conduzido em condições de campo, no delineamento em blocos ao acaso, com cinco tratamentos e cinco repetições. Os tratamentos empregados foram: T1 - água do poço (CEa de 0,8dS m-1) durante todo o ciclo; T2 - água salina (CEa de 5,0dS m-1) durante todo o ciclo; T3, T4 e T5 - água salina de zero a 22 dias após a semeadura (DAS), de 23 a 42DAS e de 43 a 62DAS, respectivamente. As plantas dos tratamentos T3, T4 e T5 foram irrigadas com água do poço nas demais fases do ciclo. Aos oito, 23, 43 e 63DAS, grupos de, no mínimo, quatro plantas foram coletadas para a determinação da produção de matéria seca e dos teores de Na, Cl, K, Ca, N, P, Fe, Cu, Zn e Mn. Foram avaliados os totais extraídos e a distribuição dos nutrientes na planta. A aplicação de água salina durante todo o ciclo (T2) e na germinação e fase inicial de crescimento (T3) inibiu e retardou, respectivamente, o crescimento da cultura. As plantas de feijão-de-corda extraíram os minerais analisados na seguinte ordem decrescente: N > K > Cl > Ca > Na > P > Fe > Mn > Zn > Cu, e a aplicação contínua de água salina (T2) reduziu a extração da maioria dos nutrientes, com exceção do Na. Os minerais Na, Cl, K, Ca, Fe e Mn permaneceram preferencialmente nas partes vegetativas, enquanto N e P foram translocados, em maiores proporções, aos frutos.

Due to the limited availability of low salinity waters, the use of water of moderate to high salinity in agriculture is a close reality in the expansion of irrigated farms. The objective of this research was to evaluate the effect of irrigation with saline water, applied at different development stages of cowpea plants, on growth and nutrient uptake. The experiment was set up in the field during the dry season. A completely randomized block design, with five treatments and five replications was adopted. The treatments studied were: T1 - groundwater with electrical conductivity (ECw) of 0.8dS m-1 during the whole crop cycle; T2 - saline water (ECw = 5.0dS m-1) during the whole crop cycle; T3, T4 and T5 - saline water from 0 to the 22nd day after sowing (DAS), from the 23rd to the 42nd DAS and from the 43rd to 62nd DAS, respectively. The plants subjected to T3, T4 and T5 were irrigated with groundwater in the other stages of the crop cycle. At 8, 23, 43 and 63DAS plants were collected for evaluation of plant growth, Na, Cl, K, Ca, N, P, Fe, Cu, Zn and Mn contents and distribution in plant parts. The application of saline water during the whole crop cycle (T2) and during the germination and initial plant development (T3) caused, respectively, inhibition and retardation of plant growth. Cowpea plants removed the minerals in the following decreasing sequence: N > K > Cl > Ca > Na > P > Fe > Mn > Zn > Cu, but the continuous use of saline water (T2) reduced the total uptake of all nutrients, except for Na. The minerals Na, Cl, K, Ca, Fe and Mn were distributed preferentially in the vegetative parts of the plant, while most of the N and P were translocated to the pods.

Cowpea ribonuclease: properties and effect of NaCl-salinity on its activation during seed germination and seedling establishment.

Pitiúba cowpea [Vigna unguiculata (L.) Walp] seeds were germinated in distilled water (control treatment) or in 100 mM NaCl solution (salt treatment), and RNase was purified from different parts of the seedlings. Seedling growth was reduced by the NaCl treatment. RNase activity was low in cotyledons of quiescent seeds, but the enzyme was activated during germination and seedling establishment. Salinity reduced cotyledon RNase activity, and this effect appeared to be due to a delay in its activation. The RNases from roots, stems, and leaves were immunologically identical to that found in cotyledons. Partially purified RNase fractions from the different parts of the seedling showed some activity with DNA as substrate. However, this DNA hydrolyzing activity was much lower than that of RNA hydrolyzing activity. The DNA hydrolyzing activity was strongly inhibited by Cu(2+), Hg(2+), and Zn(2+) ions, stimulated by MgCl(2), and slowly inhibited by EDTA. This activity from the most purified fraction was inhibited by increasing concentrations of RNA in the reaction medium. It is suggested that the major biological role of this cotyledon RNase would be to hydrolyze seed storage RNA during germination and seedling establishment, and it was discussed that it might have a protective role against abiotic stress during later part of seedling establishment.

Hydrogen peroxide pre-treatment induces salt-stress acclimation in maize plants.

The effect of exogenously applied H2O2 on salt stress acclimation was studied with regard to plant growth, lipid peroxidation, and activity of antioxidative enzymes in leaves and roots of a salt-sensitive maize genotype. Pre-treatment by addition of 1 microM H2O2 to the hydroponic solution for 2 days induced an increase in salt tolerance during subsequent exposure to salt stress. This was evidenced by plant growth, lipid peroxidation and antioxidative enzymes measurements. In both leaves and roots the variations in lipid peroxidation and antioxidative enzymes (superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, glutathione reductase, and catalase) activities of both acclimated and unacclimated plants, suggest that differences in the antioxidative enzyme activities may, at least in part, explain the increased tolerance of acclimated plants to salt stress, and that H2O2 metabolism is involved as signal in the processes of maize salt acclimation.