cGMP and ethylene are involved in maintaining ion homeostasis under salt stress in Arabidopsis roots

Cyclic guanosine monophosphate improves salt tolerance in Solanum lycopersicum

The cyclic nucleotide cyclic guanosine monophosphate (cGMP) is a powerful cell signaling molecule involved in biotic and abiotic stress perception and signal transduction. In the model plant Arabidopsis thaliana, salt and osmotic stress rapidly induce increase in cGMP which plays role by modulating the activity of monovalent cation transporters, possibly by direct binding to these proteins and by altering the expression of many abiotic stress responsive genes. In a recent study, a membrane permeable analogue of cGMP (8-bromo-cGMP) was found to have a promotive effect on soluble sugar, flavonoids and lignin content, and membrane integrity in Solanum lycopersicum seedlings under salt stress. However, it remains to be elucidated how salt stress affects the endogenous cGMP level in S. lycopersicum and if Br-cGMP-induced improvement in salt tolerance in S. lycopersicum involves altered cation fluxes. The current study was conducted to answer these questions. A rapid increase (within 30 s) in endogenous cGMP level was determined in S. lycopersicum roots after treatment with 100 mM NaCl. Addition of membrane permeable Br-cGMP in growth medium remarkably ameliorated the inhibitory effects of NaCl on seedlings' growth parameters, chlorophyll content and net photosynthesis rate. In salt stressed plants, Br-cGMP significantly decreased Na+ content by reducing its influx and increasing efflux while it improved plants K+ content by reducing its efflux and enhancing influx. Furthermore, supplementation with Br-cGMP improved plant's proline content and total antioxidant capacity, resulting in markedly decreased electrolyte leakage under salt stress. Br-cGMP increased the expression of Na+/H+ antiporter genes in roots and shoots of S. lycopersicum growing under salt stress, potentially enhancing plant's ability to sequester Na+ into the vacuole. The findings of this study provide insights into the mechanism of cGMP-induced salt stress tolerance in S. lycopersicum.

Alleviating the adverse effects of salinity stress on Salicornia persica using sodium nitroprusside and potassium nitrate

BACKGROUND

Glasswort (Salicornia persica) is identified as a halophyte plant, which is one of the most tolerant plants to salt conditions. The seed oil of the plant contains about 33% oil. In the present study, the effects of sodium nitroprusside (SNP; 0, 0.1, 0.2, and 0.4 mM) and potassium nitrate (KNO₃; 0, 0.5, and 1%) were evaluated on several characteristics of glasswort under salinity stress (0, 10, 20, and 40 dS/m).

RESULTS

morphological features, phenological traits, and yield parameters such as plant height, number of days to flowering, seed oil, biological yield, and seed yield significantly decreased in response to severe salt stress. However, the plants needed an optimal salinity concentration (20 dS/m NaCl) to obtain high amounts of seed oil and seed yield. The results also showed that a high level of salinity (40 dS/m NaCl) caused a decrease in plant oil and yield. In addition, by increasing the exogenous application of SNP and KNO₃, the seed oil and seed yield increased.

CONCLUSIONS

The application of SNP and KNO₃ were effective in protecting S. persica plants from the deleterious effects of severe salt stress (40 dS/m NaCl), thereby restoring the activity of antioxidant enzymes, increasing the proline content, and maintaining cell membrane stability. It seems that both factors, i.e. SNP and KNO₃, can be applied as mitigators of salt stress in plants.

Phenotypes of cyclic nucleotide-gated cation channel mutants: probing the nature of native channels

Plant cyclic nucleotide gated channels (CNGCs) facilitate cytosolic Ca²⁺ influx as an early step in numerous signaling cascades. CNGC-mediated Ca²⁺ elevations are essential for plant immune defense and high temperature thermosensing. In the present study, we evaluated phenotypes of CNGC2, CNGC4, CNGC6, and CNGC12 null mutants in these two pathways. It is shown CNGC2, CNGC4, and CNGC6 physically interact in vivo, whereas CNGC12 does not. CNGC involvement in immune signaling was evaluated by monitoring mutant response to elicitor peptide Pep3. Pep3 response cascades involving CNGCs included mitogen-activated kinase activation mediated by Ca²⁺ -dependent protein kinase phosphorylation. Pep3-induced reactive oxygen species generation was impaired in cngc2, cngc4, and cngc6, but not in cngc12, suggesting that CNGC2, CNGC4, and CNGC6 (which physically interact) may be components of a multimeric CNGC channel complex for immune signaling. However, unlike cngc2 and cngc4, cngc6 is not sensitive to high Ca²⁺ and displays no pleiotropic dwarfism. All four cngc mutants showed thermotolerance compared to wild-type, although CNGC12 does not interact with the other three CNGCs. These results imply that physically interacting CNGCs may, in some cases, function in a signaling cascade as components of a heteromeric channel complex, although this may not be the case in other signaling pathways.

BdGUCD1 and Cyclic GMP Are Required for Responses of Brachypodium distachyon to Fusarium pseudograminearum in the Mechanism Involving Jasmonate

Guanosine 3′,5′-cyclic monophosphate (cGMP) is an important signaling molecule in plants. cGMP and guanylyl cyclases (GCs), enzymes that catalyze the synthesis of cGMP from GTP, are involved in several physiological processes and responses to environmental factors, including pathogen infections. Using in vitro analysis, we demonstrated that recombinant BdGUCD1 is a protein with high guanylyl cyclase activity and lower adenylyl cyclase activity. In Brachypodium distachyon, infection by Fusarium pseudograminearum leads to changes in BdGUCD1 mRNA levels, as well as differences in endogenous cGMP levels. These observed changes may be related to alarm reactions induced by pathogen infection. As fluctuations in stress phytohormones after infection have been previously described, we performed experiments to determine the relationship between cyclic nucleotides and phytohormones. The results revealed that inhibition of cellular cGMP changes disrupts stress phytohormone content and responses to pathogen. The observations made here allow us to conclude that cGMP is an important element involved in the processes triggered as a result of infection and changes in its levels affect jasmonic acid. Therefore, stimuli-induced transient elevation of cGMP in plants may play beneficial roles in priming an optimized response, likely by triggering the mechanisms of feedback control.

Root Metabolite Differences in Two Maize Varieties Under Lead (Pb) Stress

To assess root metabolic differences of maize varieties in their response to lead (Pb) stress, the lead-tolerant variety Huidan No. 4 and the lead-sensitive variety Ludan No. 8 were tested under Pb-free and Pb-stressed conditions. Changes in metabolites were measured using ultra-performance liquid chromatography-mass spectrometry. Pb stress changed the levels of the amino acids proline, glutamine, lysine, and arginine in both varieties, whereas glutamate and phenylalanine levels changed only in Huidan No. 4. Pb stress altered cystine, valine, methionine, and tryptophan levels only in Ludan No. 8. Therefore, the synthesis and decomposition of amino acids may affect the response of maize to Pb stress. The degree of change in differential metabolites for Huidan No. 4 was greater than that for Ludan No. 8. In cell wall subcellular components, increases in superoxide dismutase (SOD), peroxidases (PODs), and Pb concentrations were greater in Huidan No. 4 than in Ludan No. 8. Therefore, the greater Pb tolerance of Huidan No. 4 could be due to better sequestration of Pb in cell walls and more effective removal of reactive oxygen species (ROS) from the plant. The levels of certain metabolites only increased in Ludan No. 8, indicating that Pb-sensitive varieties may use different metabolic pathways to cope with Pb stress. Both varieties showed increased levels of some metabolites related to antioxidant protection and osmotic regulation. This study provides an understanding of maize Pb tolerance mechanisms and a basis for further development of tools for use in maize breeding.

Induction of Jasmonoyl-Isoleucine (JA-Ile)-Dependent JASMONATE ZIM-DOMAIN (JAZ) Genes in NaCl-Treated Arabidopsis thaliana Roots Can Occur at Very Low JA-Ile Levels and in the Absence of the JA/JA-Ile Transporter JAT1/AtABCG16

The plant hormone jasmonoyl-isoleucine (JA-Ile) is an important regulator of plant growth and defense in response to various biotic and abiotic stress cues. Under our experimental conditions, JA-Ile levels increased approximately seven-fold in NaCl-treated Arabidopsis thaliana roots. Although these levels were around 1000-fold lower than in wounded leaves, genes of the JA-Ile signaling pathway were induced by a factor of 100 or more. Induction was severely compromised in plants lacking the JA-Ile receptor CORONATINE INSENSITIVE 1 or enzymes required for JA-Ile biosynthesis. To explain efficient gene expression at very low JA-Ile levels, we hypothesized that salt-induced expression of the JA/JA-Ile transporter JAT1/AtABCG16 would lead to increased nuclear levels of JA-Ile. However, mutant plants with different jat1 alleles were similar to wild-type ones with respect to salt-induced gene expression. The mechanism that allows COI1-dependent gene expression at very low JA-Ile levels remains to be elucidated.

A Salt-Signaling Network Involving Ethylene, Extracellular ATP, Hydrogen Peroxide, and Calcium Mediates K+/Na+ Homeostasis in Arabidopsis

This work aimed at investigating the interactive effects of salt-signaling molecules, i.e., ethylene, extracellular ATP (eATP), H₂O₂, and cytosolic Ca²⁺ ([Ca²⁺]_cyt), on the regulation of K⁺/Na⁺ homeostasis in Arabidopsisthaliana. The presence of eATP shortened Col-0 hypocotyl length under no-salt conditions. Moreover, eATP decreased relative electrolyte leakage and lengthened root length significantly in salt-treated Col-0 plants but had no obvious effects on the ethylene-insensitive mutants etr1-1 and ein3-1eil1-1. Steady-state ionic flux kinetics showed that exogenous 1-aminocyclopropane-1-carboxylic acid (ACC, an ethylene precursor) and eATP-Na₂ (an eATP donor) significantly increased Na⁺ extrusion and suppressed K⁺ loss during short-term NaCl treatment. Moreover, ACC remarkably raised the fluorescence intensity of salt-elicited H₂O₂ and cytosolic Ca²⁺. Our qPCR data revealed that during 12 h of NaCl stress, application of ACC increased the expression of AtSOS1 and AtAHA1, which encode the plasma membrane (PM) Na⁺/H⁺ antiporters (SOS1) and H⁺-ATPase (H⁺ pumps), respectively. In addition, eATP markedly increased the transcription of AtEIN3, AtEIL1, and AtETR1, and ACC treatment of Col-0 roots under NaCl stress conditions caused upregulation of AtRbohF and AtSOS2/3, which directly contribute to the H₂O₂ and Ca²⁺ signaling pathways, respectively. Briefly, ethylene was triggered by eATP, a novel upstream signaling component, which then activated and strengthened the H₂O₂ and Ca²⁺ signaling pathways to maintain K⁺/Na⁺ homeostasis under salinity.

The Central Negative Regulator of Flooding Tolerance, the PROTEOLYSIS 6 Branch of the N-degron Pathway, Adversely Modulates Salinity Tolerance in Arabidopsis

Seawater intrusion in coastal regions and waterlogging in salinized lands are serious constraints that reduce crop productivity under changing climate scenarios. Under these conditions, plants encounter flooding and salinity concurrently or sequentially. Identification and characterization of genes and pathways associated with both flooding and salinity adaptation are critical steps for the simultaneous improvement of plant tolerance to these stresses. The PROTEOLYSIS 6 (PRT6) branch of the N-degron pathway is a well-characterized process that negatively regulates flooding tolerance in plants. Here, we determined the role of the PRT6/N-degron pathway in salinity tolerance in Arabidopsis. This study demonstrates that the prt6 mutation enhances salinity tolerance at the germination, seedling, and adult plant stages. Maintenance of chlorophyll content and root growth under high salt in the prt6 mutant was linked with the restricted accumulation of sodium ions (Na⁺) in shoots and roots of the mutant genotype. The prt6 mutation also stimulated mRNA accumulation of key transcription factors in ABA-dependent and independent pathways of osmotic/salinity tolerance, accompanied by the prominent expression of their downstream genes. Furthermore, the prt6 mutant displayed increased sensitivity to ethylene and brassinosteroids, which can suppress Na⁺ uptake and promote the expression of stress-responsive genes. This study provides genetic evidence that both salinity and flooding tolerance is coordinated through a common regulatory pathway in Arabidopsis.

A brief appraisal of ethylene signaling under abiotic stress in plants

For years, ethylene has been known to humankind as the plant hormone responsible for fruit ripening. However, the multitasking aspect of ethylene is still being investigated as ever. It is one of the most diversified signaling molecules which acclimatize plant under adverse conditions. It promotes adventitious root formation, stem and petiole elongation, opening and closing of stomatal aperture, reduces salinity and metal stress, etc. Presence of ethylene checks the production and scavenging of reactive oxygen species by strengthening the antioxidant machinery. Meanwhile, it interacts with other signaling molecules and initiates a cascade of adaptive responses. In the present mini review, the biosynthesis and sources of ethylene production, interaction with other signaling molecules, and its exogenous application under different abiotic stresses have been discussed.

Characterization of tomato protein kinases embedding guanylate cyclase catalytic center motif

Guanylate cyclases (GCs) are enzymes that catalyze the reaction to produce cyclic GMP (cGMP), a key signaling molecule in eukaryotes. Nevertheless, systemic identification and functional analysis of GCs in crop plant species have not yet been conducted. In this study, we systematically identified GC genes in the economically important crop tomato (Solanum lycopersicum L.) and analyzed function of two putative tomato GC genes in disease resistance. Ninety-nine candidate GCs containing GC catalytic center (GC-CC) motif were identified in tomato genome. Intriguingly, all of them were putative protein kinases embedding a GC-CC motif within the protein kinase domain, which was thus tentatively named as GC-kinases here. Two homologs of Arabidopsis PEPRs, SlGC17 and SlGC18 exhibited in vitro GC activity. Co-silencing of SlGC17 and SlGC18 genes significantly reduced resistance to tobacco rattle virus, fungus Sclerotinia sclerotiorum, and bacterium Pseudomonas syringae pv. tomato (Pst) DC3000. Moreover, co-silencing of these two genes attenuated PAMP and DAMP-triggered immunity as shown by obvious decrease of flg22, chitin and AtPep1-elicited Ca2+ and H2O2 burst in SlGC-silenced plants. Additionally, silencing of these genes altered the expression of a set of Ca2+ signaling genes. Furthermore, co-silencing of these GC-kinase genes exhibited stronger effects on all above regulations in comparison with individual silencing. Collectively, our results suggest that GC-kinases might widely exist in tomato and the two SlPEPR-GC genes redundantly play a positive role in resistance to diverse pathogens and PAMP/DAMP-triggered immunity in tomato. Our results provide insights into composition and functions of GC-kinases in tomato.

Ethylene promotes pollen tube growth by affecting actin filament organization via the cGMP-dependent pathway in Arabidopsis thaliana

Ethylene and cGMP are key regulators of plant developmental processes. In this study, we demonstrate that ethylene or cGMP promote pollen tube growth in a dose-dependent manner. The etr1-1 mutant was found to be insensitive to ethylene with regard to pollen tube growth, while the growth-promoting effect of ethylene in etr2-2, ein4-4, or ein4-7 did not change, suggesting that ethylene signaling was mainly perceived by ETR1. However, the function of cGMP was not inhibited in etr1-1 and pollen tubes became insensitive to ethylene when the endogenous cGMP level was artificially decreased. This shows that cGMP is necessary for the control of pollen tube growth and that it might be a downstream component of ETR1 in the ethylene signaling pathway. Our study also found that ethylene or cGMP increase the actin bundles and elevated the percentage of relative amount of F-actin, while removal of cGMP decreased actin bundles abundance and altered the ratio of F-actin in the tip and base regions of pollen tubes. In conclusion, our data suggests that ethylene functions as the upstream signal of cGMP, and that both signals promote pollen germination and tube growth by regulating F-actin, which is essential for vesicular transport and cytoplasmic streaming.

cGMP signalling in plants: from enigma to main stream

All living organisms communicate with their environment, and part of this dialogue is mediated by secondary messengers such as cyclic guanosine mono phosphate (cGMP). In plants, most of the specific components that allow production and breakdown of cGMP have now been identified apart from cGMP dependent phosphodiesterases, enzymes responsible for cGMP catabolism. Irrespectively, the role of cGMP in plant signal transductions is now firmly established with involvement of this nucleotide in development, stress response, ion homeostasis and hormone function. Within these areas, several consistent themes where cGMP may be particularly relevant are slowly emerging: these include regulation of cation fluxes, for example via cyclic nucleotide gated channels and in stomatal functioning. Many details of signalling pathways that incorporate cGMP remain to be unveiled. These include downstream targets other than a small number of ion channels, in particular cGMP dependent kinases. Improved genomics tools may help in this respect, especially since many proteins involved in cGMP signalling appear to have multiple and often overlapping functional domains which hampers identification on the basis of simple homology searches. Another open question regards the topographical distribution of cGMP signals are they cell limited? Does long distance cGMP signalling occur and if so, by what mechanisms? The advent of non-disruptive fluorescent reporters with high spatial and temporal resolution will provide a tool to accelerate progress in all these areas. Automation can facilitate large scale screens of mutants or the action of effectors that impact on cGMP signalling.

Ethylene Improves Root System Development under Cadmium Stress by Modulating Superoxide Anion Concentration in Arabidopsis thaliana

This work aims at identifying the effects of ethylene on the response of Arabidopsis thaliana root system to cadmium chloride (CdCl2) stress. Two ethylene-insensitive mutants, ein2-5 and ein3-1eil1-1, were subjected to (25, 50, 75, and 100 μM) CdCl2 concentrations, from which 75 μM concentration decreased root growth by 40% compared with wild type Col-0 as a control. Ethylene biosynthesis increased in response to CdCl2 treatment. The length of primary root and root tip in ein2-5 and ein3-1eil1-1 decreased compared with wild type after CdCl2 treatment, suggesting that ethylene play a role in root system response to Cd stress. The superoxide concentration in roots of ein2-5 and ein3-1eil1-1 was greater than in wild type seedlings under Cd stress. Application of exogenous 1-aminocyclopropane-1-carboxylic acid (ACC) (a precursor of ethylene biosynthesis) in different concentrations (0.01, 0.05 and 0.5 μM) decreased superoxide accumulation in Col-0 root tips and increased the activities of superoxide dismutase (SOD) isoenzymes under Cd stress. This result was reversed with 5 μM of aminoisobutyric acid AIB (an inhibitor of ethylene biosynthesis). Moreover, it was accompanied by increase in lateral roots number and root hairs length, indicating the essential role of ethylene in modulating root system development by controlling superoxide accumulation through SOD isoenzymes activities. The suppressed Cd-induced superoxide accumulation in wild type plants decreased the occurrence of cells death while programmed cell death (PCD) was initiated in the root tip zone, altering root morphogenesis (decreased primary root length, more lateral roots and root hairs) to minimize the damage caused by Cd stress, whereas this response was absent in the ein2-5 and ein3-1eil1-1 seedlings. Hence, ethylene has a role in modulating root morphogenesis during CdCl2 stress in A. thaliana by increasing the activity of SOD isoenzymes to control superoxide accumulation.

In Search of Enzymes with a Role in 3', 5'-Cyclic Guanosine Monophosphate Metabolism in Plants

In plants, nitric oxide (NO)-mediated 3', 5'-cyclic guanosine monophosphate (cGMP) synthesis plays an important role during pathogenic stress response, stomata closure upon osmotic stress, the development of adventitious roots and transcript regulation. The NO-cGMP dependent pathway is well characterized in mammals. The binding of NO to soluble guanylate cyclase enzymes (GCs) initiates the synthesis of cGMP from guanosine triphosphate. The produced cGMP alters various cellular responses, such as the function of protein kinase activity, cyclic nucleotide gated ion channels and cGMP-regulated phosphodiesterases. The signal generated by the second messenger is terminated by 3', 5'-cyclic nucleotide phosphodiesterase (PDEs) enzymes that hydrolyze cGMP to a non-cyclic 5'-guanosine monophosphate. To date, no homologues of mammalian cGMP-synthesizing and degrading enzymes have been found in higher plants. In the last decade, six receptor proteins from Arabidopsis thaliana have been reported to have guanylate cyclase activity in vitro. Of the six receptors, one was shown to be a NO dependent guanylate cyclase enzyme (NOGC1). However, the role of these proteins in planta remains to be elucidated. Enzymes involved in the degradation of cGMP remain elusive, albeit, PDE activity has been detected in crude protein extracts from various plants. Additionally, several research groups have partially purified and characterized PDE enzymatic activity from crude protein extracts. In this review, we focus on presenting advances toward the identification of enzymes involved in the cGMP metabolism pathway in higher plants.

Analyzing serial cDNA libraries revealed reactive oxygen species and gibberellins signaling pathways in the salt response of Upland cotton (Gossypium hirsutum L.)

By comparing series full-length cDNA libraries stressed and control, the dynamic process of salt stress response in Upland cotton was studied, and reactive oxygen species and gibberellins signaling pathways were proposed. The Upland cotton is the most important fiber plant with highly salt tolerance. However, the molecular mechanism underlying salt tolerance in domesticated cotton was unclear. Here, seven full-length cDNA libraries were constructed for seedling roots of Upland cotton 'Zhong G 5' at 0, 3, 12 and 48 h after the treatment of control or 150 mM NaCl stress. About 3300 colonies in each library were selected robotically for 5'-end pyrosequencing, resulting in 20,358 expressed sequence tags (ESTs) totally. And 8516 uniESTs were then assembled, including 2914 contigs and 5602 singletons, and explored for Gene Ontology (GO) function. GO comparison between serial stress libraries and control reflected the growth regulation, stimulus response, signal transduction and biology regulation processes were conducted dynamically in response to salt stress. MYB, MYB-related, WRKY, bHLH, GRAS and ERF families of transcription factors were significantly enriched in the early response. 65 differentially expressed genes (DEGs), mainly associated with reactive oxygen species (ROS) scavenging, gibberellins (GAs) metabolism, signal transduction, transcription regulation, stress response and transmembrane transport, were identified and confirmed by quantitative real-time PCR. Overexpression of selected DEGs increased tolerance against salt stress in transgenic yeast. Results in this study supported that a ROS-GAs interacting signaling pathway of salt stress response was activated in Upland cotton. Our results provided valuable gene resources for further investigation of the molecular mechanism of salinity tolerance.

The Role of Ethylene in Plants Under Salinity Stress

Although the roles of ethylene in plant response to salinity and other stresses have been extensively studied, there are still some obscure points left to be clarified. Generally, in Arabidopsis and many other terrestrial plants, ethylene signaling is indispensable for plant rapid response and tolerance to salinity stress. However, a few studies showed that functional knock-out of some ACSs increased plant salinity-tolerance, while overexpression of them caused more sensitivity. This seems to be contradictory to the known opinion that ethylene plays positive roles in salinity response. Differently, ethylene in rice may play negative roles in regulating seedling tolerance to salinity. The main positive ethylene signaling components MHZ7/OsEIN2, MHZ6/OsEIL1, and OsEIL2 all negatively regulate the salinity-tolerance of rice seedlings. Recently, several different research groups all proposed a negative feedback mechanism of coordinating plant growth and ethylene response, in which several ethylene-inducible proteins (including NtTCTP, NEIP2 in tobacco, AtSAUR76/77/78, and AtARGOS) act as inhibitors of ethylene response but activators of plant growth. Therefore, in addition to a summary of the general roles of ethylene biosynthesis and signaling in salinity response, this review mainly focused on discussing (i) the discrepancies between ethylene biosynthesis and signaling in salinity response, (ii) the divergence between rice and Arabidopsis in regulation of salinity response by ethylene, and (iii) the possible negative feedback mechanism of coordinating plant growth and salinity response by ethylene.

Opposite extremes in ethylene/nitric oxide ratio induce cell death in suspension culture and root apices of tomato exposed to salt stress

The plant hormone ethylene or the gaseous signalling molecule nitric oxide (NO) may enhance salt stress tolerance by maintaining ion homeostasis, first of all K+/Na+ ratio of tissues. Ethylene and NO accumulation increased in the root apices and suspension culture cells of tomato at sublethal salt stress caused by 100 mM NaCl, however, the induction phase of programmed cell death (PCD) was different at lethal salt concentration. The production of ethylene by root apices and the accumulation of NO in the cells of suspension culture did not increase during the initiation of PCD after 250 mM NaCl treatment. Moreover, cells in suspension culture accumulated higher amount of reactive oxygen species which, along with NO deficiency contributed to cell death induction. The absence of ethylene in the apical root segments and the absence of NO accumulation in the cell suspension resulted in similar ion disequilibrium, namely K+/Na+ ratio of 1.41 ± 0.1 and 1.68 ± 0.3 in intact plant tissues and suspension culture cells, respectively that was not tolerated by tomato.