Plant Stress Acclimation Research Articles

Silicon nanoparticles in sustainable agriculture: synthesis, absorption, and plant stress alleviation.

Silicon (Si) is a widely recognized beneficial element in plants. With the emergence of nanotechnology in agriculture, silicon nanoparticles (SiNPs) demonstrate promising applicability in sustainable agriculture. Particularly, the application of SiNPs has proven to be a high-efficiency and cost-effective strategy for protecting plant against various biotic and abiotic stresses such as insect pests, pathogen diseases, metal stress, drought stress, and salt stress. To date, rapid progress has been made in unveiling the multiple functions and related mechanisms of SiNPs in promoting the sustainability of agricultural production in the recent decade, while a comprehensive summary is still lacking. Here, the review provides an up-to-date overview of the synthesis, uptake and translocation, and application of SiNPs in alleviating stresses aiming for the reasonable usage of SiNPs in nano-enabled agriculture. The major points are listed as following: (1) SiNPs can be synthesized by using physical, chemical, and biological (green synthesis) approaches, while green synthesis using agricultural wastes as raw materials is more suitable for large-scale production and recycling agriculture. (2) The uptake and translocation of SiNPs in plants differs significantly from that of Si, which is determined by plant factors and the properties of SiNPs. (3) Under stressful conditions, SiNPs can regulate plant stress acclimation at morphological, physiological, and molecular levels as growth stimulator; as well as deliver pesticides and plant growth regulating chemicals as nanocarrier, thereby enhancing plant growth and yield. (4) Several key issues deserve further investigation including effective approaches of SiNPs synthesis and modification, molecular basis of SiNPs-induced plant stress resistance, and systematic effects of SiNPs on agricultural ecosystem.

Proline Metabolism Process and Antioxidant Potential of Lycium ruthenicum Murr. in Response to NaCl Treatments.

Salinity influences the level of antioxidants and proline content, which are both involved in the regulation of stress responses in plants. To examine the interplay between the antioxidant system and proline metabolism in plant stress acclimation, explants of Lycium ruthenicum were subjected to NaCl treatments, and the growth characteristics, antioxidant enzyme activities, proline accumulation, and metabolic enzyme content were analyzed. The results revealed that NaCl concentrations between 50 to 150 mM have a positive effect on the growth of L. ruthenicum explants. Increasing NaCl concentrations elevated the activities of superoxide dismutase (SOD) and catalase (CAT), while hydrogen peroxide (H2O2) content was inhibited, suggesting that the elevated antioxidants play a central protective role in superoxide anion (O2•-) and H2O2 scavenging processes in response to NaCl treatments. Also, high proline levels also protect antioxidant enzyme machinery, thus protecting the plants from oxidative damage and enhancing osmotic adjustment. Increasing levels of pyrroline-5-carboxylate synthetase (P5CS), pyrroline-5-carboxylate reductase (P5CR), and ornithine-δ-aminotransferase (δ-OAT) were observed, resulting in elevated level of proline. In addition, the expression levels of LrP5CS1, -2, -3, LrOAT-1, and -2 were promoted in NaCl treatments. According to the combined analysis of metabolic enzyme activities and their relative expression, it is confirmed that the glutamate (Glu) pathway is activated in L. ruthenicum faced with different levels of NaCl concentrations. However, Glu supplied by δ-OAT is fed back into the main pathway for proline metabolism.

Ethylene and spermine attenuate chromium-inhibited photosynthetic functions by improving nitrogen and sulfur assimilation and antioxidant system in mustard

Phytohormones signaling is known for the regulation of metabolites synthesis, photosynthetic performance and abiotic stress acclimation in plants. We investigated the influence of ethylene (sourced as ethephon; 2-chloroethyl phosohonic acid, ETH) and spermine (Spm; polyamine) in alleviating the chromium (Cr)-triggered oxidative stress in the mustard (Brassica juncea cv. Varuna). The individual additions of 200 µL L−1 ETH and 1 mM Spm improved photosynthetic and growth parameters in the absence of Cr, but the Cr presence increased oxidative stress and the formation of stress (excess) ethylene, inhibiting photosynthesis, and growth. The application of ETH and Spm together in the absence of Cr, improved photosynthesis conspicuously. In the presence of Cr, ETH and Spm application improved photosynthesis by upregulation of psbA and psbB genes expression, and reducing oxidative stress, stress ethylene formation, and increase in activity of antioxidant enzymes, upregulation of ascorbate peroxidase (APX) and glutathione reductase (GR) genes, and nitrogen (N) and sulfur (S) assimilation. The possible ways in which ethylene and Spm interact in the mitigation of Cr-triggered oxidative damage are discussed. The study will provide insight into the mechanisms of N and S assimilation, antioxidant system, and regulation of ethylene production under the influence of ETH and Spm, and the strategy may be adopted for the sustainability of mustard plants under Cr stress.

Plant thiol peroxidases as redox sensors and signal transducers in abiotic stress acclimation.

The temporal and spatial patterns of reactive oxygen species (ROS) in cells and tissues decisively determine the plant acclimation response to diverse abiotic and biotic stresses. Recent progress in developing dynamic cell imaging probes provides kinetic information on changes in parameters like H2O2, glutathione (GSH/GSSG) and NAD(P)H/NAD(P)+, that play a crucial role in tuning the cellular redox state. Central to redox-based regulation is the thiol-redox regulatory network of the cell that integrates reductive information from metabolism and oxidative ROS signals. Sensitive proteomics allow for monitoring changes in redox-related posttranslational modifications. Thiol peroxidases act as sensitive peroxide and redox sensors and play a central role in this signal transduction process. Peroxiredoxins (PRX) and glutathione peroxidases (GPX) are the two main thiol peroxidases and their function in ROS sensing and redox signaling in plants is emerging at present and summarized in this review. Depending on their redox state, PRXs and GPXs act as redox-dependent binding partners, direct oxidants of target proteins and oxidants of thiol redox transmitters that in turn oxidize target proteins. With their versatile functions, the multiple isoforms of plant thiol peroxidases play a central role in plant stress acclimation, e.g. to high light or osmotic stress, but also in ROS-mediated immunity and development.

The missing link: The role of the ROS receptor HPCA1 in systemic signaling and stress acclimation in plants.

The missing link: The role of the ROS receptor HPCA1 in systemic signaling and stress acclimation in plants.

Polyamine: A Potent Ameliorator for Plant Growth Response and Adaption to Abiotic Stresses Particularly the Ammonium Stress Antagonized by Urea.

Polyamine(s) (PA, PAs), a sort of N-containing and polycationic compound synthesized in almost all organisms, has been recently paid considerable attention due to its multifarious actions in the potent modulation of plant growth, development, and response to abiotic/biotic stresses. PAs in cells/tissues occur mainly in free or (non- or) conjugated forms by binding to various molecules including DNA/RNA, proteins, and (membrane-)phospholipids, thus regulating diverse molecular and cellular processes as shown mostly in animals. Although many studies have reported that an increase in internal PA may be beneficial to plant growth under abiotic conditions, leading to a suggestion of improving plant stress adaption by the elevation of endogenous PA via supply or molecular engineering of its biosynthesis, such achievements focus mainly on PA homeostasis/metabolism rather than PA-mediated molecular/cellular signaling cascades. In this study, to advance our understanding of PA biological actions important for plant stress acclimation, we gathered some significant research data to succinctly describe and discuss, in general, PA synthesis/catabolism, as well as PA as an internal ameliorator to regulate stress adaptions. Particularly, for the recently uncovered phenomenon of urea-antagonized NH4+-stress, from a molecular and physiological perspective, we rationally proposed the possibility of the existence of PA-facilitated signal transduction pathways in plant tolerance to NH4+-stress. This may be a more interesting issue for in-depth understanding of PA-involved growth acclimation to miscellaneous stresses in future studies.

A guanosine tetraphosphate (ppGpp) mediated brake on photosynthesis is required for acclimation to nitrogen limitation in Arabidopsis.

Guanosine pentaphosphate and tetraphosphate (together referred to as ppGpp) are hyperphosphorylated nucleotides found in bacteria and the chloroplasts of plants and algae. In plants and algae artificial ppGpp accumulation can inhibit chloroplast gene expression, and influence photosynthesis, nutrient remobilization, growth, and immunity. However, it is so far unknown whether ppGpp is required for abiotic stress acclimation in plants. Here, we demonstrate that ppGpp biosynthesis is necessary for acclimation to nitrogen starvation in Arabidopsis. We show that ppGpp is required for remodeling the photosynthetic electron transport chain to downregulate photosynthetic activity and for protection against oxidative stress. Furthermore, we demonstrate that ppGpp is required for coupling chloroplastic and nuclear gene expression during nitrogen starvation. Altogether, our work indicates that ppGpp is a pivotal regulator of chloroplast activity for stress acclimation in plants.

Hydrogen peroxide-induced stress acclimation in plants.

Among all reactive oxygen species (ROS), hydrogen peroxide (H2O2) takes a central role in regulating plant development and responses to the environment. The diverse role of H2O2 is achieved through its compartmentalized synthesis, temporal control exerted by the antioxidant machinery, and ability to oxidize specific residues of target proteins. Here, we examine the role of H2O2 in stress acclimation beyond the well-studied transcriptional reprogramming, modulation of plant hormonal networks and long-distance signalling waves by highlighting its global impact on the transcriptional regulation and translational machinery.

ICE-CBF-COR Signaling Cascade and Its Regulation in Plants Responding to Cold Stress.

Cold stress limits plant geographical distribution and influences plant growth, development, and yields. Plants as sessile organisms have evolved complex biochemical and physiological mechanisms to adapt to cold stress. These mechanisms are regulated by a series of transcription factors and proteins for efficient cold stress acclimation. It has been established that the ICE-CBF-COR signaling pathway in plants regulates how plants acclimatize to cold stress. Cold stress is perceived by receptor proteins, triggering signal transduction, and Inducer of CBF Expression (ICE) genes are activated and regulated, consequently upregulating the transcription and expression of the C-repeat Binding Factor (CBF) genes. The CBF protein binds to the C-repeat/Dehydration Responsive Element (CRT/DRE), a homeopathic element of the Cold Regulated genes (COR gene) promoter, activating their transcription. Transcriptional regulations and post-translational modifications regulate and modify these entities at different response levels by altering their expression or activities in the signaling cascade. These activities then lead to efficient cold stress tolerance. This paper contains a concise summary of the ICE-CBF-COR pathway elucidating on the cross interconnections with other repressors, inhibitors, and activators to induce cold stress acclimation in plants.

Mechanical stress acclimation in plants: Linking hormones and somatic memory to thigmomorphogenesis.

A single event of mechanical stimulation is perceived by mechanoreceptors that transduce rapid transient signalling to regulate gene expression. Prolonged mechanical stress for days to weeks culminates in cellular changes that strengthen the plant architecture leading to thigmomorphogenesis. The convergence of multiple signalling pathways regulates mechanically induced tolerance to numerous biotic and abiotic stresses. Emerging evidence showed prolonged mechanical stimulation can modify the baseline level of gene expression in naive tissues, heighten gene expression, and prime disease resistance upon a subsequent pathogen encounter. The phenotypes of thigmomorphogenesis can persist throughout growth without continued stimulation, revealing somatic-stress memory. Epigenetic processes regulate TOUCH gene expression and could program transcriptional memory in differentiating cells to program thigmomorphogenesis. We discuss the early perception, gene regulatory and phytohormone pathways that facilitate thigmomorphogenesis and mechanical stress acclimation in Arabidopsis and other plant species. We provide insights regarding: (1) the regulatory mechanisms induced by single or prolonged events of mechanical stress, (2) how mechanical stress confers transcriptional memory to induce cross-acclimation to future stress, and (3) why thigmomorphogenesis might resemble an epigenetic phenomenon. Deeper knowledge of how prolonged mechanical stimulation programs somatic memory and primes defence acclimation could transform solutions to improve agricultural sustainability in stressful environments.

Chloroplast-derived photo-oxidative stress causes changes in H2O2 and EGSH in other subcellular compartments.

Metabolic fluctuations in chloroplasts and mitochondria can trigger retrograde signals to modify nuclear gene expression. Mobile signals likely to be involved are reactive oxygen species (ROS), which can operate protein redox switches by oxidation of specific cysteine residues. Redox buffers, such as the highly reduced glutathione pool, serve as reservoirs of reducing power for several ROS-scavenging and ROS-induced damage repair pathways. Formation of glutathione disulfide and a shift of the glutathione redox potential (EGSH) toward less negative values is considered as hallmark of several stress conditions. Here we used the herbicide methyl viologen (MV) to generate ROS locally in chloroplasts of intact Arabidopsis (Arabidopsis thaliana) seedlings and recorded dynamic changes in EGSH and H2O2 levels with the genetically encoded biosensors Grx1-roGFP2 (for EGSH) and roGFP2-Orp1 (for H2O2) targeted to chloroplasts, the cytosol, or mitochondria. Treatment of seedlings with MV caused rapid oxidation in chloroplasts and, subsequently, in the cytosol and mitochondria. MV-induced oxidation was significantly boosted by illumination with actinic light, and largely abolished by inhibitors of photosynthetic electron transport. MV also induced autonomous oxidation in the mitochondrial matrix in an electron transport chain activity-dependent manner that was milder than the oxidation triggered in chloroplasts by the combination of MV and light. In vivo redox biosensing resolves the spatiotemporal dynamics of compartmental responses to local ROS generation and provides a basis for understanding how compartment-specific redox dynamics might operate in retrograde signaling and stress acclimation in plants.

Phytochrome type B family: The abiotic stress responses signaller in plants

AbstractPhotoreceptors are primarily known as key photomorphogenic modulators of various physiological events during plant development. Although there are different groups of photoreceptors, the phytochrome B (phyB) family mediates developmental responses in a wide range of plant species, from seed germination to flowering. In addition, these molecules also regulate abiotic stress acclimation responses, such as salinity, drought, low/high temperature, high light and heavy metals. The signalling pathways mediated by phyB could enhance plant resistance to environmental stresses, as phyB photoreceptors reduce leaf transpiration, increase the antioxidant system, enhance protective pigments and increase the expression of genes related to plant stress acclimation. Thus, the elucidation of positive or negative roles for phyB in these stress tolerance characteristics would provide essential knowledge for genetic engineering, improving plant growth and development in critical environments. In this review, we cover the main findings on how the phyB family works to modulate abiotic stress by discussing biochemical and molecular aspects of the underlying mechanisms operated by these photoreceptors.

Involvement of homocastasterone, salicylic and abscisic acids in the regulation of drought and freezing tolerance in doubled haploid lines of winter barley

Ten doubled haploid (DH) lines of winter barley with an increased range of freezing/drought tolerance were used to identify phytohormones involved in plant stress acclimation. Cold hardening and drought stress were applied at the most critical stages of plant development on young seedlings and heading plants, respectively. The level of the phytohormones was significantly higher at heading, more than 5-fold in respect of salicylic acid (SA) and total brassinosteroids (BRs) and 1.7-fold in the case of abscisic acid (ABA). Moreover, the spectrum of detectable BRs increased from one—homocastasterone (HCS)—found in seedlings to four BRs identified in heading plants [HCS, castasterone (CS), teasterone and dolicholide], with the last one detected for the first time in cereal species. To some extent freezing tolerance seems to be determined by native hormonal status as control seedlings of tolerant DH lines contained 1.4- and 2.3-fold lower amount of ABA and HCS and 2.3-fold higher amount of SA in comparison to freezing-sensitive ones. Such dependency was not observed in heading plants as significant variation in CS content was the only detected difference. Under stress treatments, tolerant DH lines accumulated significantly lower (75–81%) amount of ABA, which probably reflected lower stress intensity resulting from another defence strategy. In contrast, stress-induced significant almost 2-fold increase in HCS/CS and 2–3-fold decrease in SA content specific for tolerant DH lines of barley suggest the involvement of these molecules in freezing/drought defence. Detected correlations suggest their interaction with nonspecific peroxidase and low molecular weight antioxidants.

Effects of early cold stress on gene expression in Chlamydomonas reinhardtii

Cold stress imposes a great impact on the growth of nearly all photosynthetic organisms, including Chlamydomonas reinhardtii (C. reinhardtii). Despite prior studies on the mechanism of stress acclimation in plants, little has been done on the early events of cold sensing in C. reinhardtii. Here, we used C. reinhardtii as a model to study early events of cold signal transduction. By analyzing transcriptomic changes of C. reinhardtii exposed to cold, we found that 3471 genes were differentially expressed after 1 h of cold exposure. These genes were associated with a wide range of biological events and processes such as protein synthesis, cell cycle and protein kinase-based phosphorylation. Besides, the promoter of one gene (named as crAP2) which belongs to AP2/EREBP family and was significantly induced by cold was cloned, and functional analysis was conducted using GUS activity analysis through Agrobacterium-mediated transient assay in tobacco leaves.

Wound-Induced Shoot-to-Root Relocation of JA-Ile Precursors Coordinates Arabidopsis Growth

Multicellular organisms rely on the movement of signaling molecules across cells, tissues, and organs to communicate among distal sites. In plants, localized leaf damage activates jasmonic acid (JA)-dependent transcriptional reprogramming in both harmed and unharmed tissues. Although it has been indicated that JA species can translocate from damaged into distal sites, the identity of the mobile compound(s), the tissues through which they translocate, and the effect of their relocation remain unknown. Here, we found that following shoot wounding, the relocation of endogenous jasmonates through the phloem is essential to initiate JA signaling and stunt growth in unharmed roots of Arabidopsis thaliana. By employing grafting experiments and hormone profiling, we uncovered that the hormone precursor cis-12-oxo-phytodienoic acid (OPDA) and its derivatives, but not the bioactive JA-Ile conjugate, translocate from wounded shoots into undamaged roots. Upon root relocation, the mobile precursors cooperatively regulated JA responses through their conversion into JA-Ile and JA signaling activation. Collectively, our findings demonstrate the existence of long-distance translocation of endogenous OPDA and its derivatives, which serve as mobile molecules to coordinate shoot-to-root responses, and highlight the importance of a controlled redistribution of hormone precursors among organs during plant stress acclimation.

Cation Specificity of Vacuolar NHX-Type Cation/H+ Antiporters.

Cation/H+ (NHX-type) antiporters are important regulators of intracellular ion homeostasis and are critical for cell expansion and plant stress acclimation. In Arabidopsis (Arabidopsis thaliana), four distinct NHX isoforms, named AtNHX1 to AtNHX4, locate to the tonoplast. To determine the concerted roles of all tonoplast NHXs on vacuolar ion and pH homeostasis, we examined multiple knockout mutants lacking all but one of the four vacuolar isoforms and quadruple knockout plants lacking any vacuolar NHX activity. The nhx triple and quadruple knockouts displayed reduced growth phenotypes. Exposure to sodium chloride improved growth while potassium chloride was deleterious to some knockouts. Kinetic analysis of K+ and Na+ transport indicated that AtNHX1 and AtNHX2 are the main contributors to both vacuolar pH and K+ and Na+ uptake, while AtNHX3 and AtNHX4 differ in Na+/K+ selectivity. The lack of any vacuolar NHX activity resulted in no K+ uptake, highly acidic vacuoles, and reduced but not abolished vacuolar Na+ uptake. Additional K+/H+ and Na+/H+ exchange activity assays in the quadruple knockout indicated Na+ uptake that was not H+ coupled, suggesting the existence of an alternative, cation/H+-independent, Na+ conductive pathway in vacuoles. These results highlight the importance of NHX-type cation/H+ antiporters in the maintenance of cellular cation homeostasis and in growth and development.

Discovery of stress responsive TATA-box binding protein associated Factor6 (TAF6) from finger millet (Eleusine coracana (L.) Gaertn)

Drought is one of the major abiotic factors affecting plant growth mainly in the tropical regions. During stress acclimation response, basal transcriptional process is critical which is coordinated by several cofactors, among which TATAbox Binding Protein (TBP) - Associated Factors (TAFs) are the major proteins involved in pre-initiation complex assembly. In search of novel transcriptional regulators, we identified one of the factors, TAF6, in a drought cDNA library generated from an adapted crop, finger millet (Eleusine coracana (L.) Gaertn). RTPCR analysis in leaf tissues of finger millet experiencing different levels of drought, revealed its stress responsiveness in three different genotypes. The gene is also induced under salt, osmotic and oxidative stresses at seedling stage. Analysis of its promoter in model plants indicated the presence of cis-elements associated with diverse stress responses. Down regulation of NbTAF6 by virus induced gene silencing (VIGS) approach resulted in a developmentally defective and stress sensitive phenotype in Nicotiana benthamiana. This is the first report on the up-regulation of EcTAF6 under abiotic stresses and the gene probably has a crucial role in safeguarding the stress related transcription process. The basal regulators like TAF6 can thus be considered as prospective candidates associated with stress acclimation in plants.

Induction of osmotic stress resistance by seed osmo-priming in winter wheat (Triticum aestivum L.) during post-germinative stages

The potential of wheat (Triticum aestivum) seed osmo-priming for post-germinative osmo-stress resistance was evaluated. Further, for comparative evaluation of seed osmo-priming and plant stress-acclimation, a set of plants was stress-acclimated prior to osmo-stress treatment. Under osmo-stress at the post-germination stage, plants from osmo-primed seeds produced 2.3% higher root biomass, 21.6% longer roots, accumulated 15.1% higher soluble sugar content and respectively, 10.4 and 12.8% higher activity of SOD and POD in the roots compared with plants from unprimed seeds. At the post-seedling establishment stage, seed-osmo-primed plants produced 29.9% higher root biomass, 15.2% longer roots, accumulated 32.7% higher soluble sugar content and 31.2% higher proline content in the roots, while the activity of SOD and POD was respectively, 19.6 and 13.1% higher than the unprimed plants. Plant stress-acclimation induced osmo-stress tolerance was exemplified by 6.9% higher root biomass, 4.6% longer roots, 52.7% higher soluble sugar content in roots, 23.9% higher proline content in the roots and 27.0% higher activity of SOD in the roots under osmo-stress when compared with the unprimed plants. The data of seedling morphology and physiological activity under control conditions showed adverse effects of stress acclimation whereas seed osmo-priming-induced stress resistance resulted in advanced germination and higher osmolyte accumulation.

Full-Length Cloning and Characterization of Abiotic Stress Responsive <i>CIPK31-Like</i> Gene from Finger Millet, a Drought-Tolerant Crop

Regulatory proteins such as protein kinases are known to play a crucial role in plant stress acclimation pathways. In this study, we report the identification and characterization of an abiotic stress responsive protein kinase called CBL Interacting Protein Kinase ( EcCIPK31-like ) from finger millet, a drought-tolerant crop. PCR-based approach was followed to clone 1350 bp coding region that encodes a 449 amino acid protein with a calculated molecular mass of 50.69 kDa. The conserved domain analysis revealed the presence of CIPK domain with 90% sequence similarity with fox tail millet SiCIPK31-like gene. We also report its upregulation under salinity, desiccation, oxidative and temperature stresses at seedling level in finger millet. Drought stress at wholeplant level significantly induced the expression of EcCIPK31-like , indicating that the gene is linked to drought signalling pathways. The stress-responsive nature of EcCIPK31-lik e to diverse stresses indicates that the gene could regulate multiple cellular tolerance traits and its further functional validation can highlight the relevance in abiotic stress acclimation in plants.

Increased drought tolerance in maize plants induced by H2O2 is closely related to an enhanced enzymatic antioxidant system and higher soluble protein and organic solutes contents

Drought stress is one of the most important agricultural problems limiting development and growth in plants. Therefore, mechanisms to alleviate drought stress have been one of the major limiting factors in production. H2O2 pretreatment has emerged as a method to induce stress acclimation in plants. In this study, the effects of H2O2 leaf pretreatment on plant growth, antioxidative enzymes, soluble protein, and organic solute content in maize plants under conditions of drought stress were analyzed. Results demonstrated that drought stress reduced shoot and root mass compared with the control, and H2O2 leaf spraying significantly improved the growth of drought-stressed plants. In general, in drought-stressed plants, CAT, APX, GPX, and SOD activities in roots and leaves were increased by H2O2 leaf spraying relative to water spraying. GPX was the main H2O2-scavenging enzyme in leaves and roots, and CAT activity was not detected in the leaves of maize plants. Increased organic solute contents (proteins, carbohydrates, soluble proline, and amino acids) were found in the leaves and mainly in the roots of H2O2-stressed plants relative to water-stressed plants. In conclusion, it was found that H2O2 leaf spraying pretreatment reduced the deleterious effects of drought stress on maize plant growth. This treatment proved to be a beneficial health strategy in plants. This effect could be attributed to the ability of H2O2 to induce antioxidant defense system activity, particularly GPX, and to increase organic solute (protein, carbohydrate, proline, and free amino acid) content in roots and leaves.