Components and processes involved in retrograde signaling from chloroplast to nucleus.

Retrograde signaling conceptually means the transfer of signals from semi-autonomous cell organelles to the nucleus to modulate nuclear gene expression. A generalized explanation is that chloroplasts are highly sensitive to environmental stimuli and quickly generate signaling molecules (retrograde signals) and transport them to the nucleus through the cytosol to reprogram nuclear gene expression for cellular/metabolic adjustments to cope with environmental fluctuations. During the past decade, substantial advancements have been made in the area of retrograde signaling, including information on putative retrograde signals. Researchers have also proposed possible mechanisms for generating retrograde signals and their transmission. However, the exact mechanisms and processes responsible for transmitting retrograde signaling from the chloroplast to the nucleus remain elusive, demanding substantial attention. This review highlights strategies employed to detect retrograde signals, their possible modes of signaling to the nucleus, and their implications for cellular processes during stress conditions. The present review also summarizes the role of ROS-mediated retrograde signaling in plastid-nucleus communication and its functional significance in co-coordinating the physiological profile of plant cells.

Genetic manipulation of photosynthesis to enhance crop productivity under changing environmental conditions.

Current global agricultural production needs to be increased to feed the unconstrained growing population. The changing climatic condition due to anthropogenic activities also makes the conditions more challenging to meet the required crop productivity in the future. The increase in crop productivity in the post green revolution era most likely became stagnant, or no major enhancement in crop productivity observed. In this review article, we discuss the emerging approaches for the enhancement of crop production along with dealing to the future climate changes like rise in temperature, increase in precipitation and decrease in snow and ice level, etc. At first, we discuss the efforts made for the genetic manipulation of chlorophyll metabolism, antenna engineering, electron transport chain, carbon fixation, and photorespiratory processes to enhance the photosynthesis of plants and to develop tolerance in plants to cope with changing environmental conditions. The application of CRISPR to enhance the crop productivity and develop abiotic stress-tolerant plants to face the current changing climatic conditions is also discussed.

MicroRNAs modulating nutrient homeostasis: a sustainable approach for developing biofortified crops.

During their lifespan, sessile plants have to cope with bioavailability of the suboptimal nutrient concentration and have to constantly sense/evolve the connecting web of signal cascades for efficient nutrient uptake, storage, and translocation for proper growth and metabolism. However, environmental fluctuations and escalating anthropogenic activities are making it a formidable challenge for plants. This is adding to (micro)nutrient-deficient crops and nutritional insecurity. Biofortification is emerging as a sustainable and efficacious approach which can be utilized to combat the micronutrient malnutrition. A biofortified crop has an enriched level of desired nutrients developed using conventional breeding, agronomic practices, or advanced biotechnological tools. Nutrient homeostasis gets hampered under nutrient stress, which involves disturbance in short-distance and long-distance cell-cell/cell-organ communications involving multiple cellular and molecular components. Advanced sequencing platforms coupled with bioinformatics pipelines and databases have suggested the potential roles of tiny signaling molecules and post-transcriptional regulators, the microRNAs (miRNAs) in key plant phenomena including nutrient homeostasis. miRNAs are seen as emerging targets for biotechnology-based biofortification programs. Thus, understanding the mechanistic insights and regulatory role of miRNAs could open new windows for exploring them in developing nutrient-efficient biofortified crops. This review discusses significance and roles of miRNAs in plant nutrition and nutrient homeostasis and how they play key roles in plant responses to nutrient imbalances/deficiencies/toxicities covering major nutrients-nitrogen (N), phosphorus (P), sulfur (S), magnesium (Mg), iron (Fe), and zinc (Zn). A perspective view has been given on developing miRNA-engineered biofortified crops with recent success stories. Current challenges and future strategies have also been discussed.

Functional properties and the oligomeric state of alkyl hydroperoxide reductase subunit F (AhpF) in Pseudomonas aeruginosa.

Alkyl hydroperoxide reductase subunit F (AhpF) is a well-known flavoprotein that transfers electrons from pyridine nucleotides to the peroxidase protein AhpC via redox-active disulfide centers to detoxify hydrogen peroxide. However, study of AhpF has historically been limited to particular eubacteria, and the connection between the functional and structural properties of AhpF remains unknown. The present study demonstrates the dual function of Pseudomonas aeruginosa AhpF (PaAhpF) as a reductase and a molecular chaperone. It was observed that the functions of PaAhpF are closely linked with its structural status. The reductase and foldase chaperone function of PaAhpF predominated for its low-molecular-weight (LMW) form, whereas the holdase chaperone function of PaAhpF was found associated with its high-molecular-weight (HMW) complex. Further, the present study also demonstrates the multiple function of PaAhpF in controlling oxidative and heat stresses in P. aeruginosa resistance to oxidative and heat stress.

Functional switching of ascorbate peroxidase 2 of rice (OsAPX2) between peroxidase and molecular chaperone.

Ascorbate peroxidase (APX) is a class I haem-containing peroxidase, which catalyses the conversion of H2O2 to H2O and O2 using ascorbate as the specific electron donor. APX plays a central role in the elimination of intracellular reactive oxygen species (ROS) and protects plants from the oxidative damage that can occur as a result of biotic and abiotic stresses. At present, the only known function of APX is as a peroxidase. However, in this study, we demonstrate that Oryza sativa APX2 also operates as a molecular chaperone in rice. The different functions of OsAPX2 correlate strongly with its structural conformation. The high-molecular-weight (HMW) complexes had chaperone activity, whereas the low-molecular-weight (LMW) forms displayed predominantly APX activity. The APX activity was effectively inhibited by sodium azide, which is an inhibitor of haem-containing enzymes, but this did not affect the protein's activity as a chaperone. Additionally, the OsAPX2 conformational changes could be regulated by salt and heat stresses and these stimulated OsAPX2 dissociation and association, respectively. Our results provide new insight into the roles of APXs.

Enhancement of the Chaperone Activity of Alkyl Hydroperoxide Reductase C from Pseudomonas aeruginosa PAO1 Resulting from a Point-Specific Mutation Confers Heat Tolerance in Escherichia coli.

Alkyl hydroperoxide reductase subunit C from Pseudomonas aeruginosa PAO1 (PaAhpC) is a member of the 2-Cys peroxiredoxin family. Here, we examined the peroxidase and molecular chaperone functions of PaAhpC using a site-directed mutagenesis approach by substitution of Ser and Thr residues with Cys at positions 78 and 105 located between two catalytic cysteines. Substitution of Ser with Cys at position 78 enhanced the chaperone activity of the mutant (S78C-PaAhpC) by approximately 9-fold compared with that of the wild-type protein (WT-PaAhpC). This increased activity may have been associated with the proportionate increase in the high-molecular-weight (HMW) fraction and enhanced hydrophobicity of S78C-PaAhpC. Homology modeling revealed that mutation of Ser(78) to Cys(78) resulted in a more compact decameric structure than that observed in WT-PaAhpC and decreased the atomic distance between the two neighboring sulfur atoms of Cys(78) in the dimer-dimer interface of S78C-PaAhpC, which could be responsible for the enhanced hydrophobic interaction at the dimer-dimer interface. Furthermore, complementation assays showed that S78C-PaAhpC exhibited greatly improved the heat tolerance, resulting in enhanced survival under thermal stress. Thus, addition of Cys at position 78 in PaAhpC modulated the functional shifting of this protein from a peroxidase to a chaperone.

Rapid activation of catalase followed by citrate efflux effectively improves aluminum tolerance in the roots of chick pea (Cicer arietinum).

The present study demonstrates the comparative response of two contrasting genotypes (aluminum (Al) tolerant and Al sensitive) of chick pea (Cicer arietinum) against Al stress. The Al-tolerant genotype (RSG 974) showed lesser inhibition of root growth as well as lower oxidative damages, measured in terms of the accumulation of H2O2 and lipid peroxidation compared to the Al-sensitive genotype (RSG 945). The accumulation of Al by roots of both genotypes was almost equal at 96 and 144 h after Al treatment; however, it was higher in Al-tolerant than Al-sensitive genotype at 48 h after Al treatment. Further, the Al-mediated induction of superoxide dismutase (SOD) activity was significantly higher in Al-tolerant than Al-sensitive genotype. Ascorbate peroxidase (APX) activity was almost similar in both genotypes. Al treatment promptly activated catalase activity in Al-tolerant genotype, and it was remarkably higher than that of Al-sensitive genotype. As another important Al detoxification mechanism, citrate efflux was almost equal in both genotypes except at 1000 µM Al treatment for 96 and 144 h. Further, citrate carrier and anion channel inhibitor experiment confirmed the contribution of citrate efflux in conferring Al tolerance in Al-tolerant genotype. Based on the available data, the present study concludes that rapid activation of catalase (also SOD) activity followed by citrate efflux effectively improves Al tolerance in chick pea.

Interaction of Mg with heavy metals (Cu, Cd) in T. aestivum with special reference to oxidative and proline metabolism.

Little effort has been made to understand the influence of Mg on cellular processes of plant cell during Cu and Cd toxicities. The present work demonstrates the influence of magnesium (Mg) on copper (Cu) and cadmium (Cd) toxicity on Triticum aestivum (Wheat). We measured a range of parameters related to oxidative stress in wheat exposed to Cu or Cd toxicity in media with different concentrations of Mg. Decreasing Mg concentration significantly exacerbated Cu and Cd toxicity and optimum supply of Mg improved the growth and decreased the toxicity-induced oxidative stress (a substantial decline in the amount of hydrogen peroxide (H2O2) and malondialdehyde (MDA) in root and shoot tissues). Activity of antioxidant enzymes-superoxide dismutase (SOD), ascorbae peroxidase (APX), catalase (CAT) was restored upon optimum Mg concentration in the presence of Cu and Cd toxicity. An increase in proline concentration in roots and shoots that was triggered by Cu and Cd exposure was partly reversed. This was due to decline in pyrroline-5-carboxylate synthetase (P5CS) and pyrroline-5-carboxylate reductase (P5CR) activity and enhanced proline dehydrogenase (PDH) activity. In conclusion, decreasing supply of Mg effectively exacerbated the toxicities of Cu and Cd in wheat.

An additional cysteine in a typical 2-Cys peroxiredoxin of Pseudomonas promotes functional switching between peroxidase and molecular chaperone.

Peroxiredoxins (Prx) have received considerable attention during recent years. This study demonstrates that two typical Pseudomonas-derived 2-Cys Prx proteins, PpPrx and PaPrx can alternatively function as a peroxidase and chaperone. The amino acid sequences of these two Prx proteins exhibit 93% homology, but PpPrx possesses an additional cysteine residue, Cys112, instead of the alanine found in PaPrx. PpPrx predominates with a high molecular weight (HMW) complex and chaperone activity, whereas PaPrx has mainly low molecular weight (LMW) structures and peroxidase activity. Mass spectrometry and structural analyses showed the involvement of Cys112 in the formation of an inter-disulfide bond, the instability of LMW structures, the formation of HMW complexes, and increased hydrophobicity leading to functional switching of Prx proteins between peroxidase and chaperone.

Site-directed mutagenesis substituting cysteine for serine in 2-Cys peroxiredoxin (2-Cys Prx A) of Arabidopsis thaliana effectively improves its peroxidase and chaperone functions.

BACKGROUND AND AIMS: The 2-Cys peroxiredoxin (Prx) A protein of Arabidopsis thaliana performs the dual functions of a peroxidase and a molecular chaperone depending on its conformation and the metabolic conditions. However, the precise mechanism responsible for the functional switching of 2-Cys Prx A is poorly known. This study examines various serine-to-cysteine substitutions on α-helix regions of 2-Cys Prx A in Arabidopsis mutants and the effects they have on the dual function of the protein. METHODS: Various mutants of 2-Cys Prx A were generated by replacing serine (Ser) with cysteine (Cys) at different locations by site-directed mutagenesis. The mutants were then over-expressed in Escherichia coli. The purified protein was further analysed by size exclusion chromatography, polyacrylamide gel electrophoresis, circular dichroism spectroscopy and transmission electron microscopy (TEM) and image analysis. Peroxidase activity, molecular chaperone activity and hydrophobicity of the proteins were also determined. Molecular modelling analysis was performed in order to demonstrate the relationship between mutation positions and switching of 2-Cys Prx A activity. KEY RESULTS: Replacement of Ser(150) with Cys(150) led to a marked increase in holdase chaperone and peroxidase activities of 2-Cys Prx A, which was associated with a change in the structure of an important domain of the protein. Molecular modelling demonstrated the relationship between mutation positions and the switching of 2-Cys Prx A activity. Examination of the α2 helix, dimer-dimer interface and C-term loop indicated that the peroxidase function is associated with a fully folded α2 helix and easy formation of a stable reduced decamer, while a more flexible C-term loop makes the chaperone function less likely. CONCLUSIONS: Substitution of Cys for Ser at amino acid location 150 of the α-helix of 2-Cys Prx A regulates/enhances the dual enzymatic functions of the 2-Cys Prx A protein. If confirmed in planta, this leads to the potential for it to be used to maximize the functional utility of 2-Cys Prx A protein for improved metabolic functions and stress resistance in plants.

The lld mutation in Pisum sativum used as a genetic tool to discern the plant leaflet/leaf developmental process.

Leaves of P. sativum the double mutant genotype tendril-less (tl) leaflet-development (lld), due to the action of lld mutation, produce many leaflets that are aborted at different stages of development. Morphological, vein pattern and histological observations showed that aborted leaflets became cup/bell/trumpet (cup) shaped because of segmental differentiation in the leaflet primordium. Cup's inside lamina surface was adaxial and outer surfaces of cup and its stem were abaxial. The lld cups were phenotypically homologous to aborted leaves described in Arabidopsis thaliana mutants, angustifolia and those which underexpressed the HD-ZIP III proteins. Leaflet primordium was found to grow and establish three dimensional polarities apex-downwards. Primordium produced lateral outgrowth on one side of midvein. Differentiation, in the outgrowth of secondary veins, whose xylem tissues faced each other, established the adaxial-abaxial polarity. Lateral outgrowth then developed a cavity which got bounded by future adaxial epidermis. Further growth, veinlet formation, differentiation of palisade parenchyma and spongy parenchyma followed. Opening of lateral outgrowth at its outer midline produced a flat leaflet with lateral lamina spans. The structural and functional correspondence between leaflet and simple leaves suggested commonality between leaf and leaflet development mechanisms. A molecular model for the lld led leaflet abortion was also provided.

Glomalin: an arbuscular mycorrhizal fungal soil protein.

Glomalin is abundant in soils and is closely correlated with aggregate water stability. Glomalin contains carbon and, hence, constitutes a non-trivial portion of the terrestrial carbon pool. Possibly far more importantly, however, stabilization of aggregates amplifies the role of glomalin in soils because carbonaceous compounds are protected from degradation inside of aggregates. Increased atmospheric CO2 can lead to increased production of glomalin because of the symbiotic association that exists between plants and producers of glomalin, arbuscular mycorrhizal fungi (AMF). Glomalin concentrations in soils are influenced by management practices, for example, in agroecosystems, further highlighting the role of this protein in carbon storage. Glomalin is an unusual molecule that has proven difficult to analyze biochemically due to its recalcitrance and complexity. Future research will be directed towards the elucidation of its structure and controls on its production.

Pisum sativum wild-type and mutant stipules and those induced by an auxin transport inhibitor demonstrate the entire diversity of laminated stipules observed in angiosperms.

About a quarter of angiosperm species are stipulate. They produce stipule pairs at stem nodes in association with leaves. Stipule morphology is treated as a species-specific characteristic. Many species bear stipules as laminated organs in a variety of configurations, including laterally free large foliaceous, small, or wholly leaf-like stipules, and as fused intrapetiolar, opposite, ochreate or interpetiolar stipules. In Pisum sativum, the wild-type and stipule-reduced and cochleata mutants are known to form free large, small, and leaf-like stipules, respectively. Auxin controls initiation and development of plant organs and perturbations in its availability and distribution in the meristems, caused by auxin transport inhibitor(s) (ATIs), lead to aberrations in leaf development. The effect(s) of ATI(s) on stipule development are unexplored. To study the effect of the ATI 1-N-naphthylphthalamic acid (NPA) on stipule morphogenesis, P. sativum explants were grown in vitro in presence of a sublethal concentration of NPA. The NPA-treated shoots produced fused stipules of all the different types described in angiosperms. The observations indicate that (a) the gene sets for stipule differentiation may be common in angiosperms and (b) the interspecies stipule architectural differences are due to mutations, affecting gene expression or activity that got selected in the course of evolution.

Physiological changes induced by chromium stress in plants: an overview.

This article presents an overview of the mechanism of chromium (Cr) stress in plants. Toxic effects of Cr on plant growth and development depend primarily on its valence state. Cr(VI) is highly toxic and mobile whereas Cr(III) is less toxic. Cr-induced oxidative stress involves induction of lipid peroxidation in plants that cause severe damage to cell membranes which includes degradation of photosynthetic pigments causing deterioration in growth. The potential of plants with the adequacy to accumulate or to stabilize Cr compounds for bioremediation of Cr contamination has gained engrossment in recent years.

A comprehensive study on dehydration-induced antioxidative responses during germination of Indian bread wheat (Triticum aestivum L. em Thell) cultivars collected from different agroclimatic zones.

To explore the adaptability of bread wheat to dehydration stress, we screened 28 cultivars collected from different agroclimatic zones, on the basis of malonaldehyde content as biochemical marker in roots of wheat seedlings during germination and classified them as highly tolerant, tolerant, sensitive and highly sensitive. From this primary screening, ten cultivars that showed differential responses to dehydration stress were selected to understand the biochemical and physiological basis of stress tolerance mechanisms. The highly tolerant cultivars showed lower levels of lipid peroxidation, less membrane damage, increased levels of antioxidants, enzymes like catalase, ascorbate peroxidase, glutathione reductase activities, and maintained higher relative water content in comparison to sensitive cultivars, indicating better protection mechanism operating in tolerant cultivars. Correspondingly, highly tolerant cultivars exhibited more accumulation of proline and less H2O2 content across different time points of polyethylene glycol treatments in comparison to sensitive ones. The above biochemical and physiological parameters were further validated through northern analysis of catalase (CAT1) gene, that showed differential expression patterns in tolerant and sensitive cultivars largely in confirmation with the biochemical and physiological analyses. Our study positively correlates the differences in the redox status and antioxidant defense system between tolerant and sensitive cultivars for the establishment of wheat seedlings in typical dehydration conditions.

Interaction of engineered nanoparticles with various components of the environment and possible strategies for their risk assessment.

Nanoparticles are the materials with at least two dimensions between 1 and 100 nm. Mostly these nanoparticles are natural products but their tremendous commercial use has boosted the artificial synthesis of these particles (engineered nanoparticles). Accelerated production and use of these engineered nanoparticles may cause their release in the environment and facilitate the frequent interactions with biotic and abiotic components of the ecosystems. Despite remarkable commercial benefits, their presence in the nature may cause hazardous biological effects. Therefore, detail understanding of their sources, release interaction with environment, and possible risk assessment would provide a basis for safer use of engineered nanoparticles with minimal or no hazardous impact on environment. Keeping all these points in mind the present review provides updated information on various aspects, e.g. sources, different types, synthesis, interaction with environment, possible strategies for risk management of engineered nanoparticles.

Distinct role of non-covalent interactions to the function and structural stability of glutaredoxins: a multifunctional redox protein.

In this work, the role of cation-pi and CH...pi interactions in the activity of glutaredoxins is analysed. Among the proteins, an average of 1 cation-pi interaction per 109 residues and an average of 1 CH...pi interaction per 16 residues were found. These interactions were influenced by long-range contacts whereas short- and medium-range contacts were found insignificant. Significant differences in these interactions were noticed when the same glutaredoxin was analysed in its glutathionylated, reduced and oxidised states. Since activities of glutaredoxins depend on its state, the role of these interactions in regulation of these proteins might be significant.

Proline improves copper tolerance in chickpea (Cicer arietinum).

The present study suggests the involvement of proline in copper tolerance of four genotypes of Cicer arietinum (chickpea). Based on the data of tolerance index and lipid peroxidation, the order for copper tolerance was as follows: RSG 888 > CSG 144 > CSG 104 > RSG 44 in the selected genotypes. The basis of differential copper tolerance in chickpea genotypes was characterized by analyzing, antioxidant enzymes (superoxide dismutase, ascorbated peroxidase and catalase), phytochelatins, copper uptake, and proline accumulation. Chickpea genotypes showed stimulated superoxide dismutase activity at all tested concentrations of copper, but H(2)O(2) decomposing enzymes especially; ascorbate peroxidase did not increase with 25 and 50 microM copper treatments. Catalase activity, however, increased at lower copper concentrations but failed to stimulate at 50 microM copper. Such divergence in responses of these enzymes minimizes their importance in protecting chickpea against copper stress. The sensitive genotypes showed greater enhancement of phytochelatins than that of tolerant genotypes. Hence, the possibility of phytochelatins in improving copper tolerance in the test plant is also excluded. Interestingly, the order of proline accumulation in the chickpea genotypes (RSG 888 > CSG 144 > CSG 104 > RSG 44) was exactly similar to the order of copper tolerance. Based on hyperaccumulation of proline in tolerant genotype (RSG 44) and the reduction and improvement of lipid peroxidation and tolerance index, respectively, by proline pretreatment, we conclude that hyperaccumulation of proline improves the copper tolerance in chickpea.