Plastidial starch phosphorylase is highly associated with starch accumulation process in developing squash (Cucurbita sp.) fruit

Potassium (K⁺) is a macronutrient known for its high mobility and positive charge, which allows efficient and fast control of the electrical balance and osmotic potential in plant cells. Such features allow K⁺ to remarkably contribute to plant stress adaptation. Some agricultural lands are deficient in K⁺, imposing a stress that reduces crop yield and makes fertilization a common practice. However, individual stress conditions in the field are rare, and crops usually face a combination of different stresses. As plant response to a stress combination cannot always be deduced from individual stress action, it is necessary to gain insights into the specific mechanisms that connect K⁺ homeostasis with other stress effects to improve plant performance in the context of climate change. Surprisingly, plant responses to environmental stresses under a K⁺‐limiting scenario are poorly understood. In the present review, we summarize current knowledge and find substantial gaps regarding specific outcomes of K⁺ deficiency in addition to other environmental stresses. In this regard, combined nutrient deficiencies of K⁺ and other macronutrients are covered in the first part of the review and interactions arising from K⁺ deficiency with salinity, drought and biotic factors in the second part. Information available so far suggests a prominent role of potassium and nitrate transport systems and their regulatory proteins in the response of plants to several stress combinations. Thus, such molecular pathways, which are located at the crossroad between K⁺ homeostasis and environmental stresses, could be considered biotechnological targets in future studies.     

Cellular and tissue distribution of potassium: Physiological relevance,mechanisms and regulation

Potassium (K⁺) is the most important cationic nutrient for all living organisms. Its cellular levels are significant (typically around 100 mM) and are highly regulated. In plants K⁺ affects multiple aspects such as growth, tolerance to biotic and abiotic stress and movement of plant organs. These processes occur at the cell, organ and whole plant level and not surprisingly, plants have evolved sophisticated mechanisms for the uptake, efflux and distribution of K⁺ both within cells and between organs.     

The role of plant cation/proton antiporter gene family in salt tolerance

Salinity is one of the major abiotic constraints to agriculture. The physiological and molecular mechanisms of salt tolerance have been studied in plants for many years. The regulation of osmosis and ion homeostasis is crucial. A lot of important components involved in plant responses to salt stress have been identified. Among them, ion transporters and channels take an essential role in ion homeostasis, mainly for Na⁺, Cl^-, and K⁺. Until now, many cation antiporters important for salt tolerance in plants have been characterized. Among them, the monovalent cation/proton antiporters (CPA) family is one of the most important families, including sodium proton exchangers (NHXs), K⁺-efflux antiporters (KEAs), and cation/H⁺ exchangers (CHXs). Here, the current knowledge of the plant CPA family in responses to salt stress is reviewed. The regulation mechanisms were also included and discussed.     

Expression of animal CED-9 anti-apoptotic gene in tobacco modifies plasma membrane ion fluxes in response to salinity and oxidative stress

Apoptosis, one form of programmed cell death (PCD), plays an important role in mediating plant adaptive responses to the environment. Recent studies suggest that expression of animal anti-apoptotic genes in transgenic plants may significantly improve a plant’s ability to tolerate a variety of biotic and abiotic stresses. The underlying cellular mechanisms of this process remain unexplored. In this study, we investigated specific ion flux “signatures” in Nicotiana benthamiana plants transiently expressing CED-9 anti-apoptotic gene and undergoing salt- and oxidative stresses. Using a range of electrophysiological techniques, we show that expression of CED-9 increased plant salt and oxidative stress tolerance by altering K⁺ and H⁺ flux patterns across the plasma membrane. Our data shows that PVX/CED-9 plants are capable of preventing stress-induced K⁺ efflux from mesophyll cells, so maintaining intracellular K⁺ homeostasis. We attribute these effects to the ability of CED-9 to control at least two types of K⁺-permeable channels; outward-rectifying depolarization-activating K⁺ channels (KOR) and non-selective cation channels (NSCC). A possible scenario linking CED-9 expression and ionic relations in plant cell is suggested. To the best of our knowledge, this study is the first to link “ion flux signatures” and mechanisms involved in regulation of PCD in plants.     

The combined effect of salinity and heat reveals a specific physiological,biochemical and molecular response in tomato plants

Many studies have described the response mechanisms of plants to salinity and heat applied individually; however, under field conditions some abiotic stresses often occur simultaneously. Recent studies revealed that the response of plants to a combination of two different stresses is specific and cannot be deduced from the stresses applied individually. Here, we report on the response of tomato plants to a combination of heat and salt stress. Interestingly, and in contrast to the expected negative effect of the stress combination on plant growth, our results show that the combination of heat and salinity provides a significant level of protection to tomato plants from the effects of salinity. We observed a specific response of plants to the stress combination that included accumulation of glycine betaine and trehalose. The accumulation of these compounds under the stress combination was linked to the maintenance of a high K⁺ concentration and thus a lower Na⁺/K⁺ ratio, with a better performance of the cell water status and photosynthesis as compared with salinity alone. Our findings unravel new and unexpected aspects of the response of plants to stress combination and provide a proposed list of enzymatic targets for improving crop tolerance to the abiotic field environment.     

The molecular mechanism of plasma membrane H+-ATPases in plant responses to abiotic stress

Plasma membrane H⁺-ATPases (PM H⁺-ATPases) are critical proton pumps that export protons from the cytoplasm to the apoplast. The resulting proton gradient and difference in electrical potential energize various secondary active transport events. PM H⁺-ATPases play essential roles in plant growth, development, and stress responses. In this review, we focus on recent studies of the mechanism of PM H⁺-ATPases in response to abiotic stresses in plants, such as salt and high pH, temperature, drought, light, macronutrient deficiency, acidic soil and aluminum stress, as well as heavy metal toxicity. Moreover, we discuss remaining outstanding questions about how PM H⁺-ATPases contribute to abiotic stress responses.     

An Appraisal of Ancient Molecule GABA in Abiotic Stress Tolerance in Plants,and Its Crosstalk with Other Signaling Molecules

Gamma-aminobutyric acid (GABA), a non-proteinaceous amino acid, is reported in prokaryotes and eukaryotes, since ancient times. However, it has gained attention in the present time because of its rapid accumulation during stressed conditions in plants as well as in the cyanobacteria. In plants, it regulates the number of physiological processes such as pollen tube growth, root growth, TCA cycle, N₂-metabolism, and osmoregulation. Several biotic and abiotic stresses prevail in the environment, which lead to enhanced accumulation of reactive oxygen species (ROS) thus causing oxidative damage. However, a rapid increase in the accumulation of GABA during stress in various plant forms like bacteria, cyanobacteria, fungi, and plants indicates its putative role in stress regulation and acclimation. This review summarizes the biosynthesis of GABA, its role in abiotic stress tolerance, and its crosstalk with ROS, nitric oxide, Ca⁺² ions, phytohormones, and polyamines in stress acclimation.

     

Herbivore exposure alters ion fluxes and improves salt tolerance in a desert shrub

Plants have evolved complex mechanisms that allow them to withstand multiple environmental stresses, including biotic and abiotic stresses. Here, we investigated the interaction between herbivore exposure and salt stress of Ammopiptanthus nanus, a desert shrub. We found that jasmonic acid (JA) was involved in plant responses to both herbivore attack and salt stress, leading to an increased NaCl stress tolerance for herbivore-pretreated plants and increase in K⁺/Na⁺ ratio in roots. Further evidence revealed the mechanism by which herbivore improved plant NaCl tolerance. Herbivore pretreatment reduced K⁺ efflux and increased Na⁺ efflux in plants subjected to long-term, short-term, or transient NaCl stress. Moreover, herbivore pretreatment promoted H⁺ efflux by increasing plasma membrane H⁺-adenosine triphosphate (ATP)ase activity. This H⁺ efflux creates a transmembrane proton motive force that drives the Na⁺/H⁺ antiporter to expel excess Na⁺ into the external medium. In addition, high cytosolic Ca²⁺ was observed in the roots of herbivore-treated plants exposed to NaCl, and this effect may be regulated by H⁺-ATPase. Taken together, herbivore exposure enhance s A. nanus tolerance to salt stress by activating the JA-signalling pathway, increasing plasma membrane H ⁺ - ATPase activity, promoting cytosolic Ca²⁺ accumulation, and then restricting K⁺ leakage and reducing Na⁺ accumulation in the cytosol.     

Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> and K<Superscript>+</Superscript>/H<Superscript>+</Superscript> antiporters AtNHX1 and AtNHX3 from <Emphasis Type="Italic">Arabidopsis</Emphasis> improve salt and drought tolerance in transgenic poplar

The tonoplast and plasma membrane localized sodium (potassium)/proton antiporters have been shown to play an important role in plant resistance to salt stress. In this study, AtNHX1 and AtNHX3, two tonoplast Na⁺(K⁺)/H⁺ antiporter encoding genes from Arabidopsis thaliana, were expressed in poplar to investigate their biological functions in the resistance to abiotic stresses in woody plants. Transgenic poplar plants expressing either gene exhibited increased resistance to both salt and water-deficit stresses. Compared to the wild type (WT) plants, transgenic plants accumulated more sodium and potassium ions in the presence of 100 mM NaCl and showed reduced electrolyte leakage in the leaves under water stress. Furthermore, the proton-translocating and cation-dependent H⁺ (Na⁺/H⁺ or K⁺/H⁺) exchange activities in the tonoplast vesicles isolated from the leaves of transgenic plants were higher than in those isolated from WT plants. Therefore, constitutive expression of either AtNHX1 or AtNHX3 genetically modified the salt and water stress tolerance of transgenic poplar plants, providing a potential tool for engineering tree species with enhanced resistance to multiple abitotic stresses.     

Fatty acid desaturase-6 (Fad6) is required for salt tolerance in Arabidopsis thaliana

Fatty acid desaturases play important role in plant responses to abiotic stresses including cold, high temperature, drought, and osmotic stress. In this work, we provide the evidence that Fad6, a chloroplast desaturase, is required for salt tolerance during the early seedling development of Arabidopsis. Expression of Fad6 was responsive to salt and osmotic stress. Compared with the wild-type plants, the fad6 mutant showed reduced tolerance to salt stress, and accumulated more Na⁺ and less K⁺ under high NaCl stress condition. Furthermore, cellular oxidative damage was more severe in fad6 when treated with high concentrations of NaCl, as indicated by increased electrolyte leakage rate and malondialdehyde production, as well as by decreased activities of anti-oxidative enzymes. All these results suggest that Fad6 is required for salt resistance in Arabidopsis.     

Evidence that arbuscular mycorrhizal and phosphate-solubilizing fungi alleviate NaCl stress in the halophyte Kosteletzkya virginica: nutrient uptake and ion distribution within root tissues

The effects of an arbuscular mycorrhizal (AM) fungus, Glomus mosseae, and a phosphate-solubilizing microorganism (PSM), Mortierella sp., and their interactions, on nutrient (N, P and K) uptake and the ionic composition of different root tissues of the halophyte Kosteletzkya virginica (L.), cultured with or without NaCl, were evaluated. Plant biomass, AM colonization and PSM populations were also assessed. Salt stress adversely affected plant nutrient acquisition, especially root P and K, resulting in an important reduction in shoot dry biomass. Inoculation of the AM fungus or/and PSM strongly promoted AM colonization, PSM populations, plant dry biomass, root/shoot dry weight ratio and nutrient uptake by K. virginica, regardless of salinity level. Ion accumulation in root tissues was inhibited by salt stress. However, dual inoculation of the AM fungus and PSM significantly enhanced ion (e.g., Na⁺, Cl^?, K⁺, Ca²⁺, Mg²⁺) accumulation in different root tissues, and maintained lower Na⁺/K⁺ and Ca²⁺/Mg²⁺ ratios and a higher Na⁺/Ca²⁺ ratio, compared to non-inoculated plants under 100 mM NaCl conditions. Correlation coefficient analysis demonstrated that plant (shoot or root) dry biomass correlated positively with plant nutrient uptake and ion (e.g., Na⁺, K⁺, Mg²⁺ and Cl^?) concentrations of different root tissues, and correlated negatively with Na⁺/K⁺ ratios in the epidermis and cortex. Simultaneously, root/shoot dry weight ratio correlated positively with Na⁺/Ca²⁺ ratios in most root tissues. These findings suggest that combined AM fungus and PSM inoculation alleviates the deleterious effects of salt on plant growth by enabling greater nutrient (e.g., P, N and K) absorption, higher accumulation of Na⁺, K⁺, Mg²⁺ and Cl^? in different root tissues, and maintenance of lower root Na⁺/K⁺ and higher Na⁺/Ca²⁺ ratios when salinity is within acceptable limits.     

Alleviation of cadmium toxicity by potassium supplementation involves various physiological and biochemical features in <Emphasis Type="Italic">Nicotiana tabacum</Emphasis> L.

Potassium (K⁺) plays important roles in the development of plants and the response to various environmental stresses. However, the involvement of potassium in alleviating heavy metal stress in tobacco remains elusive. Greenhouse hydroponic experiments were conducted to evaluate the alleviating effects of K⁺ on tobacco subjected to cadmium (Cd) toxicity using four different K⁺ levels. Dose-dependent increases of plant biomass were found in both 0-μM Cd and 5-μM Cd treatments under different K⁺ levels, with the exception of the 1-mM KHCO₃ (K3) treatment. The best mitigation effect was recorded with the 0.5-mM K⁺ (K2) treatment, which greatly alleviated Cd-induced growth inhibition, photosynthesis reduction, and oxidative stress. Compared with K0 treatment (no KHCO₃ addition), K2 treatment significantly reduced Cd uptake and translocation after 5 and 10 days of Cd treatment. Moreover, the net photosynthetic rate, intracellular CO₂ concentration, stomatal conductance, and transpiration rate as well as K⁺, zinc, manganese, copper, and iron concentrations in both shoots and roots after 10 days of Cd treatment significantly improved under the K2 treatment, and malondialdehyde accumulation in both shoots and roots was repressed, compared with K0 + Cd. Superoxide dismutase was found to play key roles in alleviating Cd-induced oxidative pressure in shoots of plants in K2 treatment under Cd treatment. Our findings advocate a positive role for K⁺ in reducing pollutant residues for safe production, especially in soils slightly or moderately polluted with Cd.     

Plant protein phosphatases 2C: from genomic diversity to functional multiplicity and importance in stress management

Protein phosphatases (PPs) counteract kinases in reversible phosphorylation events during numerous signal transduction pathways in eukaryotes. Type 2C PPs (PP2Cs) represent the major group of PPs in plants, and recent discovery of novel abscisic acid (ABA) receptors (ABARs) has placed the PP2Cs at the center stage of the major signaling pathway regulating plant responses to stresses and plant development. Several studies have provided deep insight into vital roles of the PP2Cs in various plant processes. Global analyses of the PP2C gene family in model plants have contributed to our understanding of their genomic diversity and conservation, across plant species. In this review, we discuss the genomic and structural accounts of PP2Cs in plants. Recent advancements in their interaction paradigm with ABARs and sucrose nonfermenting related kinases 2 (SnRK2s) in ABA signaling are also highlighted. In addition, expression analyses and important roles of PP2Cs in the regulation of biotic and abiotic stress responses, potassium (K⁺) deficiency signaling, plant immunity and development are elaborated. Knowledge of functional roles of specific PP2Cs could be exploited for the genetic manipulation of crop plants. Genetic engineering using PP2C genes could provide great impetus in the agricultural biotechnology sector in terms of imparting desired traits, including a higher degree of stress tolerance and productivity without a yield penalty.     

AtCPK23 functions in <Emphasis Type="Italic">Arabidopsis</Emphasis> responses to drought and salt stresses

Calcium-dependent protein kinases (CDPKs) are unique serine/threonine kinases in plants and there are 34 CDPKs in Arabidopsis genome alone. Although several CDPKs have been demonstrated to be critical calcium signaling mediators for plant responses to various environmental stresses, the biological functions of most CDPKs in stress signaling remain unclear. In this study, we provide the evidences to demonstrate that AtCPK23 plays important role in Arabidopsis responses to drought and salt stresses. The cpk23 mutant, a T-DNA insertion mutant for AtCPK23 gene, showed greatly enhanced tolerance to drought and salt stresses, while the AtCPK23 overexpression lines became more sensitive to drought and salt stresses and the complementary line of the cpk23 mutant displayed similar phenotype as wild-type plants. The results of stomatal aperture measurement showed that the disruption of AtCPK23 expression reduced stomatal apertures, while overexpression of AtCPK23 increased stomatal apertures. The alteration of stomatal apertures by changes in AtCPK23 expression may account, at least in partial, for the modified Arabidopsis response to drought stress. In consistent with the enhanced salt-tolerance by disruption of AtCPK23 expression, K⁺ content in the cpk23 mutant was not reduced under high NaCl stress compared with wild-type plants, which indicates that the AtCPK23 may also regulate plant K⁺-uptake. The possible mechanisms by which AtCPK23 mediates drought and salt stresses signaling are discussed.