Chloroplasts of salt-grown Arabidopsis seedlings are impaired in structure, genome copy number and transcript levels

Hydrogen sulfide-mitigated salinity stress impact in sunflower seedlings was associated with improved photosynthesis performance and osmoregulation

BACKGROUND

Salinity is one major abiotic stress affecting photosynthesis, plant growth, and development, resulting in low-input crops. Although photosynthesis underlies the substantial productivity and biomass storage of crop yield, the response of the sunflower photosynthetic machinery to salinity imposition and how H2S mitigates the salinity-induced photosynthetic injury remains largely unclear. Seed priming with 0.5 mM NaHS, as a donor of H2S, was adopted to analyze this issue under NaCl stress. Primed and nonprime seeds were established in nonsaline soil irrigated with tape water for 14 d, and then the seedlings were exposed to 150 mM NaCl for 7 d under controlled growth conditions.

RESULTS

Salinity stress significantly harmed plant growth, photosynthetic parameters, the structural integrity of chloroplasts, and mesophyll cells. H2S priming improved the growth parameters, relative water content, stomatal density and aperture, photosynthetic pigments, photochemical efficiency of PSII, photosynthetic performance, soluble sugar as well as soluble protein contents while reducing proline and ABA under salinity. H2S also boosted the transcriptional level of ribulose 1,5-bisphosphate carboxylase small subunit gene (HaRBCS). Further, the transmission electron microscope showed that under H2S priming and salinity stress, mesophyll cells maintained their cell membrane integrity and integrated chloroplasts with well-developed thylakoid membranes.

CONCLUSION

The results underscore the importance of H2S priming in maintaining photochemical efficiency, Rubisco activity, and preserving the chloroplast structure which participates in salinity stress adaptation, and possibly sunflower productivity under salinity imposition. This underpins retaining and minimizing the injury to the photosynthetic machinery to be a crucial trait in response of sunflower to salinity stress.

Sensitivity and responses of chloroplasts to salt stress in plants

Chloroplast, the site for photosynthesis and various biochemical reactions, is subject to many environmental stresses including salt stress, which affects chloroplast structure, photosynthetic processes, osmotic balance, ROS homeostasis, and so on. The maintenance of normal chloroplast function is essential for the survival of plants. Plants have developed different mechanisms to cope with salt-induced toxicity on chloroplasts to ensure the normal function of chloroplasts. The salt tolerance mechanism is complex and varies with plant species, so many aspects of these mechanisms are not entirely clear yet. In this review, we explore the effect of salinity on chloroplast structure and function, and discuss the adaptive mechanisms by which chloroplasts respond to salt stress. Understanding the sensitivity and responses of chloroplasts to salt stress will help us understand the important role of chloroplasts in plant salt stress adaptation and lay the foundation for enhancing plant salt tolerance.

The Salt Tolerance-Related Protein (STRP) Is a Positive Regulator of the Response to Salt Stress in Arabidopsis thaliana

Salt stress is a major abiotic stress limiting plant survival and crop productivity. Plant adaptation to salt stress involves complex responses, including changes in gene expression, regulation of hormone signaling, and production of stress-responsive proteins. The Salt Tolerance-Related Protein (STRP) has been recently characterized as a Late Embryogenesis Abundant (LEA)-like, intrinsically disordered protein involved in plant responses to cold stress. In addition, STRP has been proposed as a mediator of salt stress response in Arabidopsis thaliana, but its role has still to be fully clarified. Here, we investigated the role of STRP in salt stress responses in A. thaliana. The protein rapidly accumulates under salt stress due to a reduction of proteasome-mediated degradation. Physiological and biochemical responses of the strp mutant and STRP-overexpressing (STRP OE) plants demonstrate that salt stress impairs seed germination and seedling development more markedly in the strp mutant than in A. thaliana wild type (wt). At the same time, the inhibitory effect is significantly reduced in STRP OE plants. Moreover, the strp mutant has a lower ability to counteract oxidative stress, cannot accumulate the osmocompatible solute proline, and does not increase abscisic acid (ABA) levels in response to salinity stress. Accordingly, the opposite effect was observed in STRP OE plants. Overall, obtained results suggest that STRP performs its protective functions by reducing the oxidative burst induced by salt stress, and plays a role in the osmotic adjustment mechanisms required to preserve cellular homeostasis. These findings propose STRP as a critical component of the response mechanisms to saline stress in A. thaliana.

Salt Stress Inhibits Photosynthesis and Destroys Chloroplast Structure by Downregulating Chloroplast Development-Related Genes in Robinia pseudoacacia Seedlings

Soil salinization is an important factor limiting food security and ecological stability. As a commonly used greening tree species, Robinia pseudoacacia often suffers from salt stress that can manifest as leaf yellowing, decreased photosynthesis, disintegrated chloroplasts, growth stagnation, and even death. To elucidate how salt stress decreases photosynthesis and damages photosynthetic structures, we treated R. pseudoacacia seedlings with different concentrations of NaCl (0, 50, 100, 150, and 200 mM) for 2 weeks and then measured their biomass, ion content, organic soluble substance content, reactive oxygen species (ROS) content, antioxidant enzyme activity, photosynthetic parameters, chloroplast ultrastructure, and chloroplast development-related gene expression. NaCl treatment significantly decreased biomass and photosynthetic parameters, but increased ion content, organic soluble substances, and ROS content. High NaCl concentrations (100-200 mM) also led to distorted chloroplasts, scattered and deformed grana lamellae, disintegrated thylakoid structures, irregularly swollen starch granules, and larger, more numerous lipid spheres. Compared to control (0 mM NaCl), the 50 mM NaCl treatment significantly increased antioxidant enzyme activity while upregulating the expression of the ion transport-related genes Na⁺/H⁺ exchanger 1(NHX 1) and salt overly sensitive 1 (SOS 1) and the chloroplast development-related genes psaA, psbA, psaB, psbD, psaC, psbC, ndhH, ndhE, rps7, and ropA. Additionally, high concentrations of NaCl (100-200 mM) decreased antioxidant enzyme activity and downregulated the expression of ion transport- and chloroplast development-related genes. These results showed that although R. pseudoacacia can tolerate low concentrations of NaCl, high concentrations (100-200 mM) can damage chloroplast structure and disturb metabolic processes by downregulating gene expression.

COP1 controls salt stress tolerance by modulating sucrose content

The E3 ubiquitin ligase Constitutive Photomorphogenic 1 (COP1) plays evolutionarily conserved and divergent roles. In plants, COP1 regulates a large number of developmental processes including photomorphogenesis, seedling emergence, and gravitropism. Nevertheless, its function in abiotic stress tolerance remains largely unknown. Here, we demonstrate the role of COP1 in salt stress tolerance in Arabidopsis thaliana. In soil, cop1-4 and cop1-6 mutants were more tolerant to high salinity than wild-type (WT) plants during vegetative growth. However, in high salt-containing Murashige and Skoog (MS) medium, cop1-4 and cop1-6 seedlings exhibited significantly impaired growth compared with WT plants. Notably, cop1-4 and cop1-6 seedlings recovered their growth to the WT level upon exogenous sucrose treatment even under high salinity conditions. Compared with WT plants, the sucrose content of cop1-4 mutants was much higher at the vegetative growth stage but similar at the seedling stage. Upon exogenous sucrose supply, root elongation was significantly stimulated in cop1-4 seedlings but only slightly stimulated in WT plants. Thus, no significant difference was observed in root length between the two genotypes. Altogether, our data indicate that cop1 mutants are more tolerant to salt stress than WT plants, and the salt tolerance of cop1 mutants is correlated with their sucrose content.

Engineered Nanomaterial Exposure Affects Organelle Genetic Material Replication in Arabidopsis thaliana

Mitochondria and chloroplasts not only are cellular energy sources but also have important regulatory and developmental roles in cell function. CeO₂, FeOx ENMs, ZnS, CdS QDs, and relative metal salts were utilized in Murashige-Skoog (MS) synthetic growth medium at different concentrations (80-500 mg L^-1) and times of exposures (0-20 days). Analysis of physiological and molecular response of A. thaliana chloroplasts and mitochondrion demonstrates that ENMs increase or decrease functionality and organelle genome replication. Exposure to nanoscale CeO₂ and FeOx causes an 81-105% increase in biomass, whereas ZnS and CdS QDs yielded neutral or a 59% decrease in growth, respectively. Differential effects between ENMs and their corresponding metal salts highlight nanoscale-specific response pathways, which include energy production and oxidative stress response. Differences may be ascribed to ENM and the metal salt dissolution rate and the toxicity of the metal ion, which suggests eventual biotransformation processes occurring within the plant. With regard to specific effects on plastid (pt) and mitochondrial (mt) DNA, CdS QD exposure triggered potential variations at the substoichiometric level in the two organellar genomes, while nanoscale FeOx and ZnS QDs caused a 1- to 3-fold increase in ptDNA and mtDNA copy numbers. Nanoparticle CeO₂ exposure did not affect ptDNA and mtDNA stoichiometry. These findings suggest that modification in stoichiometry is a potential morpho-functional adaptive response to ENM exposure, triggered by modifications of bioenergetic redox balance, which leads to reducing the photosynthesis or cellular respiration rate.

Association of jasmonic acid priming with multiple defense mechanisms in wheat plants under high salt stress

Salinity is a global conundrum that negatively affects various biometrics of agricultural crops. Jasmonic acid (JA) is a phytohormone that reinforces multilayered defense strategies against abiotic stress, including salinity. This study investigated the effect of JA (60 μM) on two wheat cultivars, namely ZM9 and YM25, exposed to NaCl (14.50 dSm^-1) during two consecutive growing seasons. Morphologically, plants primed with JA enhanced the vegetative growth and yield components. The improvement of growth by JA priming is associated with increased photosynthetic pigments, stomatal conductance, intercellular CO₂, maximal photosystem II efficiency, and transpiration rate of the stressed plants. Furthermore, wheat cultivars primed with JA showed a reduction in the swelling of the chloroplast, recovery of the disintegrated thylakoids grana, and increased plastoglobuli numbers compared to saline-treated plants. JA prevented dehydration of leaves by increasing relative water content and water use efficiency via reducing water and osmotic potential using proline as an osmoticum. There was a reduction in sodium (Na⁺) and increased potassium (K⁺) contents, indicating a significant role of JA priming in ionic homeostasis, which was associated with induction of the transporters, viz., SOS1, NHX2, and HVP1. Exogenously applied JA mitigated the inhibitory effect of salt stress in plants by increasing the endogenous levels of cytokinins and indole acetic acid, and reducing the abscisic acid (ABA) contents. In addition, the oxidative stress caused by increasing hydrogen peroxide in salt-stressed plants was restrained by JA, which was associated with increased α-tocopherol, phenolics, and flavonoids levels and triggered the activities of superoxide dismutase and ascorbate peroxidase activity. This increase in phenolics and flavonoids could be explained by the induction of phenylalanine ammonia-lyase activity. The results suggest that JA plays a key role at the morphological, biochemical, and genetic levels of stressed and non-stressed wheat plants which is reflected in yield attributes. Hierarchical cluster analysis and principal component analyses showed that salt sensitivity was associated with the increments of Na⁺, hydrogen peroxide, and ABA contents. The regulatory role of JA under salinity stress was interlinked with increased JA level which consequentially improved ion transporting, osmoregulation, and antioxidant defense.

Different responses of two Chinese cabbage (Brassica rapa L. ssp. pekinensis) cultivars in photosynthetic characteristics and chloroplast ultrastructure to salt and alkali stress

Salt and alkali stress affected the photosynthetic characteristics of Chinese cabbages. A salt-tolerant cultivar maintained its tolerance by ensuring the high ability of photosynthesis. The synthesis of organic acids and carbohydrates in leaves played important roles in improving the photosynthetic capacity of alkali-tolerant plants. Soil salinization has become an increasingly serious ecological problem, which limits the quality and yield of crops. As an important economic vegetable in winter, however, little is known about the response of Chinese cabbage to salt, alkali and salt-alkali stress in photosynthetic characteristics and chloroplast ultrastructure. Thus, two Chinese cabbage cultivars, 'Qinghua' (salt-tolerant-alkali-sensitive) and 'Biyu' (salt-sensitive-alkali-tolerant) were investigated under stresses to clarify the similarities and differences between salt tolerance and alkali tolerance pathways in Chinese cabbage. We found that the root of Qinghua, the leaf ultrastructure and net photosynthetic rate (Pn), stomatal conductance (Gs), water use efficiency (WUE), maximum photochemical quantum yield of PSII (Fv/Fm) and nonphotochemical quenching (NPQ) were not affected by salt stress. However, Biyu was seriously affected under salt stress. Its growth indexes decreased by between 60 and 30% compared with the control and the photosynthetic indexes were also seriously affected under salt stress. This indicated that the salt-tolerant cultivar Qinghua improved the photosynthetic fluorescence ability to promote the synthesis of organic matter resulting in salt tolerance. In contrast, under alkali treatment, the root of Biyu was affected by alkali stress, but could still maintain good growth, and root and leaf structure were not seriously affected and could maintain the normal operations. Biyu improved its tolerance by improving the water use efficiency, regulating the synthesis of organic acids and carbohydrates, ensuring the synthesis of organic matter and ensured the normal growth of the plant.

Impact of foliar spray of zinc oxide nanoparticles on the photosynthesis of Pisum sativum L. under salt stress

This study investigates the impacts of zinc oxide nanoparticles: bare (ZnO NPs) and ZnO NPs coated with silicon shell (ZnO-Si NPs), on Pisum sativum L. under physiological and salt stress conditions. The experimental results revealed that the foliar spray with ZnO-Si NPs and 200 mg/L ZnO NPs did not influence the stomata structure, the membrane integrity, and the functions of both photosystems under physiological conditions, while 400 mg/L ZnO-Si NPs had beneficial effects on the effective quantum yield of photosystem II (PSII) and the photochemistry of photosystem I (PSI). On the contrary, small phytotoxic effects were registered after spraying with 400 mg/L ZnO NPs accompanied by stimulation of the cyclic electron flow around PSI and an increase of the non-photochemical quenching (NPQ). The results also showed that both types of NPs (with exception of 400 mg/L ZnO NPs) decrease the negative effects of 100 mM NaCl on the photochemistry of PSI (P700 photooxidation) and PSII (qp, Fv/Fm, Fv/Fo, ΦPSII, Φexc), as well as on the pigment content, stomata closure and membrane integrity. The protective effect was stronger after spraying with ZnO-Si NPs in comparison to ZnO NPs, which could be due to the presence of Si coating shell. The role of Si shell is discussed.

Effects of Salinity Stress on Chloroplast Structure and Function

Salinity is a growing problem affecting soils and agriculture in many parts of the world. The presence of salt in plant cells disrupts many basic metabolic processes, contributing to severe negative effects on plant development and growth. This review focuses on the effects of salinity on chloroplasts, including the structures and function of these organelles. Chloroplasts house various important biochemical reactions, including photosynthesis, most of which are considered essential for plant survival. Salinity can affect these reactions in a number of ways, for example, by changing the chloroplast size, number, lamellar organization, lipid and starch accumulation, and interfering with cross-membrane transportation. Research has shown that maintenance of the normal chloroplast physiology is necessary for the survival of the entire plant. Many plant species have evolved different mechanisms to withstand the harmful effects of salt-induced toxicity on their chloroplasts and its machinery. The differences depend on the plant species and growth stage and can be quite different between salt-sensitive (glycophyte) and salt-tolerant (halophyte) plants. Salt stress tolerance is a complex trait, and many aspects of salt tolerance in plants are not entirely clear yet. In this review, we discuss the different mechanisms of salt stress tolerance in plants with a special focus on chloroplast structure and its functions, including the underlying differences between glycophytes and halophytes.

Genome-wide identification of the BASS gene family in four Gossypium species and functional characterization of GhBASSs against salt stress

Bile acid sodium symporter (BASS) family proteins encode a class of sodium/solute symporters. Even though the sodium transporting property of BASSs in mammals was well studied, their sodium transportability and functional roles in plant salt tolerance remained largely unknown. Here, BASS family members from 4 cotton species, as well as 30 other species were identified. Then, they were designated as members of BASS1 to BASS5 subfamilies according to their sequence similarity and phylogenetic relationships. There were 8, 11, 16 and 18 putative BASS genes in four cotton species. While whole-genome duplications (WGD) and segmental duplications rendered the expansion of the BASS gene family in cotton, BASS gene losses occurred in the tetraploid cotton during the evolution from diploids to allotetraploids. Concerning functional characterizations, the transcript profiling of GhBASSs revealed that they not only preferred tissue-specific expression but also were differently induced by various stressors and phytohormones. Gene silencing and overexpression experiments showed that GhBASS1 and GhBASS3 positively regulated, whereas GhBASS2, GhBASS4 and GhBASS5 negatively regulated plant salt tolerance. Taken together, BASS family genes have evolved before the divergence from the common ancestor of prokaryotes and eukaryotes, and GhBASSs are plastidial sodium-dependent metabolite co-transporters that can influence plant salt tolerance.

Ectopic overexpression of a cotton plastidial Na+ transporter GhBASS5 impairs salt tolerance in Arabidopsis via increasing Na+ loading and accumulation

GhBASS5 is a member of the bile acid sodium symporter (BASS) gene family from cotton and a plastid-localized Na⁺ transporter that negatively regulates salt tolerance of plants. Soil salinization is a major constraint on global cotton production, and Na⁺ is the most dominant toxic ion in salinity stress. Hence, insights into the identities and properties of transporters that catalyze Na⁺ movement between different tissues and within the cell compartments are vital to understand the salt-tolerant mechanisms of plants. Here, we identified the GhBASS5 gene, a member of the bile acid sodium symporter (BASS) gene family from cotton, served as a plastidic Na⁺ transporter. GhBASS5 encodes a membrane protein localized in the plastid envelope. It was highly expressed in cotton roots and predominantly existed in the vascular cylinder. Heterogenous expression of GhBASS5 in Arabidopsis chloroplasts promoted Na⁺ uptake into chloroplasts, which contributed to an increased cytoplasmic Na⁺ concentration. And GhBASS5-overexpressed transgenic plants showed an increase in Na⁺ translocation from roots to shoots and an elevated Na⁺ content in both roots and shoots, but a dramatic decrease in the Na⁺ efflux from root tissues and the K⁺/Na⁺ ratio, especially under salt stress conditions. Furthermore, overexpressing GhBASS5 greatly damaged plastid functions and enhanced salt sensitivity in transgenic Arabidopsis when compared with wild-type plants under salt stress. Additionally, the salt-responsive transporter genes that regulate K⁺/Na⁺ homeostasis were dramatically expressed in GhBASS5-overexpressed lines, especially under salt stress conditions. Taken together, our results suggest that GhBASS5 is a plastid-localized Na⁺ transporter, and high expression of GhBASS5 impairs salt tolerance of plants via increasing Na⁺ transportation and accumulation at both cell and tissue levels.

Arabidopsis Plastid-RNA Polymerase RPOTp Is Involved in Abiotic Stress Tolerance

Plastid gene expression (PGE) must adequately respond to changes in both development and environmental cues. The transcriptional machinery of plastids in land plants is far more complex than that of prokaryotes. Two types of DNA-dependent RNA polymerases transcribe the plastid genome: a multimeric plastid-encoded polymerase (PEP), and a monomeric nuclear-encoded polymerase (NEP). A single NEP in monocots (RPOTp, RNA polymerase of the T3/T7 phage-type) and two NEPs in dicots (plastid-targeted RPOTp, and plastid- and mitochondrial-targeted RPOTmp) have been hitherto identified. To unravel the role of PGE in plant responses to abiotic stress, we investigated if Arabidopsis RPOTp could function in plant salt tolerance. To this end, we studied the sensitivity of T-DNA mutants scabra3-2 (sca3-2) and sca3-3, defective in the RPOTp gene, to salinity, osmotic stress and the phytohormone abscisic acid (ABA) required for plants to adapt to abiotic stress. sca3 mutants were hypersensitive to NaCl, mannitol and ABA during germination and seedling establishment. Later in development, sca3 plants displayed reduced sensitivity to salt stress. A gene ontology (GO) analysis of the nuclear genes differentially expressed in the sca3-2 mutant (301) revealed that many significantly enriched GO terms were related to chloroplast function, and also to the response to several abiotic stresses. By quantitative RT-PCR (qRT-PCR), we found that genes LHCB1 (LIGHT-HARVESTING CHLOROPHYLL a/b-BINDING1) and AOX1A (ALTERNATIVE OXIDASE 1A) were respectively down- and up-regulated in the Columbia-0 (Col-0) salt-stressed plants, which suggests the activation of plastid and mitochondria-to-nucleus retrograde signaling. The transcript levels of genes RPOTp, RPOTmp and RPOTm significantly increased in these salt-stressed seedlings, but this enhanced expression did not lead to the up-regulation of the plastid genes solely transcribed by NEP. Similar to salinity, carotenoid inhibitor norflurazon (NF) also enhanced the RPOTp transcript levels in Col-0 seedlings. This shows that besides salinity, inhibition of chloroplast biogenesis also induces RPOTp expression. Unlike salt and NF, the NEP genes were significantly down-regulated in the Col-0 seedlings grown in ABA-supplemented media. Together, our findings demonstrate that RPOTp functions in abiotic stress tolerance, and RPOTp is likely regulated positively by plastid-to-nucleus retrograde signaling, which is triggered when chloroplast functionality is perturbed by environmental stresses, e.g., salinity or NF. This suggests the existence of a compensatory mechanism, elicited by impaired chloroplast function. To our knowledge, this is the first study to suggest the role of a nuclear-encoded plastid-RNA polymerase in salt stress tolerance in plants.

Transcriptional and Post-transcriptional Regulation of Organellar Gene Expression (OGE) and Its Roles in Plant Salt Tolerance

Given their endosymbiotic origin, chloroplasts and mitochondria genomes harbor only between 100 and 200 genes that encode the proteins involved in organellar gene expression (OGE), photosynthesis, and the electron transport chain. However, as the activity of these organelles also needs a few thousand proteins encoded by the nuclear genome, a close coordination of the gene expression between the nucleus and organelles must exist. In line with this, OGE regulation is crucial for plant growth and development, and is achieved mainly through post-transcriptional mechanisms performed by nuclear genes. In this way, the nucleus controls the activity of organelles and these, in turn, transmit information about their functional state to the nucleus by modulating nuclear expression according to the organelles' physiological requirements. This adjusts organelle function to plant physiological, developmental, or growth demands. Therefore, OGE must appropriately respond to both the endogenous signals and exogenous environmental cues that can jeopardize plant survival. As sessile organisms, plants have to respond to adverse conditions to acclimate and adapt to them. Salinity is a major abiotic stress that negatively affects plant development and growth, disrupts chloroplast and mitochondria function, and leads to reduced yields. Information on the effects that the disturbance of the OGE function has on plant tolerance to salinity is still quite fragmented. Nonetheless, many plant mutants which display altered responses to salinity have been characterized in recent years, and interestingly, several are affected in nuclear genes encoding organelle-localized proteins that regulate the expression of organelle genes. These results strongly support a link between OGE and plant salt tolerance, likely through retrograde signaling. Our review analyzes recent findings on the OGE functions required by plants to respond and tolerate salinity, and highlights the fundamental role that chloroplast and mitochondrion homeostasis plays in plant adaptation to salt stress.

Ion-dependent metabolic responses of Vicia faba L. to salt stress

Salt-affected farmlands are increasingly burdened by chlorides, carbonates, and sulfates of sodium, calcium, and magnesium. Intriguingly, the underlying physiological processes are studied almost always under NaCl stress. Two faba bean cultivars were subjected to low- and high-salt treatments of NaCl, Na₂ SO₄ , and KCl. Assimilation rate and leaf water vapor conductance were reduced to approximately 25-30% without biomass reduction after 7 days salt stress, but this did not cause severe carbon shortage. The equimolar treatments of Na⁺ , K⁺ , and Cl^- showed comparable accumulation patterns in leaves and roots, except for SO₄^2- which did not accumulate. To gain a detailed understanding of the effects caused by the tested ion combinations, we performed nontargeted gas chromatography-mass spectrometry-based metabolite profiling. Metabolic responses to various salts were in part highly linearly correlated, but only a few metabolite responses were common to all salts and in both cultivars. At high salt concentrations, only myo-inositol, allantoin, and glycerophosphoglycerol were highly significantly increased in roots under all tested conditions. We discovered several metabolic responses that were preferentially associated with the presence of Na⁺ , K⁺ , or Cl^- . For example, increases of leaf proline and decreases of leaf fumaric acid and malic acid were apparently associated with Cl^- accumulation.

Chloroplast function and ion regulation in plants growing on saline soils: lessons from halophytes

Salt stress impacts multiple aspects of plant metabolism and physiology. For instance it inhibits photosynthesis through stomatal limitation, causes excessive accumulation of sodium and chloride in chloroplasts, and disturbs chloroplast potassium homeostasis. Most research on salt stress has focused primarily on cytosolic ion homeostasis with few studies of how salt stress affects chloroplast ion homeostasis. This review asks the question whether membrane-transport processes and ionic relations are differentially regulated between glycophyte and halophyte chloroplasts and whether this contributes to the superior salt tolerance of halophytes. The available literature indicates that halophytes can overcome stomatal limitation by switching to CO2 concentrating mechanisms and increasing the number of chloroplasts per cell under saline conditions. Furthermore, salt entry into the chloroplast stroma may be critical for grana formation and photosystem II activity in halophytes but not in glycophytes. Salt also inhibits some stromal enzymes (e.g. fructose-1,6-bisphosphatase) to a lesser extent in halophyte species. Halophytes accumulate more chloride in chloroplasts than glycophytes and appear to use sodium in functional roles. We propose the molecular identities of candidate transporters that move sodium, chloride and potassium across chloroplast membranes and discuss how their operation may regulate photochemistry and photosystem I and II activity in chloroplasts.

Salt-stress-responsive chloroplast proteins in Brassica juncea genotypes with contrasting salt tolerance and their quantitative PCR analysis

Brassica juncea is mainly cultivated in the arid and semi-arid regions of India where its production is significantly affected by soil salinity. Adequate knowledge of the mechanisms underlying the salt tolerance at sub-cellular levels must aid in developing the salt-tolerant plants. A proper functioning of chloroplasts under salinity conditions is highly desirable to maintain crop productivity. The adaptive molecular mechanisms offered by plants at the chloroplast level to cope with salinity stress must be a prime target in developing the salt-tolerant plants. In the present study, we have analyzed differential expression of chloroplast proteins in two Brassica juncea genotypes, Pusa Agrani (salt-sensitive) and CS-54 (salt-tolerant), under the effect of sodium chloride. The chloroplast proteins were isolated and resolved using 2DE, which facilitated identification and quantification of 12 proteins that differed in expression in the salt-tolerant and salt-sensitive genotypes. The identified proteins were related to a variety of chloroplast-associated molecular processes, including oxygen-evolving process, PS I and PS II functioning, Calvin cycle and redox homeostasis. Expression analysis of genes encoding differentially expressed proteins through real time PCR supported our findings with proteomic analysis. The study indicates that modulating the expression of chloroplast proteins associated with stabilization of photosystems and oxidative defence plays imperative roles in adaptation to salt stress.

Abscisic acid affects transcription of chloroplast genes via protein phosphatase 2C-dependent activation of nuclear genes: repression by guanosine-3'-5'-bisdiphosphate and activation by sigma factor 5

Abscisic acid (ABA) represses the transcriptional activity of chloroplast genes (determined by run-on assays), with the exception of psbD and a few other genes in wild-type Arabidopsis seedlings and mature rosette leaves. Abscisic acid does not influence chloroplast transcription in the mutant lines abi1-1 and abi2-1 with constitutive protein phosphatase 2C (PP2C) activity, suggesting that ABA affects chloroplast gene activity by binding to the pyrabactin resistance (PYR)/PYR1-like or regulatory component of ABA receptor protein family (PYR/PYL/RCAR) and signaling via PP2Cs and sucrose non-fermenting protein-related kinases 2 (SnRK2s). Further we show by quantitative PCR that ABA enhances the transcript levels of RSH2, RSH3, PTF1 and SIG5. RelA/SpoT homolog 2 (RSH2) and RSH3 are known to synthesize guanosine-3'-5'-bisdiphosphate (ppGpp), an inhibitor of the plastid-gene-encoded chloroplast RNA polymerase. We propose, therefore, that ABA leads to an inhibition of chloroplast gene expression via stimulation of ppGpp synthesis. On the other hand, sigma factor 5 (SIG5) and plastid transcription factor 1 (PTF1) are known to be necessary for the transcription of psbD from a specific light- and stress-induced promoter (the blue light responsive promoter, BLRP). We demonstrate that ABA activates the psbD gene by stimulation of transcription initiation at BLRP. Taken together, our data suggest that ABA affects the transcription of chloroplast genes by a PP2C-dependent activation of nuclear genes encoding proteins involved in chloroplast transcription.