Signal transduction during cold, salt, and drought stresses in plants

Abiotic stresses, especially cold, salinity and drought, are the primary causes of crop loss worldwide. Plant adaptation to environmental stresses is dependent upon the activation of cascades of molecular networks involved in stress perception, signal transduction, and the expression of specific stress-related genes and metabolites. Plants have stress-specific adaptive responses as well as responses which protect the plants from more than one environmental stress. There are multiple stress perception and signaling pathways, some of which are specific, but others may cross-talk at various steps. In this review article, we first expound the general stress signal transduction pathways, and then highlight various aspects of biotic stresses signal transduction networks. On the genetic analysis, many cold induced pathways are activated to protect plants from deleterious effects of cold stress, but till date, most studied pathway is ICE-CBF-COR signaling pathway. The Salt-Overly-Sensitive (SOS) pathway, identified through isolation and study of the sos1, sos2, and sos3 mutants, is essential for maintaining favorable ion ratios in the cytoplasm and for tolerance of salt stress. Both ABA-dependent and -independent signaling pathways appear to be involved in osmotic stress tolerance. 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 and the ROS signaling networks can control growth, development, and stress response. Finally, we talk about the common regulatory system and cross-talk among biotic stresses, with particular emphasis on the MAPK cascades and the cross-talk between ABA signaling and biotic signaling.     

Genetic analysis of salt tolerance in arabidopsis. Evidence for a critical role of potassium nutrition.

A large genetic screen for sos (for salt overly sensitive) mutants was performed in an attempt to isolate mutations in any gene with an sos phenotype. Our search yielded 28 new alleles of sos1, nine mutant alleles of a newly identified locus, SOS2, and one allele of a third salt tolerance locus, SOS3. The sos2 mutations, which are recessive, were mapped to the lower arm of chromosome V, approximately 2.3 centimorgans away from the marker PHYC. Growth measurements demonstrated that sos2 mutants are specifically hypersensitive to inhibition by Na+ or Li+ and not hypersensitive to general osmotic stresses. Interestingly, the SOS2 locus is also necessary for K+ nutrition because sos2 mutants were unable to grow on a culture medium with a low level of K+. The expression of several salt-inducible genes was superinduced in sos2 plants. The salt tolerance of sos1, sos2, and sos3 mutants correlated with their K+ tissue content but not their Na+ tissue content. Double mutant analysis indicated that the SOS genes function in the same pathway. Based on these results, a genetic model for salt tolerance mechanisms in Arabidopsis is presented in which SOS1, SOS2, and SOS3 are postulated to encode regulatory components controlling plant K+ nutrition that in turn is essential for salt tolerance.     

An enhancer mutant of Arabidopsis salt overly sensitive 3 mediates both ion homeostasis and the oxidative stress response

The myristoylated calcium sensor SOS3 and its interacting protein kinase, SOS2, play critical regulatory roles in salt tolerance. Mutations in either of these proteins render Arabidopsis thaliana plants hypersensitive to salt stress. We report here the isolation and characterization of a mutant called enh1-1 that enhances the salt sensitivity of sos3-1 and also causes increased salt sensitivity by itself. ENH1 encodes a chloroplast-localized protein with a PDZ domain at the N-terminal region and a rubredoxin domain in the C-terminal part. Rubredoxins are known to be involved in the reduction of superoxide in some anaerobic bacteria. The enh1-1 mutation causes enhanced accumulation of reactive oxygen species (ROS), particularly under salt stress. ROS also accumulate to higher levels in sos2-1 but not in sos3-1 mutants. The enh1-1 mutation does not enhance sos2-1 phenotypes. Also, enh1-1 and sos2-1 mutants, but not sos3-1 mutants, show increased sensitivity to oxidative stress. These results indicate that ENH1 functions in the detoxification of reactive oxygen species resulting from salt stress by participating in a new salt tolerance pathway that may involve SOS2 but not SOS3.     

Arabidopsis SOS3 plays an important role in salt tolerance by mediating calcium-dependent microfilament reorganization

Key message

SOS3 mediates calcium dependent actin filament reorganization that plays important roles in plant responses to salt stress.

Abstract

Arabidopsis salt overly sensitive 3 (SOS3) plays an important role in plant salt tolerance by regulation of Na⁺/K⁺ homeostasis. Plants lacking SOS3 are hypersensitive to salt stress and this phenomenon can be partially rescued by the addition of calcium. However the mechanism underlying remains elusive. We here report that the organization of actin filaments in sos3 mutant differs from that in wild-type plant. Under salt stress abnormal actin assembly and arrangement in sos3 are more pronounced, which can be partially complemented by addition of external calcium or low concentration of latrunculin A, an actin monomer-sequestering agent. The effects of calcium and Lat A on actin filament organization of sos3 mutant are accordant with their effects on sos3 salt sensitivity under salt stress. These findings indicate that the salt-hypersensitivity of sos3 mutant partially results from its disordered actin filaments, and SOS3 mediated actin filament reorganization plays important roles in plant responses to salt stress.     

Critical role of divalent cations and Na+ efflux in Arabidopsis thaliana salt tolerance

Detrimental effects of salinity on plants are known to be partially alleviated by external Ca²⁺. Previous work demonstrated that the Arabidopsis SOS3 locus encodes a Ca²⁺‐binding protein with similarities to CnB, the regulatory subunit of protein phosphatase 2B (calcineurin). In this study, we further characterized the role of SOS3 in salt tolerance. We found that reduced root elongation of sos3 mutants in the presence of high concentrations of either NaCl or LiCl is specifically rescued by Ca²⁺ and not Mg²⁺, whereas root growth is rescued by both Ca²⁺ and Mg²⁺ in the presence of high concentrations of KCl. Phenocopies of sos3 mutants were obtained in wild‐type plants by the application of calmodulin and calcineurin inhibitors. These data provide further evidence that SOS3 is a calcineurin‐like protein and that calmodulin plays an important role in the signalling pathways involved in plant salt tolerance. The origin of the elevated Na : K ratio in sos3 mutants was investigated by comparing Na⁺ efflux and influx in both mutant and wild type. No difference in Na⁺ influx was recorded between wild type and sos3; however, sos3 plants showed a markedly lower Na⁺ efflux, a property that would contribute to the salt‐oversensitive phenotype of sos3 plants.     

HbCIPK2, a novel CBL-interacting protein kinase from halophyte Hordeum brevisubulatum, confers salt and osmotic stress tolerance

Protein kinases play an important role in regulating the response to abiotic stress in plant. CIPKs are plant‐specific signal transducers, and some members have been identified. However, the precise functions of novel CIPKs still remain unknown. Here we report that HbCIPK2 is a positive regulator of salt and osmotic stress tolerance. HbCIPK2 was screened out of the differentially expressed fragments from halophyte Hordeum brevisubulatum by cDNA‐AFLP technique, and was a single‐copy gene without intron. Expression of HbCIPK2 was increased by salt, drought and ABA treatment. HbCIPK2 is mainly localized to the plasma membrane and nucleus. Ectopic expression of 35S:HbCIPK2 not only rescued the salt hypersensitivity in Arabidopsis mutant sos2‐1, but also enhanced salt tolerance in Arabidopsis wild type, and exhibited tolerance to osmotic stress during germination. The HbCIPK2 contributed to the ability to prevent K⁺ loss in root and to accumulate less Na⁺ in shoot resulting in K⁺/Na⁺ homeostasis and protection of root cell from death, which is consistent with the gene expression profile of HbCIPK2‐overexpressing lines. These findings imply possible novel HbCIPK2‐mediated salt signalling pathways or networks in H. brevisubulatum.     

Overexpression of PP2A‐C5 that encodes the catalytic subunit 5 of protein phosphatase 2A in Arabidopsis confers better root and shoot development under salt conditions

Protein phosphatase 2A (PP2A) is an enzyme consisting of three subunits: a scaffolding A subunit, a regulatory B subunit and a catalytic C subunit. PP2As were shown to play diverse roles in eukaryotes. In this study, the function of the Arabidopsis PP2A‐C5 gene that encodes the catalytic subunit 5 of PP2A was studied using both loss‐of‐function and gain‐of‐function analyses. Loss‐of‐function mutant pp2a‐c5‐1 displayed more impaired growth during root and shoot development, whereas overexpression of PP2A‐C5 conferred better root and shoot growth under different salt treatments, indicating that PP2A‐C5 plays an important role in plant growth under salt conditions. Double knockout mutants of pp2a‐c5‐1 and salt overly sensitive (sos) mutants sos1‐1, sos2‐2 or sos3‐1 showed additive sensitivity to NaCl, indicating that PP2A‐C5 functions in a pathway different from the SOS signalling pathway. Using yeast two‐hybrid analysis, four vacuolar membrane chloride channel (CLC) proteins, AtCLCa, AtCLCb, AtCLCc and AtCLCg, were found to interact with PP2A‐C5. Moreover, overexpression of AtCLCc leads to increased salt tolerance and Cl^? accumulation in transgenic Arabidopsis plants. These data indicate that PP2A‐C5‐mediated better growth under salt conditions might involve up‐regulation of CLC activities on vacuolar membranes and that PP2A‐C5 could be used for improving salt tolerance in crops.     

Upstream kinases of plant SnRKs are involved in salt stress tolerance

Sucrose non‐fermenting 1‐related protein kinases (SnRKs) are important for plant growth and stress responses. This family has three clades: SnRK1, SnRK2 and SnRK3. Although plant SnRKs are thought to be activated by upstream kinases, the overall mechanism remains obscure. Geminivirus Rep‐Interacting Kinase (GRIK)1 and GRIK2 phosphorylate SnRK1s, which are involved in sugar/energy sensing, and the grik1‐1 grik2‐1 double mutant shows growth retardation under regular growth conditions. In this study, we established another Arabidopsis mutant line harbouring a different allele of gene GRIK1 (grik1‐2 grik2‐1) that grows similarly to the wild‐type, enabling us to evaluate the function of GRIKs under stress conditions. In the grik1‐2 grik2‐1 double mutant, phosphorylation of SnRK1.1 was reduced, but not eliminated, suggesting that the grik1‐2 mutation is a weak allele. In addition to high sensitivity to glucose, the grik1‐2 grik2‐1 mutant was sensitive to high salt, indicating that GRIKs are also involved in salinity signalling pathways. Salt Overly Sensitive (SOS)2, a member of the SnRK3 subfamily, is a critical mediator of the response to salinity. GRIK1 phosphorylated SOS2 in vitro, resulting in elevated kinase activity of SOS2. The salt tolerance of sos2 was restored to normal levels by wild‐type SOS2, but not by a mutated form of SOS2 lacking the T168 residue phosphorylated by GRIK1. Activation of SOS2 by GRIK1 was also demonstrated in a reconstituted system in yeast. Our results indicate that GRIKs phosphorylate and activate SnRK1 and other members of the SnRK3 family, and that they play important roles in multiple signalling pathways in vivo.     

Increased tolerance to salt stress in the phosphate-accumulating <Emphasis Type="Italic">Arabidopsis</Emphasis> mutants <Emphasis Type="Italic">siz1</Emphasis> and <Emphasis Type="Italic">pho2</Emphasis>

High salinity is an environmental factor that inhibits plant growth and development, leading to large losses in crop yields. We report here that mutations in SIZ1 or PHO2, which cause more accumulation of phosphate compared with the wild type, enhance tolerance to salt stress. The siz1 and pho2 mutations reduce the uptake and accumulation of Na⁺. These mutations are also able to suppress the Na⁺ hypersensitivity of the sos3-1 mutant, and genetic analyses suggest that SIZ1 and SOS3 or PHO2 and SOS3 have an additive effect on the response to salt stress. Furthermore, the siz1 mutation cannot suppress the Li⁺ hypersensitivity of the sos3-1 mutant. These results indicate that the phosphate-accumulating mutants siz1 and pho2 reduce the uptake and accumulation of Na⁺, leading to enhanced salt tolerance, and that, genetically, SIZ1 and PHO2 are likely independent of SOS3-dependent salt signaling.     

Role of salicylic acid on physiological and biochemical mechanism of salinity stress tolerance in plants

Salinity stress is one of the major abiotic stresses affecting plant growth and productivity globally. In order to improve the yields of plants growing under salt stress bear remarkable importance to supply sustainable agriculture. Acclimation of plants to salinized condition depends upon activation of cascade of molecular network involved in stress sensing/perception, signal transduction, and the expression of specific stress-related genes and metabolites. Isolation of salt overly sensitive (SOS) genes by sos mutants shed us light on the relationship between ion homeostasis and salinity tolerance. Regulation of antioxidative system to maintain a balance between the overproduction of reactive oxygen species and their scavenging to keep them at signaling level for reinstating metabolic activity has been elucidated. However, osmotic adaptation and metabolic homeostasis under abiotic stress environment is required. Recently, role of phytohormones like Abscisic acid, Jasmonic acid, and Salicylic acid in the regulation of metabolic network under osmotic stress condition has emerged through crosstalk between chemical signaling pathways. Thus, abiotic stress signaling and metabolic balance is an important area with respect to increase crop yield under suboptimal conditions. This review focuses on recent developments on improvement in salinity tolerance aiming to contribute sustainable plant yield under saline conditions in the face of climate change.     

The battle of two ions: Ca2+ signalling against Na+ stress

Soil salinity adversely affects plant growth, crop yield and the composition of ecosystems. Salinity stress impacts plants by combined effects of Na⁺ toxicity and osmotic perturbation. Plants have evolved elaborate mechanisms to counteract the detrimental consequences of salinity. Here we reflect on recent advances in our understanding of plant salt tolerance mechanisms. We discuss the embedding of the salt tolerance‐mediating SOS pathway in plant hormonal and developmental adaptation. Moreover, we review newly accumulating evidence indicating a crucial role of a transpiration‐dependent salinity tolerance pathway, that is centred around the function of the NADPH oxidase RBOHF and its role in endodermal and Casparian strip differentiation. Together, these data suggest a unifying and coordinating role for Ca²⁺ signalling in combating salinity stress at the cellular and organismal level.     

Calcium- and salt-stress signaling in plants: shedding light on SOS pathway

As salt stress imposes a major environmental threat to agriculture, understanding the basic physiology and genetics of cell under salt stress is crucial for developing any transgenic strategy. Salt Overly Sensitive (SOS) genes (SOS1-SOS3) were isolated through positional cloning. Since sos mutants are hypersensitive to salt, their characterization resulted in the discovery of a novel pathway, which has helped in our understanding the mechanism of salt-stress tolerance in plants. Genetic analysis confirmed that SOS1-SOS3 function in a common pathway of salt tolerance. This pathway also emphasizes the significance of Ca²⁺ signal in reinstating cellular ion homeostasis. SOS3, a Ca²⁺ sensor, transduces the signal downstream after activating and interacting with SOS2 protein kinase. This SOS3-SOS2 complex activates the Na⁺/H⁺ antiporter activity of SOS1 thereby reestablish cellular ion homeostasis. Recently, SOS4 and SOS5 have also been characterized. SOS4 encodes a pyridoxal (PL) kinase that is involved in the biosynthesis of pyridoxal-5-phosphate (PLP), an active form of vitamin B6. SOS5 has been shown to be a putative cell surface adhesion protein that is required for normal cell expansion. Under salt stress, the normal growth and expansion of a plant cell becomes even more important and SOS5 helps in the maintenance of cell wall integrity and architecture. In this review we focus on the recent advances in salt stress and SOS signaling pathway. A broad coverage of the discovery of SOS mutants, structural aspect of these genes and the latest developments in the field of SOS1-SOS5 has been described.     

Arabidopsis sos1 mutant in a salt-tolerant accession revealed an importance of salt acclimation ability in plant salt tolerance

An analysis of the salinity tolerance of 354 Arabidopsis thaliana accessions showed that some accessions were more tolerant to salt shock than the reference accession, Col-0, when transferred from 0 to 225 mM NaCl. In addition, several accessions, including Zu-0, showed marked acquired salt tolerance after exposure to moderate salt stress. It is likely therefore that Arabidopsis plants have at least two types of tolerance, salt shock tolerance and acquired salt tolerance. To evaluate a role of well-known salt shock tolerant gene SOS1 in acquired salt tolerance, we isolated a sos1 mutant from ion-beam-mutagenized Zu-0 seedlings. The mutant showed severe growth inhibition under salt shock stress owing to a single base deletion in the SOS1 gene and was even more salt sensitive than Col-0. Nevertheless, it was able to survive after acclimation on 100 mM NaCl for 7 d followed by 750 mM sorbitol for 20 d, whereas Col-0 became chlorotic under the same conditions. We propose that genes for salt acclimation ability are different from genes for salt shock tolerance and play an important role in the acquisition of salt or osmotic tolerance.     

SOS2 promotes salt tolerance in part by interacting with the vacuolar H+-ATPase and upregulating its transport activity

The salt overly sensitive (SOS) pathway is critical for plant salt stress tolerance and has a key role in regulating ion transport under salt stress. To further investigate salt tolerance factors regulated by the SOS pathway, we expressed an N-terminal fusion of the improved tandem affinity purification tag to SOS2 (NTAP-SOS2) in sos2-2 mutant plants. Expression of NTAP-SOS2 rescued the salt tolerance defect of sos2-2 plants, indicating that the fusion protein was functional in vivo. Tandem affinity purification of NTAP-SOS2-containing protein complexes and subsequent liquid chromatography-tandem mass spectrometry analysis indicated that subunits A, B, C, E, and G of the peripheral cytoplasmic domain of the vacuolar H⁺-ATPase (V-ATPase) were present in a SOS2-containing protein complex. Parallel purification of samples from control and salt-stressed NTAP-SOS2/sos2-2 plants demonstrated that each of these V-ATPase subunits was more abundant in NTAP-SOS2 complexes isolated from salt-stressed plants, suggesting that the interaction may be enhanced by salt stress. Yeast two-hybrid analysis showed that SOS2 interacted directly with V-ATPase regulatory subunits B1 and B2. The importance of the SOS2 interaction with the V-ATPase was shown at the cellular level by reduced H⁺ transport activity of tonoplast vesicles isolated from sos2-2 cells relative to vesicles from wild-type cells. In addition, seedlings of the det3 mutant, which has reduced V-ATPase activity, were found to be severely salt sensitive. Our results suggest that regulation of V-ATPase activity is an additional key function of SOS2 in coordinating changes in ion transport during salt stress and in promoting salt tolerance.     

SOS3 mediates lateral root development under low salt stress through regulation of auxin redistribution and maxima in Arabidopsis

? The SOS signaling pathway plays an important role in plant salt tolerance. However, little is known about how the SOS pathway modulates organ development in response to salt stress. Here, the involvement of SOS signaling in NaCl-induced lateral root (LR) development in Arabidopsis was assessed. ? Wild-type and sos3-1 mutant seedlings on iso-osmotic concentrations of NaCl and mannitol were analyzed. The marker lines for auxin accumulation, auxin transport, cell division activity and stem cells were also examined. ? The results showed that ionic effect alleviates the inhibitory effects of osmotic stress on LR development. LR development of the sos3-1 mutant showed increased sensitivity specifically to low salt. Under low-salt conditions, auxin in cotyledons and LR primordia (LRP) of the sos3-1 mutant was markedly reduced. Decreases in auxin polar transport of mutant roots may cause insufficient auxin supply, resulting in defects not only in LR initiation but also in cell division activity in LRP. ? Our data uncover a novel role of the SOS3 gene in modulation of LR developmental plasticity and adaptation in response to low salt stress, and reveal a new mechanism for plants to sense and adapt to small changes of salt.     

Molecular cloning and functional characterization of a novel apple MdCIPK6L gene reveals its involvement in multiple abiotic stress tolerance in transgenic plants

CBL-interacting protein kinases (CIPKs) are involved in many aspects of plant responses to abiotic stresses. However, their functions are poorly understood in fruit trees. In this study, a salt-induced MdCIPK6L gene was isolated from apple. Its expression was positively induced by abiotic stresses, stress-related hormones and exogenous Ca(2+). MdCIPK6L was not homologous to AtSOS2, however, its ectopic expression functionally complemented Arabidopsis sos2 mutant. Furthermore, yeast two-hybrid assay showed that MdCIPK6L protein interacted with AtSOS3, indicating that it functions in salt tolerance partially like AtSOS2 through SOS pathway. As a result, the overexpression of both MdCIPK6L and MdCIPK6LT175D remarkably enhanced the tolerance to salt, osmotic/drought and chilling stresses, but did not affect root growth, in transgenic Arabidopsis and apple. Also, T-to-D mutation to MdCIPK6L at Thr175 did not affect its function. These differences between MdCIPK6L and other CIPKs, especially CIPK6s, indicate that MdCIPK6L encodes a novel CIPK in apple. Finally, MdCIPK6L overexpression also conferred tolerance to salt, drought and chilling stresses in transgenic tomatoes. Therefore, MdCIPK6L functions in stress tolerance crossing the species barriers, and is supposed to be a potential candidate gene to improve stress tolerance by genetic manipulation in apple and other crops.