Aquaporin functionality in relation to H+-ATPase activity in root cells of Capsicum annuum grown under salinity

An environmental field assessment of soil quality and phytoremediation of toxic metals from saline soil by selected halophytes

The current study has aims to investigate the soil quality and phytoextraction of cadmium (Cd), chromium (Cr), and lead (Pb) from saline soils using Alhagi maurorum (camelthorn), Tamarix aphylla (saltcedar), Salvadora persica (mustard bush), and Suaeda nigra (bush seep weed). The saline bulk soil, rhizospheric soil, and different parts of selected plants were oxidized using the acid mixture and determined Cd, Cr, and Pb by atomic absorption spectrometry. The bio-concentration factor (BCF) and translocation factor (T_F) of also examined. The quality parameters of soil like pH (< 8.5), and electrical conductivity (EC; > 4.00 dS m^-1) indicated the soil is saline. The salinity of soil was lower the organic matters, and total nitrogen contents in studied saline bulk soil due to deterioration condition of soils. However, the rhizospheric soil showed the improved quality of saline soil reflected the good phytoextraction of salts from saline soil. The high contents of Cd in roots and shoots (1.02 and 0.65 µg g^-1) of Alhagi maurorum, Cr in the roots and shoots (6.20, and 6.75 µg g^-1) of Tamarix aphylla and Pb in the roots and shoots (5.63, and 5.75 µg g^-1) of Suaeda nigra. The BCF and T_F showed the Tamarix aphylla and Alhagi maurorum for Pb, Alhagi maurorum, and Salvadora persica for Cr considered as hyperaccumulator plants. Based on BCF and T_F values of Alhagi maurorum, Tamarix aphylla for Cd, and Salvadora persica for Cr and Pb have the efficiency to uptake toxic metals from saline soil. Thus, it can be concluded that selected plant species may have ability for the phytoextraction the Cd, Cr and Pb from saline soil.

Root hydraulics adjustment is governed by a dominant cell-to-cell pathway in Beta vulgaris seedlings exposed to salt stress

Soil salinity reduces root hydraulic conductivity (L_pr) of several plant species. However, how cellular signaling and root hydraulic properties are linked in plants that can cope with water restriction remains unclear. In this work, we exposed the halotolerant species red beet (Beta vulgaris) to increasing concentrations of NaCl to determine the components that might be critical to sustaining the capacity to adjust root hydraulics. Our strategy was to use both hydraulic and cellular approaches in hydroponically grown seedlings during the first osmotic phase of salt stress. Interestingly, L_pr presented a bimodal profile response apart from the magnitude of the imposed salt stress. As well as L_pr, the PIP2-aquaporin profile follows an unphosphorylated/phosphorylated pattern when increasing NaCl concentration while PIP1 aquaporins remain constant. L_pr also shows high sensitivity to cycloheximide. In low NaCl concentrations, L_pr was high and 70 % of its capacity could be attributed to the CHX-inhibited cell-to-cell pathway. More interestingly, roots can maintain a constant spontaneous exudated flow that is independent of the applied NaCl concentration. In conclusion, Beta vulgaris root hydraulic adjustment completely lies in a dominant cell-to-cell pathway that contributes to satisfying plant water demands.

Discerning the mechanism of the multiwalled carbon nanotubes effect on root cell water and nutrient transport

Multiwalled carbon nanotubes (MWCNTs) are tubular carbon structures that are able to enter cells through holes in the plasma membrane and produce changes in gene expression. In this work, we compared the functionality of carbon nanotubes with the electroporation that perforates membranes, in Brassica oleracea var. Italica (broccoli) root protoplasts. For this, we combined those treatments with control conditions and abiotic stress (salinity) in order to elucidate if the response is related to conditions optimal for the plant. The measurement of the osmotic water permeability (Pf), mineral concentrations and expression levels of aquaporins (PIP1s and PIP2s) revealed that the physiological action of the nanotubes was similar to that achieved with electroporation for both Pf and the concentrations of nutrients in the protoplasts. On the other hand, PIP1s and PIP2s expression was increased in the protoplasts receiving the control plus MWCNTs treatment but not in those treated with electroporation. This opens new and interesting lines, as it shows that nanotubes are able to modulate the expression of aquaporins.

Mechanisms of Silicon-Mediated Amelioration of Salt Stress in Plants

Silicon (Si), the second most predominant element in the earth crust consists of numerous benefits to plant. Beneficial effect of Si has been apparently visible under both abiotic and biotic stress conditions in plants. Supplementation of Si improved physiology and yield on several important agricultural and horticultural crops. Salinity is one of the major abiotic stresses that affect growth and yield. The presence of high concentration of salt in growing medium causes oxidative, osmotic, and ionic stresses to plants. In extreme conditions salinity affects soil, ground water, and limits agricultural production. Si ameliorates salt stress in several plants. The Si mediated stress mitigation involves various regulatory mechanisms such as photosynthesis, detoxification of harmful reactive oxygen species using antioxidant and non-antioxidants, and proper nutrient management. In the present review, Si mediated alleviation of salinity stress in plants through the regulation of photosynthesis, root developmental changes, redox homeostasis equilibrium, and regulation of nutrients have been dealt in detail.

Silicon-mediated Improvement in Plant Salinity Tolerance: The Role of Aquaporins

Silicon (Si) is an abundant and differentially distributed element in soils that is believed to have important biological functions. However, the benefits of Si and its essentiality in plants are controversial due to differences among species in their ability to take up this element. Despite this, there is a consensus that the application of Si improves the water status of plants under abiotic stress conditions. Hence, plants treated with Si are able to maintain a high stomatal conductance and transpiration rate under salt stress, suggesting that a reduction in Na⁺ uptake occurs due to deposition of Si in the root. In addition, root hydraulic conductivity increases when Si is applied. As a result, a Si-mediated upregulation of aquaporin (PIP) gene expression is observed in relation to increased root hydraulic conductivity and water uptake. Aquaporins of the subclass nodulin 26-like intrinsic proteins are further involved in allowing Si entry into the cell. Therefore, on the basis of available published results and recent developments, we propose a model to explain how Si absorption alleviates stress in plants grown under saline conditions through the conjugated action of different aquaporins.

Intrinsic stability of Brassicaceae plasma membrane in relation to changes in proteins and lipids as a response to salinity

Changes in plasma membrane lipids, such as sterols and fatty acids, have been observed as a result of salt stress. These alterations, together with modification of the plasma membrane protein profile, confer changes in the physical properties of the membrane to be taken into account for biotechnological uses. In our experiments, the relationship between lipids and proteins in three different Brassicaceae species differing in salinity tolerance (Brassica oleracea, B. napus and Cakile maritima) and the final plasma membrane stability were studied. The observed changes in the sterol (mainly an increase in sitosterol) and fatty acid composition (increase in RUFA) in each species led to physical adaptation of the plasma membrane to salt stress. The in vitro vesicles stability was higher in the less tolerant (B. oleracea) plants together with low lipoxygenase activity. These results indicate that the proteins/lipids ratio and lipid composition is an important aspect to take into account for the use of natural vesicles in plant biotechnology.

Some rootstocks improve pepper tolerance to mild salinity through ionic regulation

Grafting has been proposed as an interesting strategy that improves the responses of crops under salinity. In pepper, we reported increased fruit yield of the commercial 'Adige' cultivar under salinity when grafted onto accessions Capsicum chinense Jacq. 'ECU-973' (12) and Capsicum baccatum L. var. pendulum 'BOL-58' (14), whereas no effect was observed when grafted onto accession Capsicum annuum L var. 'Serrano' (5). We also analysed the physiological and biochemical mechanisms related to the tolerance conferred by these rootstocks. Responses to salinity (40 mM NaCl) were studied in the different plant combinations for 30 days by determining water relations, mineral content, proline accumulation, photosynthetic parameters, nitrate reductase activity and antioxidant capacity. Higher salt tolerance was achieved when the 'Adige' cultivar was grafted onto the 12 genotype, which allowed not only lower Na(+) and Cl(-) accumulation in the scion, but also ion selectivity maintenance, particularly Na(+)/K(+) discrimination. These traits led to a minor negative impact on photosynthesis, nitrate reductase activity and lipid peroxidation in grafted scion leaves. This work suggests that using tolerant pepper rootstocks that maintain the scion's ion homeostasis is a promising strategy to provide salinity tolerance and can consequently improve crop yield.

Involvement of a glucosinolate (sinigrin) in the regulation of water transport in Brassica oleracea grown under salt stress

Members of the Brassicaceae are known for their contents of nutrients and health-promoting phytochemicals, including glucosinolates. The concentrations of these chemopreventive compounds (glucosinolate-degradation products, the bioactive isothiocyanates) may be modified under salinity. In this work, the effect of the aliphatic glucosinolate sinigrin (2-propenyl-glucosinolate) on plant water balance, involving aquaporins, was explored under salt stress. For this purpose, water uptake and its transport through the plasma membrane were determined in plants after NaCl addition, when sinigrin was also supplied. We found higher hydraulic conductance (L0 ) and water permeability (Pf ) and increased abundance of PIP2 aquaporins after the direct administration of sinigrin, showing the ability of the roots to promote cellular water transport across the plasma membrane in spite of the stress conditions imposed. The higher content of the allyl-isothiocyanate and the absence of sinigrin in the plant tissues suggest that the isothiocyanate is related to water balance; in fact, a direct effect of this nitro-sulphate compound on water uptake is proposed. This work provides the first evidence that the addition of a glucosinolate can regulate aquaporins and water transport: this effect and the mechanism(s) involved merit further investigation.

The physiological response of Populus tremula x alba leaves to the down-regulation of PIP1 aquaporin gene expression under no water stress

In order to study the role of PIP1 aquaporins in leaf water and CO2 transport, several lines of PIP1-deficient transgenic Populus tremula x alba were generated using a reverse genetic approach. These transgenic lines displayed no visible developmental or morphological phenotypes when grown under conditions of no water stress. Major photosynthetic parameters were also not affected by PIP1 down regulation. However, low levels of PIP1 expression resulted in greater leaf hydraulic resistance (an increase of 27%), which effectively implicated PIP1 role in water transport. Additionally, the expression level of PIP1 genes in the various transgenic lines was correlated with reductions in mesophyll conductance to CO2 (gm), suggesting that in poplar, these aquaporins influenced membrane permeability to CO2. Overall, although analysis showed that PIP1 genes contributed to the mass transfer of water and CO2 in poplar leaves, their down-regulation did not dramatically impair the physiological needs of this fast growing tree when cultivated under conditions of no stress.

Boric acid and salinity effects on maize roots. Response of aquaporins ZmPIP1 and ZmPIP2, and plasma membrane H+-ATPase, in relation to water and nutrient uptake

Under saline conditions, an optimal cell water balance, possibly mediated by aquaporins, is important to maintain the whole-plant water status. Furthermore, excessive accumulation of boric acid in the soil solution can be observed in saline soils. In this work, the interaction between salinity and excess boron with respect to the root hydraulic conductance (L(0)), abundance of aquaporins (ZmPIP1 and ZmPIP2), ATPase activity and root sap nutrient content, in the highly boron- and salt-tolerant Zea mays L. cv. amylacea, was evaluated. A downregulation of root ZmPIP1 and ZmPIP2 aquaporin contents were observed in NaCl-treated plants in agreement with the L(0) measurements. However, in the H3BO3-treated plants differences in the ZmPIP1 and ZmPIP2 abundance were observed. The ATPase activity was related directly to the amount of ATPase protein and Na+ concentration in the roots, for which an increase in NaCl- and H3BO3+ NaCl-treated plants was observed with respect to untreated and H3BO3-treated plants. Although nutrient imbalance may result from the effect of salinity or H3BO3 alone, an ameliorative effect was observed when both treatments were applied together. In conclusion, our results suggest that under salt stress, the activity of specific membrane components can be influenced directly by boric acid, regulating the functions of certain aquaporin isoforms and ATPase as possible components of the salinity tolerance mechanism.

Changes in phosphate content and phosphatase activities in rice seedlings exposed to arsenite

The effect of arsenite (As2O3) in situ on the level of the phosphate pool and activities of phosphohydrolytic enzymes was examined in rice (Oryza sativa L.) seedlings grown for 5-20 d in sand cultures. The effects were manifested via a decline in phosphate content and inhibition of the activities of key phosphatases. Application of 50 µM As2O3 in situ resulted in 34 to 77% inhibition of acid phosphatase activity in roots and about 38 to 50% inhibition of activity in shoots of 15-20-d-old seedlings. Similarly, alkaline phosphatase activity was inhibited in shoots under in situ As (III) toxicity. Varietal as well as organ specific differences were observed in the response of inorganic pyrophosphatase activity to in situ As (III) treatment. A moderately toxic in situ As2O3 level of 25 µM as well as a highly toxic level of 50 µM inhibited mitochondrial-ATPase activity whereas 25 µM As (III) stimulated the chloroplastic isoform of ATPase but at a higher level (50 µM) As (III) was inhibitory. The results suggest that exposure of rice plants to arsenite leads to lowering of the phosphate pool and alteration in the activities of key phosphohydrolytic enzymes which might contribute to metabolic perturbations and decreased growth of rice plants in an As (III) polluted environment.

Different cation stresses affect specifically osmotic root hydraulic conductance, involving aquaporins, ATPase and xylem loading of ions in Capsicum annuum, L. plants

In order to study the effect of nutrient stress on water uptake in pepper plants (Capsicum annuum L.), the excess or deficiency of the main cations involved in plant nutrition (K(+), Mg(2+), Ca(2+)) and two different degrees of salinity were related to the activity of plasma membrane H(+)-ATPase, the pH of the xylem sap, nutrient flux into the xylem (J(s)) and to a number of parameters related to water relations, such as root hydraulic conductance (L(0)), stomatal conductance (g(s)) and aquaporin activity. Excess of K(+), Ca(+) and NaCl produced a toxic effect on L(0) while Mg(2+) starvation produced a positive effect, which was in agreement with aquaporin functionality, but not with ATPase activity. The xylem pH was altered only by Ca treatments. The results obtained with each treatment could suggest that detection of the quality of the nutrient supply being received by roots can be related to aquaporins functionality, but also that each cation stress triggers specific responses that have to be assessed individually.

Plant aquaporins: new perspectives on water and nutrient uptake in saline environment

The mechanisms of salt stress and tolerance have been targets for genetic engineering, focusing on ion transport and compartmentation, synthesis of compatible solutes (osmolytes and osmoprotectants) and oxidative protection. In this review, we consider the integrated response to salinity with respect to water uptake, involving aquaporin functionality. Therefore, we have concentrated on how salinity can be alleviated, in part, if a perfect knowledge of water uptake and transport for each particular crop and set of conditions is available.

New evidence about the relationship between water channel activity and calcium in salinity-stressed pepper plants

This study, of how Ca2+ availability (intracellular, extracellular or linked to the membrane) influences the functionality of aquaporins of pepper (Capsicum annuum L.) plants grown under salinity stress, was carried out in plants treated with NaCl (50 mM), CaCl2 (10 mM), and CaCl2 (10 mM) + NaCl (50 mM). For this, water transport through the plasma membrane of isolated protoplasts, and the involvement of aquaporins and calcium (extracellular, intracellular and linked to the membrane) has been determined. After these treatments, it could be seen that the calcium concentration was reduced in the apoplast, in the cells and on the plasma membrane of roots of pepper plants grown under saline conditions; these concentrations were increased or restored when extra calcium was added to the nutrient solution. Protoplasts extracted from plants grown under Ca2+ starvation showed no aquaporin functionality. However, for the protoplasts to which calcium was added, an increase of aquaporin functionality of the plasma membrane was observed [osmotic water permeability (Pf) inhibition after Hg addition]. Interestingly, when verapamil (a Ca2+ channel blocker) was added, no functionality was observed, even when Ca2+ was added with verapamil. Therefore, calcium seems to be involved in plasma membrane aquaporin regulation via a chain of processes within the cell but not by alteration of the stability of the plasma membrane.