Salt stress impact on the molecular structure and function of the photosynthetic apparatus--the protective role of polyamines

Accumulation of astaxanthin in Microcystis aeruginosa under NaCl and KCl stresses

Astaxanthin is a high-value natural antioxidant, and can be accumulated in Microcystis aeruginosa. To enhance astaxanthin accumulation in the microalgae by using salt stress, the cell growth, photosynthetic abilities, reactive oxygen species (ROS) levels, astaxanthin and its precursor content, and gene expression were investigated under NaCl and KCl stresses. The two salt stresses inhibited the cell growth by lowering photosynthetic abilities and raising ROS levels. During the 6-day treatment, the two salt stresses improved the levels of astaxanthin, precursors (β-carotene and zeaxanthin) and carotenoids, which might be caused by the raised ROS up-regulating expression of 7 related genes. At the same concentration, KCl stress showed stronger inducing effect on astaxanthin and its precursor production than NaCl stress, due to higher expression of related genes. Therefore, NaCl and KCl stresses have obvious ion differences on astaxanthin accumulation, of which KCl stress is more suitable for the high-value antioxidant production from microalgae.

Zinc regulation of chlorophyll fluorescence and carbohydrate metabolism in saline-sodic stressed rice seedlings

Saline-sodic stress can limit the absorption of available zinc in rice, subsequently impacting the normal photosynthesis and carbohydrate metabolism of rice plants. To investigate the impact of exogenous zinc application on photosynthesis and carbohydrate metabolism in rice grown in saline-sodic soil, this study simulated saline-sodic stress conditions using two rice varieties, 'Changbai 9' and 'Tonghe 899', as experimental materials. Rice seedlings at 4 weeks of age underwent various treatments including control (CT), 2 μmol·L-1 zinc treatment alone (Z), 50 mmol·L-1 saline-sodic treatment (S), and 50 mmol·L-1 saline-sodic treatment with 2 μmol·L-1 zinc (Z + S). We utilized JIP-test to analyze the variations in excitation fluorescence and MR820 signal in rice leaves resulting from zinc supplementation under saline-sodic stress, and examined the impact of zinc supplementation on carbohydrate metabolism in both rice leaves and roots under saline-sodic stress. Research shows that zinc increased the chloroplast pigment content, specific energy flow, quantum yield, and performance of active PSII reaction centers (PIABS), as well as the oxidation (VOX) and reduction rate (Vred) of PSI in rice leaves under saline-sodic stress. Additionally, it decreased the relative variable fluorescence (WK and VJ) and quantum energy dissipation yield (φDO) of the rice. Meanwhile, zinc application can reduce the content of soluble sugars and starch in rice leaves and increasing the starch content in the roots. Therefore, the addition of zinc promotes electron and energy transfer in the rice photosystem under saline-sodic stress. It enhances rice carbohydrate metabolism, improving the rice plants' ability to withstand saline-sodic stress and ultimately promoting rice growth and development.

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.

Genome-Wide Identification and Comprehensive Analysis of the GASA Gene Family in Peanuts (Arachis hypogaea L.) under Abiotic Stress

Peanut (Arachis hypogaea L.) is a globally cultivated crop of significant economic and nutritional importance. The role of gibberellic-acid-stimulated Arabidopsis (GASA) family genes is well established in plant growth, development, and biotic and abiotic stress responses. However, there is a gap in understanding the function of GASA proteins in cultivated peanuts, particularly in response to abiotic stresses such as drought and salinity. Thus, we conducted comprehensive in silico analyses to identify and verify the existence of 40 GASA genes (termed AhGASA) in cultivated peanuts. Subsequently, we conducted biological experiments and performed expression analyses of selected AhGASA genes to elucidate their potential regulatory roles in response to drought and salinity. Phylogenetic analysis revealed that AhGASA genes could be categorized into four distinct subfamilies. Under normal growth conditions, selected AhGASA genes exhibited varying expressions in young peanut seedling leaves, stems, and roots tissues. Notably, our findings indicate that certain AhGASA genes were downregulated under drought stress but upregulated under salt stress. These results suggest that specific AhGASA genes are involved in the regulation of salt or drought stress. Further functional characterization of the upregulated genes under both drought and salt stress will be essential to confirm their regulatory roles in this context. Overall, our findings provide compelling evidence of the involvement of AhGASA genes in the mechanisms of stress tolerance in cultivated peanuts. This study enhances our understanding of the functions of AhGASA genes in response to abiotic stress and lays the groundwork for future investigations into the molecular characterization of AhGASA genes.

Drought and salinity stresses induced physio-biochemical changes in sugarcane: an overview of tolerance mechanism and mitigating approaches

Sugarcane productivity is being hampered globally under changing environmental scenarios like drought and salinity. The highly complex nature of the plant responses against these stresses is determined by a variety of factors such as genotype, developmental phase of the plant, progression rate and stress, intensity, and duration. These factors influence plant responses and can determine whether mitigation approaches associated with acclimation are implemented. In this review, we attempt to summarize the effects of drought and salinity on sugarcane growth, specifically on the plant's responses at various levels, viz., physiological, biochemical, and metabolic responses, to these stresses. Furthermore, mitigation strategies for dealing with these stresses have been discussed. Despite sugarcane's complex genomes, conventional breeding approaches can be utilized in conjunction with molecular breeding and omics technologies to develop drought- and salinity-tolerant cultivars. The significant role of plant growth-promoting bacteria in sustaining sugarcane productivity under drought and salinity cannot be overlooked.

The Utilisation of Antarctic Microalgae Isolated from Paradise Bay (Antarctic Peninsula) in the Bioremediation of Diesel

Research has confirmed that the utilisation of Antarctic microorganisms, such as bacteria, yeasts and fungi, in the bioremediation of diesel may provide practical alternative approaches. However, to date there has been very little attention towards Antarctic microalgae as potential hydrocarbon degraders. Therefore, this study focused on the utilisation of an Antarctic microalga in the bioremediation of diesel. The studied microalgal strain was originally obtained from a freshwater ecosystem in Paradise Bay, western Antarctic Peninsula. When analysed in systems with and without aeration, this microalgal strain achieved a higher growth rate under aeration. To maintain the growth of this microalga optimally, a conventional one-factor-at a-time (OFAT) analysis was also conducted. Based on the optimized parameters, algal growth and diesel degradation performance was highest at pH 7.5 with 0.5 mg/L NaCl concentration and 0.5 g/L of NaNO₃ as a nitrogen source. This currently unidentified microalga flourished in the presence of diesel, with maximum algal cell numbers on day 7 of incubation in the presence of 1% v/v diesel. Chlorophyll a, b and carotenoid contents of the culture were greatest on day 9 of incubation. The diesel degradation achieved was 64.5% of the original concentration after 9 days. Gas chromatography analysis showed the complete mineralisation of C₇-C₁₃ hydrocarbon chains. Fourier transform infrared spectroscopy analysis confirmed that strain WCY_AQ5_3 fully degraded the hydrocarbon with bioabsorption of the products. Morphological and molecular analyses suggested that this spherical, single-celled green microalga was a member of the genus Micractinium. The data obtained confirm that this microalga is a suitable candidate for further research into the degradation of diesel in Antarctica.

Exogenous SA Applications Alleviate Salinity Stress via Physiological and Biochemical changes in St John's Wort Plants

The plant St. John's wort contains high levels of melatonin, an important biochemical that has both beneficial and adverse effects on stress. Therefore, a method for increasing melatonin levels in plants without adversely affecting their growth is economically important. In this study, we investigated the regulation of melatonin levels in St. John's wort by exposing samples to salinity stress (150 mM) and salicylic acid (0.25 mM) to augment stress tolerance. The results indicated that salinity stress significantly reduced the plant chlorophyll content and damaged the photosystem, plant growth and development. Additionally, these were reconfirmed with biochemical indicators; the levels of abscisic acid (ABA) and proline were increased and the activities of antioxidants were reduced. However, a significant increase was found in melatonin content under salinity stress through upregulation in the relative expression of tryptophan decarboxylase (TDC), tryptamine 5-hydroxylase (T5H), serotonin N-acetyltransferase (SNAT), and N-acetylserotonin methyltransferase (ASMT). The salicylic acid (SA) treatment considerably improved their photosynthetic activity, the maximum photochemical quantum yield (133%), the potential activity of PSⅡ (294%), and the performance index of electron flux to the final PS I electron acceptors (2.4%). On the other hand, SA application reduced ABA levels (32%); enhanced the activity of antioxidant enzymes, such as superoxide dismutase (SOD) (15.4%) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) (120%); and increased polyphenol (6.4%) and flavonoid (75.4%) levels in salinity-stressed St. John's wort plants. Similarly, SA application under NaCl stress significantly modulated the melatonin content in terms of ion balance; the level of melatonin was reduced after SA application on salt-treated seedlings but noticeably higher than on only SA-treated and non-treated seedlings. Moreover, the proline content was reduced considerably and growth parameters, such as plant biomass, shoot length, and chlorophyll content, were enhanced following treatment of salinity-stressed St. John's wort plants with salicylic acid. These findings demonstrate the beneficial impact of salt stress in terms of a cost-effective approach to extract melatonin in larger quantities from St. John's wort. They also suggest the efficiency of salicylic acid in alleviating stress tolerance and promoting growth of St. John's wort plants.

Photosynthetic electron transport rate and root dynamics of finger millet in response to Trichoderma harzianum

Finger millet (ragi) is the main food grain for many people, especially in the arid and semiarid regions of developing countries in Asia and Africa. The grains contain an exceptionally higher amount of Ca (>300 mg/100 g) when compared to other major cereals. For sustainable production of ragi in the current scenario of climate change, this study aimed to evaluate the impact of Trichoderma harzianum (TRI) on ragi performance. The performance of photosynthetic pigment pool, photosynthetic apparatus, and root dynamics of three varieties of ragi (PRM-1, PRM-701, and PRM-801) in response to four treatments viz., C (soil), S+ TRI (soil + Trichoderma), farmyard manure (soil+ FYM), and FYM+TRI (Soil + FYM + Trichoderma) were studied. Results have shown a significant increase in the photosynthetic pigment pool and optimized functional and structural integrity of the photosynthetic apparatus in response to the combination of farmyard manure (FYM) with TRI. Higher yield parameters viz., φ(Po) and φ(Eo), δ(Ro), efficiency ψ(Eo), performance indices - PI_abs and PI_total, and enhanced root canopy and biomass were observed in all three varieties. Improved electron transport from PSII to PSI, root canopy and biomass, may also suitably favor biological carbon sequestration to retain soil health and plant productivity in case grown in association with FYM and TRI.

Salt-Induced Changes in Cytosolic pH and Photosynthesis in Tobacco and Potato Leaves

Salinity is one of the most common factors limiting the productivity of crops. The damaging effect of salt stress on many vital plant processes is mediated, on the one hand, by the osmotic stress caused by large concentrations of Na⁺ and Cl^- outside the root and, on the other hand, by the toxic effect of these ions loaded in the cell. In our work, the influence of salinity on the changes in photosynthesis, transpiration, water content and cytosolic pH in the leaves of two important crops of the Solanaceae family-tobacco and potato-was investigated. Salinity caused a decrease in photosynthesis activity, which manifested as a decrease in the quantum yield of photosystem II and an increase in non-photochemical quenching. Along with photosynthesis limitation, there was a slight reduction in the relative water content in the leaves and a decrease in transpiration, determined by the crop water stress index. Furthermore, a decrease in cytosolic pH was detected in tobacco and potato plants transformed by the gene of pH-sensitive protein Pt-GFP. The potential mechanisms of the salinity influence on the activity of photosynthesis were analyzed with the comparison of the parameters' dynamics, as well as the salt content in the leaves.

Micro-Raman spectroscopy of the light-harvesting pigments in Chlamydomonas reinhardtii under salinity stress

Microalgae are a rich source of carotenoids with enhanced yields during biotic or abiotic stresses, which often impose survival challenges on the cells. Using a non-invasive pigment profiling approach with micro-Raman spectroscopy, we have analyzed the effect of salinity stress on carotenoids in autotrophic Chlamydomonas reinhardtii. Raman spectral analysis of ν(C = C) mode indicates an increase in the carotenoids with lower conjugation length (lutein and zeaxanthin) compared to β-carotene, as the function of culture age and salinity stress, but especially when salinity stress was imposed in two-stage mode (stress imposed on 2nd day, D2_100, and 4th day, D4_100, during exponential phase). Population-scale heterogeneities in carotenoid Raman mode peak center, quantified with heterogeneity index (HI), were highest during the stationary phase of the cultures and under salinity stress. Although the Raman signal was obtained from a randomly selected small focal volume in the cell, a decrease in chlorophyll Raman mode intensities with age and salinity stress was well corroborated by single-cell population fraction measurements by microscopy. Raman intensity fluctuations (I_f) were high for both chlorophyll and carotenoid modes under salinity stress, which can arise due to variations in chlorophyll/carotenoid content and composition, or conformational changes in the pigments in C. reinhardtii cells. Interestingly, in all growth conditions, chlorophyll a Raman mode intensity was found to show a high correlation to that of β-carotene, pointing out a high degree of cooperativity in the light-harvesting complex pigments even during salinity stress. Thus, we demonstrate the usefulness of non-invasive pigment profiling with micro-Raman spectroscopy for developing an optimization for salinity stress conditions for high biomass yield and proper harvest time to obtain carotenoids with desired chemical composition.

Light Spectral Composition Modifies Polyamine Metabolism in Young Wheat Plants

Although light-emitting diode (LED) technology has extended the research on targeted photomorphogenic, physiological, and biochemical responses in plants, there is not enough direct information about how light affects polyamine metabolism. In this study, the effect of three spectral compositions (referred to by their most typical characteristic: blue, red, and the combination of blue and red [pink] lights) on polyamine metabolism was compared to those obtained under white light conditions at the same light intensity. Although light quality induced pronounced differences in plant morphology, pigment contents, and the expression of polyamine metabolism-related genes, endogenous polyamine levels did not differ substantially. When exogenous polyamines were applied, their roborative effect were detected under all light conditions, but these beneficial changes were correlated with an increase in polyamine content and polyamine metabolism-related gene expression only under blue light. The effect of the polyamines on leaf gene expression under red light was the opposite, with a decreasing tendency. Results suggest that light quality may optimize plant growth through the adjustment of polyamine metabolism at the gene expression level. Polyamine treatments induced different strategies in fine-tuning of polyamine metabolism, which were induced for optimal plant growth and development under different spectral compositions.

Physiological Comparison of Wheat and Maize Seedlings Responses to Water Stresses

The aim of the study was to investigate specific responses of spring wheat (C3 photosynthesis) and maize (C4 photosynthesis) to drought and flooding stress. Analyses of water content, gas exchange intensity, photosynthetic apparatus activity, chlorophyll content, plant height and biological membrane integrity were performed on the 10th day of drought and flooding in both species at the third leaf stage. A specific response of wheat under both drought and flooding conditions involved an increase in ETo/RC ratio, describing electron transport flux converted into a single reaction center in PSII. Correlations between electrolyte leakage and the probability of electron transport beyond the plastoquinone QA, and the amount of energy used for the electron transport were also found. A specific response of maize during flooding was the increase of stomatal conductance. Additionally, a significant correlation between PN/Ci and relative water content was exhibited. Furthermore, the parameters differentiating the studied species only under stressful conditions were rendered. The application of such parameters can be widely used, e.g., for studying the reaction of a potential cultivars to drought and flooding. Providing such information to potential farmers can help better select cultivars for their environmental conditions.

A Novel Approach for the Biological Desalination of Major Anions in Seawater Using Three Microalgal Species: A Kinetic Study

The global water shortage alert has been upgraded to a higher risk level. Consequently, a sustainable approach for ecofriendly, energy efficient water desalination is required for agricultural and municipal water reuse. In this study, an energy-efficient biological desalination process was used to treat chloride anions, which are the most abundant anion salt in seawater. Three algal species were studied: Scenedismus arcuatusa (S. arcuatusa), Chlorella vulgaris (C. vulgaris), and Spirulina maxima (Sp. maxima), under different operating conditions (saline concentrations, contact time, high light intensity, and CO2 supply), and two kinetic models were used. It was identified that under a high light intensity and CO2 supply, S. arcuatusa enhanced chloride removal from 32.42 to 48.93%; the daily bioaccumulation capacity (Qe), according to the kinetic models, was enhanced from 124 to 210 mg/g/day; and the net biomass production was enhanced from 0.02 to 0.740 g/L. The EDX analysis proved that salt bioaccumulation may be attributed to the replacement of Ca2+ and Mg2+ with Na+ and K+ through algal cells. The study’s findings provide promising data that can be used in the search for novel energy-efficient alternative ecofriendly desalination technologies based on algae biological systems with biomass byproducts that can be reused in a variety of ways.

Changes in the growth rate of Chlamydomonas reinhardtii under long-term selection by temperature and salinity: Acclimation vs. evolution

We investigated the roles of acclimation and different components involved in evolution (adaptation, chance and history) on the changes in the growth rate of the model freshwater microalga Chlamydomonas reinhardtii P. A. Dang. exposed to selective temperature and salinity. Three C. reinhardtii strains previously grown during one year in freshwater medium and 20 °C were exposed to 5 °C temperature increase and a salinity of 5 g L^-1 NaCl. Cultures under each selective scenario and in combination (increase of salinity and temperature), were propagated until growth rate achieved an invariant mean value for 6 months (100-350 generations, varying as a function of scenario and strain). The changes of the growth rate under increased temperature were due to both adaptation and acclimation, as well as history. However, acclimation was the only mechanism detected under salinity increase as well as in the selective scenario of both temperature and salinity, suggesting that genetic variability would not allow survival at salinity higher than that to which experimental populations were exposed. Therefore, it could be hypothesized that under a global change scenario an increase in salinity would be a greater challenge than warming for some freshwater phytoplankton.

Change in the photochemical and structural organization of thylakoids from pea (Pisum sativum) under salt stress

Salt can induce adverse effects, primarily on the photosynthetic process, ultimately influencing plant productivity. Still, the impact of salt on the photosynthesis process in terms of supercomplexes organization of thylakoid structure and function is not understood in Pea (Pisum sativum). To understand the structure and function in the leaves and thylakoids under salt (NaCl) treatment, we used various biophysical and biochemical techniques like infrared gas analyzer, chlorophyll a fluorescence, circular dichroism, electron microscopy, blue native gels, and western blots. The net photosynthetic rate, transpiration rate, and stomatal conductance were reduced significantly, whereas the water use efficiency was enhanced remarkably under high salt conditions (200 mM NaCl). The photochemical efficiency of both photosystem (PS) I and II was reduced in high salt by inhibiting their donor and acceptor sides. Interestingly the non-photochemical quenching (NPQ) is reduced in high salt; however, the non-regulated energy dissipation (NO) of PSII increased, leading to inactivation of PSII. The obtained results exhibit inhibition of NAD(P)H dehydrogenase (NDH) mediated pathway-dependent cyclic electron transport under salinity caused a decrease in proton motive force of ΔpH and Δψ. Further, the electron micrographs show the disorganization of grana thylakoids under salt stress. Furthermore, the macro-organization and supercomplexes of thylakoids were significantly affected by high salt. Specifically, the mega complexes, PSII-LHCII, PSI-LHCI, and NDH complexes were notably reduced, ultimately altering the electron transport. The reaction center proteins of oxygen-evolving complexes, D1 and D2 proteins were affected to high salt indicating changes in photochemical activities.

Bioaugmentation Improves Phytoprotection in Halimione portulacoides Exposed to Mild Salt Stress: Perspectives for Salinity Tolerance Improvement

Plant growth-promoting rhizobacteria (PGPR) can promote plant growth through mechanisms such as mineral phosphates solubilization, biological N2 fixation and siderophores and phytohormones production. The present work aims to evaluate the physiological fitness improvement by PGPR in Halimione portulacoides under mild and severe salt stress. PGPR-inoculated plants showed improved energy use efficiencies, namely in terms of the trapped and electron transport energy fluxes, and reduced energy dissipation. Allied to this, under mild stress, inoculated plants exhibited a significant reduction of the Na and Cl root concentrations, accompanied by a significant increase in K and Ca leaf content. This ion profile reshaping was intrinsically connected with an increased leaf proline content in inoculated plants. Moreover, bioaugmented plants showed an increased photoprotection ability, through lutein and zeaxanthin leaf concentration increase, allowing plants to cope with potentially photoinhibition conditions. Reduced Na leaf uptake in inoculated plants, apparently reduced the oxidative stress degree as observed by the superoxide dismutase and peroxidase activity reduction. Additionally, a reduced lipid peroxidation degree was observed in inoculated plants, while compared to their non-inoculated counterparts. These results, point out an important role of bioaugmentation in promoting plant fitness and improving salt tolerance, with a great potential for applications in biosaline agriculture and salinized soil restoration.

Exogenous Putrescine Increases Heat Tolerance in Tomato Seedlings by Regulating Chlorophyll Metabolism and Enhancing Antioxidant Defense Efficiency

Crops around the world are facing a diversity of environmental problems, of which high temperatures are proving to be the most serious threat to crops. Polyamine putrescine (Put) acts as a master growth regulator that contributes to optimal plant growth and development and increased stress tolerance. Here, the current study aimed to elucidate how Put functions in regulating chlorophyll (Chl) metabolism, oxidative stress, and antioxidant defense, as well as to characterize the expression of genes related to heat stress in tomato seedlings under such stress. The results revealed that Put treatment significantly attenuates heat-induced damage by promoting biomass production, increasing photosynthetic efficiency, and inhibiting excessive production of oxidative stress markers. Heat stress markedly decreased the Chl content in the tomato leaf and accelerated the leaf yellowing process. However, Put-treated tomato seedlings showed a higher Chl content, which could be associated with the functions of Put in elevating PBGD activity (Chl biosynthesis enzyme) and suppressing the activity of the Chl catabolic enzyme (Chlase and MDCase). Under high-temperature stress, the expression levels of the gene encoding factors involved in Chl biosynthesis and Chl catabolism were significantly down- and upregulated, respectively, and this trend was reversed in Put-treated heat-stressed seedlings. In addition, exogenous application of Put boosted the activity of antioxidant enzymes, along with the levels of expression of their encoding genes, only in plants that were heat stressed. Furthermore, the expression levels of heat-shock-related genes (HSP90, HSP70, and HsfA1) were elevated in Put-treated, high-temperature-stressed tomato seedlings. Taken together, our results indicate that Put treatment significantly increases the heat tolerance of tomato seedlings, by elevating Chl concentrations and suppressing Chl catabolic enzyme activity, modulating endogenous free PA content, increasing antioxidant defense efficiency, and upregulating the expression of heat-shock-related genes.

Assessment of the Photosynthetic Apparatus Functions by Chlorophyll Fluorescence and P700 Absorbance in C3 and C4 Plants under Physiological Conditions and under Salt Stress

Functions of the photosynthetic apparatus of C3 (Pisum sativum L.) and C4 (Zea mays L.) plants under physiological conditions and after treatment with different NaCl concentrations (0-200 mM) were investigated using chlorophyll a fluorescence (pulse-amplitude-modulated (PAM) and JIP test) and P700 photooxidation measurement. Data revealed lower density of the photosynthetic structures (RC/CSo), larger relative size of the plastoquinone (PQ) pool (N) and higher electron transport capacity and photosynthetic rate (parameter RFd) in C4 than in C3 plants. Furthermore, the differences were observed between the two studied species in the parameters characterizing the possibility of reduction in the photosystem (PSI) end acceptors (REo/RC, REo/CSo and δRo). Data revealed that NaCl treatment caused a decrease in the density of the photosynthetic structures and relative size of the PQ pool as well as decrease in the electron transport to the PSI end electron acceptors and the probability of their reduction as well as an increase in the thermal dissipation. The effects were stronger in pea than in maize. The enhanced energy losses after high salt treatment in maize were mainly from the increase in the regulated energy losses (ΦNPQ), while in pea from the increase in non-regulated energy losses (ΦNO). The reduction in the electron transport from QA to the PSI end electron acceptors influenced PSI activity. Analysis of the P700 photooxidation and its decay kinetics revealed an influence of two PSI populations in pea after treatment with 150 mM and 200 mM NaCl, while in maize the negligible changes were registered only at 200 mM NaCl. The experimental results clearly show less salt tolerance of pea than maize.

Polyamines: Α bioenergetic smart switch for plant protection and development

The present review highlights the bioenergetic role of polyamines in plant protection and development and proposes a universal model for describing polyamine-mediated stress responses. Any stress condition induces an excitation pressure on photosystem II by reforming the photosynthetic apparatus. To control this phenomenon, polyamines act directly on the molecular structure and function of the photosynthetic apparatus as well as on the components of the chemiosmotic proton-motive force (ΔpH/Δψ), thus regulating photochemical (qP) and non-photochemical quenching (NPQ) of energy. The review presents the mechanistic characteristics that underline the key role of polyamines in the structure, function, and bioenergetics of the photosynthetic apparatus upon light adaptation and/or under stress conditions. By following this mechanism, it is feasible to make stress-sensitive plants to be tolerant by simply altering their polyamine composition (especially the ratio of putrescine to spermine), either chemically or by light regulation.

Salt Stress Induced Changes in Photosynthesis and Metabolic Profiles of One Tolerant ('Bonica') and One Sensitive ('Black Beauty') Eggplant Cultivars (Solanum melongena L.)

The impact of salinity on the physiological and biochemical parameters of tolerant (‘Bonica’) and susceptible (‘Black Beauty’) eggplant varieties (Solanum melongena L.) was determined. The results revealed that the increase in salinity contributes to a significant decline in net photosynthesis (An) in both varieties; however, at the highest salt concentration (160 mM NaCl), the decrease in photorespiration (Rl) was less pronounced in the tolerant cultivar ‘Bonica’. Stomatal conductance (gs) was significantly reduced in ‘Black Beauty’ following exposure to 40 mM NaCl. However, gs of ‘Bonica’ was only substantially reduced at the highest level of NaCl (160 mM). In addition, a significant decrease in Chla, Chlb, total Chl, Chla/b and carotenoids (p > 0.05) was found in ‘Black Beauty’, and soluble carbohydrates accumulation and electrolyte leakage (EL) were more pronounced in ‘Black Beauty’ than in ‘Bonica’. The total phenols increase in ‘Bonica’ was 65% higher than in ‘Black Beauty’. In ‘Bonica’, the roots displayed the highest enzyme scavenging activity compared to the leaves. Salt stress contributes to a significant augmentation of root catalase and guaiacol peroxidase activities. In ‘Bonica’, the Na concentration was higher in roots than in leaves, whereas in ‘Black Beauty‘, the leaves accumulated more Na. Salt stress significantly boosted the Na/K ratio in ‘Black Beauty’, while no significant change occurred in ‘Bonica’. ACC deaminase activity was significantly higher in ‘Bonica’ than in ‘Black Beauty’.

Abiotic Stress Induces Morphological, Physiological, and Genetic Changes in Orthosiphon stamineus Benth. in In Vitro Cultures

Orthosiphon stamineus is a well-known medicinal herb that has long been used as a traditional treatment for a variety of ailments and disorders. In this study, in vitro grown O. stamineus plantlets were subjected to various stress factors to elucidate the effects of these stresses on the occurrence of somaclonal variation in this species. Murashige and Skoog (MS) media supplemented with 2.0 mg/L kinetin plus 0.5 mg/L IAA was identified as the optimum in vitro regeneration media (OM). The data analysis confirmed the clonal nature of the resulting plantlets. The plantlets were also exposed to various stress factors such as high salinity, abscisic acid (ABA), and high cytokinin concentrations, by subculturing the plantlets on OM added with various concentrations of NaCl, ABA, and high kinetin levels. Exposure to the various stress factors was observed to result in the occurrence of somaclonal variation in O. stamineus, where the incidence of hyperhydricity was found to be the most prominent. An ISSR analysis revealed 70.12% polymorphism among the variants. The results showed that the chlorophyll content and photosynthetic ability of plantlets were reduced when the abiotic stress levels increased.

Biotechnology under extreme conditions: Lichens after extreme UVB radiation and extreme temperatures produce large amounts of hydrogen

The present study demonstrates biotechnological applications of the lichen Pleurosticta acetabulum, specifically the production of large amounts of hydrogen even after the lichen exposure to extreme conditions such as a) extreme UVB radiation (1.7 mW/cm² = 1000 J m^-2 min^-1) over different time periods (4, 20 & 70 h) and b) combined exposure of the lichen to high intensity UVB radiation and extreme low (-196 °C) or extreme high temperatures (+70 °C). The results highlight that the extremophilic and polyextremophilic behavior of lichens both in dehydrated and in regenerated form, under extreme conditions not necessarily recorded on earth, is compatible with their biotechnological uses. The lichen viability was measured using fluorescence induction techniques (OJIP-test), which record changes in the molecular structure and function of the photosynthetic mechanism, while its ability to produce molecular hydrogen was measured through thermal conductivity gas chromatography (GC-TCD) analysis. Hydrogen is a promising fuel for the future. The exciting result of a lichen micro-ecosystem is its ability to expel its moisture and remain in an inactive state, protecting itself from extreme conditions and maintaining its ability to high yield hydrogen production in a closed system, with the sole addition of water and without the need for additional energy. Our results expand the potential use of lichens for future biotechnological applications in extreme Earth environments, but also in environments on other planets, such as Mars, thus paving the way for astrobiotechnological applications.

Polyamine Metabolism under Different Light Regimes in Wheat

Although the relationship between polyamines and photosynthesis has been investigated at several levels, the main aim of this experiment was to test light-intensity-dependent influence of polyamine metabolism with or without exogenous polyamines. First, the effect of the duration of the daily illumination, then the effects of different light intensities (50, 250, and 500 μmol m^–2 s^–1) on the polyamine metabolism at metabolite and gene expression levels were investigated. In the second experiment, polyamine treatments, namely putrescine, spermidine and spermine, were also applied. The different light quantities induced different changes in the polyamine metabolism. In the leaves, light distinctly induced the putrescine level and reduced the 1,3-diaminopropane content. Leaves and roots responded differently to the polyamine treatments. Polyamines improved photosynthesis under lower light conditions. Exogenous polyamine treatments influenced the polyamine metabolism differently under individual light regimes. The fine-tuning of the synthesis, back-conversion and terminal catabolism could be responsible for the observed different polyamine metabolism-modulating strategies, leading to successful adaptation to different light conditions.

Treatment of clean in place (CIP) wastewater using microalgae: Nutrient upcycling and value-added byproducts production

CIP wastewater is one of the major wastewater streams from the food industry, and its treatment is generally expensive, requiring a large effort to reduce its typically high nitrogen (N), and phosphorus (P) contents. Microalgae-based wastewater treatment is increasingly explored as a more sustainable alternative to the conventional methods, due to the added benefit of nutrient upcycling and value-added biomass production. For the first time, four microalgae species were used to treat CIP wastewater high in N (565.5 mg NO₃^--N/l) and P (98.0 mg PO₄^3--P/l). An intermittent biomass harvesting strategy was adopted in this study to enhance the purification of CIP water and redirection of nutrients into algal biomass. Over 93 days operation, N removal efficiency was 52.1 ± 2.9%, 54.8 ± 2.5%, 50.0 ± 2.3% and 48.3 ± 0.5%, and P removal efficiency was 65.5 ± 10.0%, 79.4 ± 6.1%, 61.8 ± 2.5% and 69.1 ± 7.7% for Chlamydomonas reinhardtii, Chlorella vulgaris, Scenedesmus obliquus and wastewater borne microalgae, respectively. After the first (acclimatization) and second growth cycles, cell growth and nutrient removal slowed down but increased again after adding trace nutrients, indicating the lack of trace elements after the first two growth cycles. In the fourth and fifth batch runs, both algal growth rate and nutrient removal rate decreased despite adding trace nutrients and/or increasing light intensity, this being a consequence of the excreted soluble algal products accumulating during long-term operation. S. obliquus had the highest protein concentration of 44.5 ± 9.8% DW, while C. vulgaris accumulated the highest total lipid content (15.6 ± 0.9%, DW). In this proof-of-concept study, the cultivation of microalgae in CIP wastewater with an intermittent harvest of the accumulated algal biomass is demonstrated and it outlines the potential of microalgae to sustainably treat effluents with extremely high nutrients concentration while producing the food-grade algae biomass.

Physiological and Molecular Analysis Reveals the Differences of Photosynthesis between Colored and Green Leaf Poplars

Leaf coloration changes evoke different photosynthetic responses among different poplar cultivars. The aim of this study is to investigate the photosynthetic difference between a red leaf cultivar (ZHP) and a green leaf (L2025) cultivar of Populus deltoides. In this study, ‘ZHP’ exhibited wide ranges and huge potential for absorption and utilization of light energy and CO₂ concentration which were similar to those in ‘L2025’ and even showed a stronger absorption for weak light. However, with the increasing light intensity and CO₂ concentration, the photosynthetic capacity in both ‘L2025’ and ‘ZHP’ was gradually restricted, and the net photosynthetic rate (Pn) in ‘ZHP’ was significantly lower than that in ‘L2025’under high light or high CO₂ conditions, which was mainly attributed to stomatal regulation and different photosynthetic efficiency (including the light energy utilization efficiency and photosynthetic CO₂ assimilation efficiency) in these two poplars. Moreover, the higher anthocyanin content in ‘ZHP’ than that in ‘L2025’ was considered to be closely related to the decreased photosynthetic efficiency in ‘ZHP’. According to the results from the JIP-test, the capture efficiency of the reaction center for light energy in ‘L2025’ was significantly higher than that in ‘ZHP’. Interestingly, the higher levels of light quantum caused relatively higher accumulation of Q_A^- in ‘L2025’, which blocked the electron transport and weakened the photosystem II (PSII) performance as compared with ‘ZHP’; however, the decreased capture of light quantum also could not promote the utilization of light energy, which was the key to the low photosynthetic efficiency in ‘ZHP’. The differential expressions of a series of photosynthesis-related genes further promoted these specific photosynthetic processes between ‘L2025’ and ‘ZHP’.

Unfinished story of polyamines: Role of conjugation, transport and light-related regulation in the polyamine metabolism in plants

Polyamines play a fundamental role in the functioning of all cells. Their regulatory role in plant development, their function under stress conditions, and their metabolism have been well documented as regards both synthesis and catabolism in an increasing number of plant species. However, the majority of these studies concentrate on the levels of the most abundant polyamines, sometimes providing data on the enzyme activity or gene expression levels during polyamine synthesis, but generally making no mention of the fact that changes in the polyamine pool are very dynamic, and that other processes are also involved in the regulation of actual polyamine levels. Differences in the distribution of individual polyamines and their conjugation with other compounds were described some time ago, but these have been given little attention. In addition, the role of polyamine transporters in plants is only now being recognised. The present review highlights the importance of conjugated polyamines and also points out that investigations should not only deal with the polyamine metabolism itself, but should also cover other important questions, such as the relationship between light perception and the polyamine metabolism, or the involvement of polyamines in the circadian rhythm.

Halotolerant bacteria mitigate the effects of salinity stress on soybean growth by regulating secondary metabolites and molecular responses

BACKGROUND

Salinity is a major threat to the agriculture industry due to the negative impact of salinity stress on crop productivity. In the present study, we isolated rhizobacteria and evaluated their capacities to promote crop growth under salt stress conditions.

RESULTS

We isolated rhizospheric bacteria from sand dune flora of Pohang beach, Korea, and screened them for plant growth-promoting (PGP) traits. Among 55 bacterial isolates, 14 produced indole-3-acetic acid (IAA), 10 produced siderophores, and 12 produced extracellular polymeric and phosphate solubilization. Based on these PGP traits, we selected 11 isolates to assess for salinity tolerance. Among them, ALT29 and ALT43 showed the highest tolerance to salinity stress. Next, we tested the culture filtrate of isolates ALT29 and ALT43 for IAA and organic acids to confirm the presence of these PGP products. To investigate the effects of ALT29 and ALT43 on salt tolerance in soybean, we grew seedlings in 0 mM, 80 mM, 160 mM, and 240 mM NaCl treatments, inoculating half with the bacterial isolates. Inoculation with ALT29 and ALT43 significantly increased shoot length (13%), root length (21%), shoot fresh and dry weight (44 and 35%), root fresh and dry weight (9%), chlorophyll content (16-24%), Chl a (8-43%), Chl b (13-46%), and carotenoid (14-39%) content of soybean grown under salt stress. Inoculation with ALT29 and ALT43 also significantly decreased endogenous ABA levels (0.77-fold) and increased endogenous SA contents (6-16%), increased total protein (10-20%) and glutathione contents, and reduced lipid peroxidation (0.8-5-fold), superoxide anion (21-68%), peroxidase (12.14-17.97%), and polyphenol oxidase (11.76-27.06%) contents in soybean under salinity stress. In addition, soybean treated with ALT29 and ALT43 exhibited higher K+ uptake (9.34-67.03%) and reduced Na+ content (2-4.5-fold). Genes involved in salt tolerance, GmFLD19 and GmNARK, were upregulated under NaCl stress; however, significant decreases in GmFLD19 (3-12-fold) and GmNARK (1.8-3.7-fold) expression were observed in bacterial inoculated plants.

CONCLUSION

In conclusion, bacterial isolates ALT29 and ALT43 can mitigate salinity stress and increase plant growth, providing an eco-friendly approach for addressing saline conditions in agricultural production systems.

Acclimation potential of Noni (Morinda citrifolia L.) plant to temperature stress is mediated through photosynthetic electron transport rate

Noni (Morindacitrifolia L.), a tropical, medicinal plant of the family Rubiaceae utilized since 2000 y ago by the Polynesians, is currently facing a major challenge in production vis-a-vis climate change. The worldwide average temperatures continue to fluctuate, resulting in extremely cold winters and hot summers that reduce plant productivity. Photosynthetic apparatus is an exceptionally sensitive component to estimate the degree of damage at contrasting temperatures. The present study was aimed to evaluate the temperature stress response of Noni plant using the chlorophyll a fluorescence OJIP transients (OJIP transients). Results showed the declined photosynthetic pigment pool and reduced functional and structural integrity of the photosynthetic apparatus under very low- and high-temperature treatments. Drastically lower yield parameters such as φ(Po) and φ(Eo), efficiency ψ(Eo) and performance indices - PI_abs and PI_total, and accumulation of inactive reaction centers were observed. Consecutively, a lower level of calculated electron transport from PSII to PSI was observed. In contrast, the enhanced δRo indicates that PSI is more thermo-tolerant as compared to PSII. Additionally, very low and high temperatures cause an increase in antenna size (ABS/RC) and the decrease in the amplitude of I to P phase of fluorescence transient. Overall, the photosynthetic apparatus of leaf tissue was more sensitive to low and high temperatures than the developing fruit. The findings of the present study demonstrated the potential role of thylakoid components of the photosynthetic apparatus, which might be crucial in regulating the temperature stress response in the Noni plant, and thereby crop improvement.

Effect of cold stress on polyamine metabolism and antioxidant responses in chickpea

Metabolic and genomic characteristics of polyamines (PAs) may be associated with the induction of cold tolerance (CT) responses in plants. Characteristics of PAs encoding genes in chickpea (Cicer arietinum L.) and their function under cold stress (CS) are currently unknown. In this study, the potential role of PAs along with the antioxidative defense systems were assessed in two chickpea genotypes (Sel96th11439, cold-tolerant and ILC533, cold-sensitive) under CS conditions. Six days after exposure to CS, the leaf H₂O₂ content and electrolyte leakage index increased in the sensitive genotype by 47.7 and 59 %, respectively, while these values decreased or remained unchanged, respectively, in the tolerant genotype. In tolerant genotype, the enhanced activity of superoxide dismutase (SOD) (by 50 %) was accompanied by unchanged activities of ascorbate peroxidase (APX), guaiacol peroxidase (GPX) and catalase (CAT) as well as the accumulation of glutathione (GSH) (by 43 %) on the sixth day of CS. Higher levels of putrescine (Put) (322 %), spermidine (Spd) (45 %), spermine (Spm) (69 %) and the highest ratio of Put/(Spd + Spm) were observed in tolerant genotype compared to the sensitive one on the sixth day of CS. Gamma-aminobutyric acid (GABA) accumulation was 74 % higher in tolerant genotype compared to the sensitive one on the sixth day of CS. During CS, the activity of diamine oxidase (DAO) and polyamine oxidase (PAO) increased in tolerant (by 3.02- and 2.46-fold) and sensitive (by 2.51- and 2.8-fold) genotypes, respectively, in comparison with the respective non-stressed plants (normal conditions). The highest activity of DAO and PAO in the tolerant genotype was accompanied by PAs decomposition and a peak in GABA content on the sixth day of CS. The analysis of chickpea genome revealed the presence of five PAs biosynthetic genes, their chromosomal locations, and cis-regulatory elements. A significant increase in transcript levels of arginine decarboxylase (ADC) (24.26- and 7.96-fold), spermidine synthase 1 (SPDS1) (3.03- and 1.53-fold), SPDS2 (5.5- and 1.62-fold) and spermine synthase (SPMS) (3.92- and 1.65-fold) genes was detected in tolerant and sensitive genotypes, respectively, whereas the expression of ornithine decarboxylase (ODC) genes decreased significantly under CS conditions in both genotypes. Leaf chlorophyll and carotenoid contents exhibited declining trends in the sensitive genotype, while these photosynthetic pigments were stable in the tolerant genotype due to the superior performance of defensive processes under CS conditions. Overall, these results suggested the specific roles of putative PAs genes and PAs metabolism in development of effective CT responses in chickpea.

Stem Photosynthesis-A Key Element of Grass Pea (Lathyrus sativus L.) Acclimatisation to Salinity

Grass pea (Lathyrus sativus) is a leguminous plant of outstanding tolerance to abiotic stress. The aim of the presented study was to describe the mechanism of grass pea (Lathyrus sativus L.) photosynthetic apparatus acclimatisation strategies to salinity stress. The seedlings were cultivated in a hydroponic system in media containing various concentrations of NaCl (0, 50, and 100 mM), imitating none, moderate, and severe salinity, respectively, for three weeks. In order to characterise the function and structure of the photosynthetic apparatus, Chl a fluorescence, gas exchange measurements, proteome analysis, and Fourier-transform infrared spectroscopy (FT-IR) analysis were done inter alia. Significant differences in the response of the leaf and stem photosynthetic apparatus to severe salt stress were observed. Leaves became the place of harmful ion (Na+) accumulation, and the efficiency of their carboxylation decreased sharply. In turn, in stems, the reconstruction of the photosynthetic apparatus (antenna and photosystem complexes) activated alternative electron transport pathways, leading to effective ATP synthesis, which is required for the efficient translocation of Na+ to leaves. These changes enabled efficient stem carboxylation and made them the main source of assimilates. The observed changes indicate the high plasticity of grass pea photosynthetic apparatus, providing an effective mechanism of tolerance to salinity stress.

Protection of PSI and PSII complexes of wheat from toxic effect of anthracene by Bacillus subtilis (NCIM 5594)

Contamination of polycyclic aromatic hydrocarbons (PAHs) in environment indicates a serious problem to the present era. These are carcinogenic and mutagenic compounds and pose a potential risk to photosynthetic organisms. The present study illustrates the protection of Photosystem I and Photosystem II complexes of wheat plant by Bacillus subtilis (NCIM 5594) from toxic effects of anthracene (ANT). Initially, Chl a fluorescence induction curve measurement revealed declined J-I and I-P phase in ANT-treated plants. Efficiency of light absorption, trapping, and electron transport was reduced in ANT-treated plants, while in ANT + Bacillus subtilis (NCIM 5594)-treated plants value of these parameters was restored. Effect of ANT and ANT + Bacillus subtilis (NCIM 5594) on energy conversion of Photosystem I and Photosystem II was measured. Quantum yield of Photosystem I (YI) and Photosystem II (YII) was decreased in the presence of ANT, while these values were recovered in ANT + Bacillus subtilis (NCIM 5594)-treated plants. Reduction in Y(II) was associated with an increase in non-regulated energy dissipation NO. Likewise the reduction of Y(I) was induced due to donor-side and acceptor-side limitation of Photosystem I caused by toxic effect of ANT. Toxic effects of ANT on electron transport rate (ETR_I and ETR_II) were found to be reduced in ANT + Bacillus subtilis (NCIM 5594)-treated plants. Activation of Cyclic electron flow around Photosystem I in ANT-treated plants was recovered by bacteria. It was concluded that toxic effect of ANT on Photosystem I and Photosystem II complexes was recovered by Bacillus subtilis (NCIM 5594) strain, and thus it is useful strain for crop improvement in ANT-polluted soil.

Drought and Salinity Stress Responses and Microbe-Induced Tolerance in Plants

Drought and salinity are among the most important environmental factors that hampered agricultural productivity worldwide. Both stresses can induce several morphological, physiological, biochemical, and metabolic alterations through various mechanisms, eventually influencing plant growth, development, and productivity. The responses of plants to these stress conditions are highly complex and depend on other factors, such as the species and genotype, plant age and size, the rate of progression as well as the intensity and duration of the stresses. These factors have a strong effect on plant response and define whether mitigation processes related to acclimation will occur or not. In this review, we summarize how drought and salinity extensively affect plant growth in agriculture ecosystems. In particular, we focus on the morphological, physiological, biochemical, and metabolic responses of plants to these stresses. Moreover, we discuss mechanisms underlying plant-microbe interactions that confer abiotic stress tolerance.

Transcriptome profiling at osmotic and ionic phases of salt stress response in bread wheat uncovers trait-specific candidate genes

BACKGROUND

Bread wheat is one of the most important crops for the human diet, but the increasing soil salinization is causing yield reductions worldwide. Improving salt stress tolerance in wheat requires the elucidation of the mechanistic basis of plant response to this abiotic stress factor. Although several studies have been performed to analyze wheat adaptation to salt stress, there are still some gaps to fully understand the molecular mechanisms from initial signal perception to the onset of responsive tolerance pathways. The main objective of this study is to exploit the dynamic salt stress transcriptome in underlying QTL regions to uncover candidate genes controlling salt stress tolerance in bread wheat. The massive analysis of 3'-ends sequencing protocol was used to analyze leave samples at osmotic and ionic phases. Afterward, stress-responsive genes overlapping QTL for salt stress-related traits in two mapping populations were identified.

RESULTS

Among the over-represented salt-responsive gene categories, the early up-regulation of calcium-binding and cell wall synthesis genes found in the tolerant genotype are presumably strategies to cope with the salt-related osmotic stress. On the other hand, the down-regulation of photosynthesis-related and calcium-binding genes, and the increased oxidative stress response in the susceptible genotype are linked with the greater photosynthesis inhibition at the osmotic phase. The specific up-regulation of some ABC transporters and Na+/Ca2+ exchangers in the tolerant genotype at the ionic stage indicates their involvement in mechanisms of sodium exclusion and homeostasis. Moreover, genes related to protein synthesis and breakdown were identified at both stress phases. Based on the linkage disequilibrium blocks, salt-responsive genes within QTL intervals were identified as potential components operating in pathways leading to salt stress tolerance. Furthermore, this study conferred evidence of novel regions with transcription in bread wheat.

CONCLUSION

The dynamic transcriptome analysis allowed the comparison of osmotic and ionic phases of the salt stress response and gave insights into key molecular mechanisms involved in the salt stress adaptation of contrasting bread wheat genotypes. The leveraging of the highly contiguous chromosome-level reference genome sequence assembly facilitated the QTL dissection by targeting novel candidate genes for salt tolerance.

Exogenous Polyamines Only Indirectly Induce Stress Tolerance in Wheat Growing in Hydroponic Culture under Polyethylene Glycol-Induced Osmotic Stress

The aim of the present study was to evaluate the effect of osmotic stress caused by polyethylene glycol (PEG) 6000 in hydroponic culture on wheat seedlings of drought-resistant Chinese Spring (CS) and drought-susceptible SQ1 cultivar, and to examine the alleviative role of exogenous polyamines (PAs) applied to the medium. The assessment was based on physiological (chlorophyll a fluorescence kinetics, chlorophyll and water content) as well as biochemical (content of carbohydrates, phenols, proline, salicylic and abscisic acid, activity of low molecular weight antioxidants) parameters, measured after supplementation with PAs (putrescine, spermidine and spermine) on the 3rd, 5th and 7th day of the treatment. The results indicate that PAs ameliorate the effects of stress, indirectly and conditionally inducing stress tolerance of wheat seedlings. In contrast to the susceptible SQ1, the resistant CS cultivar activated its protective mechanisms, adjusting the degree of their activation to the level of the stress, depending on the genetic resources of the plant. Increased accumulation of antioxidants in the resistant CS in response to stress after the application of PAs confirms the hypothesis that PAs are involved in the signaling pathway determining the antioxidative response and the tolerance of wheat plants to drought stress.

The Application of a Commercially Available Citrus-Based Extract Mitigates Moderate NaCl-Stress in Arabidopsis thaliana Plants

AIMS

The aim of this study was to assess the effect of BC204 as a plant biostimulant on Arabidopsis thaliana plants under normal and NaCl-stressed conditions.

METHODS

For this study, ex vitro and in vitro growth experiments were conducted to assess the effect of both NaCl and BC204 on basic physiological parameters such as biomass, chlorophyll, proline, malondialdehyde, stomatal conductivity, Fv/Fm and the expression of four NaCl-responsive genes.

RESULTS

This study provides preliminary evidence that BC204 mitigates salt stress in Arabidopsis thaliana. BC204 treatment increased chlorophyll content, fresh and dry weights, whilst reducing proline, anthocyanin and malondialdehyde content in the presence of 10 dS·m^-1 electroconductivity (EC) salt stress. Stomatal conductivity was also reduced by BC204 and NaCl in source leaves. In addition, BC204 had a significant effect on the expression of salinity-related genes, stimulating the expression of salinity-related genes RD29A and SOS1 independently of NaCl-stress.

CONCLUSIONS

BC204 stimulated plant growth under normal growth conditions by increasing above-ground shoot tissue and root and shoot growth in vitro. BC204 also increased chlorophyll content while reducing stomatal conductivity. BC204 furthermore mitigated moderate to severe salt stress (10-20 dS·m^-1) in A. thaliana. Under salt stress conditions, BC204 reduced the levels of proline, anthocyanin and malondialdehyde. The exact mechanism by which this occurs is unknown, but the results in this study suggest that BC204 may act as a priming agent, stimulating the expression of genes such as SOS1 and RD29A.

Low-Temperature Adaptation of the Snow Alga Chlamydomonas nivalis Is Associated With the Photosynthetic System Regulatory Process

The alga Chlamydomonas nivalis thrives in polar snow fields and on high-altitude mountain tops, and contributes significantly on primary production in the polar regions, however, the mechanisms underlying this adaptation to low temperatures are unknown. Here, we compared the growth, photosynthetic activity, membrane lipid peroxidation, and antioxidant activity of C. nivalis with those of the model alga C. reinhardtii, under grow temperature and low temperatures. C. nivalis maintained its photosynthetic activity in these conditions by reducing the light-harvesting ability of photosystem II and enhancing the cyclic electron transfer around photosystem I, both of which limited damage to the photosystem from excess light energy and resulted in ATP production, supporting cellular growth and other physiological processes. Furthermore, the increased cyclic electron transfer rate, carotenoid content, and antioxidant enzyme activities jointly regulated the reactive oxygen species levels in C. nivalis, enabling recovery from excess excitation energy and reduced photooxidative damage to the cell. Therefore, we propose a model in which adaptive mechanisms related to photosynthetic regulation promote the survival and even blooming of C. nivalis under polar environment, suggesting that C. nivalis can provide organic carbon sources as an important primary producer for other surrounding life in the polar regions for maintaining ecosystem.

Response of Scenedesmus quadricauda (Chlorophyceae) to Salt Stress Considering Nutrient Enrichment and Intracellular Proline Accumulation

Aquatic organisms are exposed to a wide range of salinity, which could critically affect their survival and growth. However, their survival and growth response to salinity stress remain unclear. This study evaluates the growth response and intracellular proline accumulation of green algae, Scenedesmus quadricauda, isolated from brackish water, against dissolved salts stress with N and P enrichment. We tested a hypothesis that nutrient enrichment can relieve the dissolved salts stress of algae by accumulating intracellular proline, thereby improving survival and growth. Four levels of salinity (0, 3, 6, 12 psu) were experimentally manipulated with four levels of nutrient stoichiometry (N:P ratio = 2, 5, 10, 20) at constant N (1 mgN/L) or P levels (0.05 and 0.5 mgP/L). In each set of experiments, growth rate and intracellular proline content were measured in triplicate. The highest level of salinity inhibited the growth rate of S. quadricauda, regardless of the nutrient levels. However, with nutrient enrichment, the alga showed tolerance to dissolved salts, reflecting intracellular proline synthesis. Proline accumulation was most prominent at the highest salinity level, and its maximum value appeared at the highest N:P ratio (i.e., highest N level) in all salinity treatments, regardless of P levels. Therefore, the effects of P and N on algal response to salt stress differ.

γ-Aminobutyric acid confers cadmium tolerance in maize plants by concerted regulation of polyamine metabolism and antioxidant defense systems

Gamma-Aminobutyric acid (GABA) accumulates in plants following exposure to heavy metals. To investigate the role of GABA in cadmium (Cd) tolerance and elucidate the underlying mechanisms, GABA (0, 25 and 50 µM) was applied to Cd-treated maize plants. Vegetative growth parameters were improved in both Cd-treated and control plants due to GABA application. Cd uptake and translocation were considerably inhibited by GABA. Antioxidant enzyme activity was enhanced in plants subjected to Cd. Concurrently GABA caused further increases in catalase and superoxide dismutase activities, which led to a significant reduction in hydrogen peroxide, superoxide anion and malondealdehyde contents under stress conditions. Polyamine biosynthesis-responsive genes, namely ornithine decarboxylase and spermidine synthase, were induced by GABA in plants grown under Cd shock. GABA suppressed polyamine oxidase, a gene related to polyamine catabolism, when plants were exposed to Cd. Consequently, different forms of polyamines were elevated in Cd-exposed plants following GABA application. The maximum quantum efficiency of photosystem II (Fv/Fm) was decreased by Cd-exposed plants, but was completely restored by GABA to the same value in the control. These results suggest a multifaceted contribution of GABA, through regulation of Cd uptake, production of reactive oxygen species and polyamine metabolism, in response to Cd stress.

β-Aminobutyric Acid Pretreatment Confers Salt Stress Tolerance in Brassica napus L. by Modulating Reactive Oxygen Species Metabolism and Methylglyoxal Detoxification

Salinity is a serious environmental hazard which limits world agricultural production by adversely affecting plant physiology and biochemistry. Hence, increased tolerance against salt stress is very important. In this study, we explored the function of β-aminobutyric acid (BABA) in enhancing salt stress tolerance in rapeseed (Brassica napus L.). After pretreatment with BABA, seedlings were exposed to NaCl (100 and 150 mM) for 2 days. Salt stress increased Na content and decreased K content in shoot and root. It disrupted the antioxidant defense system by producing reactive oxygen species (ROS; H2O2 and O2•-), methylglyoxal (MG) content and causing oxidative stress. It also reduced the growth and photosynthetic pigments of seedlings but increased proline (Pro) content. However, BABA pretreatment in salt-stressed seedlings increased ascorbate (AsA) and glutathione (GSH) contents; GSH/GSSG ratio; and the activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione peroxidase (GPX), superoxide dismutase (SOD), catalase (CAT), glyoxalase I (Gly I), and glyoxalase II (Gly II) as well as the growth and photosynthetic pigments of plants. In addition, compared to salt stress alone, BABA increased Pro content, reduced the H2O2, MDA and MG contents, and decreased Na content in root and increased K content in shoot and root of rapeseed seedlings. Our findings suggest that BABA plays a double role in rapeseed seedlings by reducing Na uptake and enhancing stress tolerance through upregulating the antioxidant defense and glyoxalase systems.

Influence of Mixed Substrate and Arbuscular Mycorrhizal Fungi on Photosynthetic Efficiency, Nutrient and Water Status and Yield in Tomato Plants Irrigated with Saline Reclaimed Waters

The use of reclaimed water (RW) is considered as a means of maintaining agricultural productivity under drought conditions. However, RW may contain high concentrations of salts. The use of some practices, such as biofertilizers and organic substrates, is also becoming increasingly important in agricultural. production. The aim of this study was to evaluate the application of a mixed substrate (with coconut fibre) and arbuscular mycorrhizal fungi (AMF) on water relations, nutrient uptake and productivity in tomato plants irrigated with saline RW in a commercial greenhouse. Saline RW on its own caused a nutrient imbalance and negatively affected several physiological parameters. However, the high water-holding capacity of coconut fibre in the mixed substrate increased water and nutrient availability for the plants. As a consequence, leaf water potential, gas exchange, some fluorescence parameters (PhiPSII, Fv’/Fm’, qP and ETR) and fruit size and weight improved, even in control irrigation conditions. The use of AMF improved only some parameters because of the low percentage of colonization, suggesting that AMF effectiveness in commercial field conditions is slower and dependent of several factors.

Photosynthesis enhancement in four marine microalgal species exposed to expanded polystyrene leachate

Due to its widespread use, large amounts of expanded polystyrene (EPS) have been released into the marine environment, where it is broken down into small pieces with large surface areas. As such, chemical additives may be released into the environment, which can affect marine organisms; however, studies of the effects of such additives are lacking. We assessed the effects of leachate from EPS on the photosynthetic activities of four microalgal species (Dunaliella salina, Scenedesmus rubescens, Chlorella saccharophila, and Stichococcus bacillaris). They were exposed to EPS leachate for seven days and their photosynthetic activities were analyzed based on seven parameters. Overall, leachate exposure increased photosynthetic activity in all four species, albeit to different degrees and showing slightly different trends among the seven parameters. Based on chemical analysis, hexabromocyclododecane concentrations were higher in small-fragment leachate, whereas UV326 concentrations were higher in low-concentration-large-sphere leachate; bisphenol-A and total organic carbon showed no major differences among leachates. Thus, we speculate that exposure to trace chemicals influenced microalgal photosynthesis and overall growth. These results support further investigation of the impacts of plastic debris and chemical additives on marine ecosystems and organisms.

Using Rapid Chlorophyll Fluorescence Transients to Classify Vitis Genotypes

When a dark-adapted leaf is illuminated with saturating light, a fast polyphasic rise of fluorescence emission (Kautsky effect) is observed. The shape of the curve is dependent on the molecular organization of the photochemical apparatus, which in turn is a function of the interaction between genotype and environment. In this paper, we evaluate the potential of rapid fluorescence transients, aided by machine learning techniques, to classify plant genotypes. We present results of the application of several machine learning algorithms (k-nearest neighbors, decision trees, artificial neural networks, genetic programming) to rapid induction curves recorded in different species and cultivars of vine grown in the same environmental conditions. The phylogenetic relations between the selected Vitis species and Vitis vinifera cultivars were established with molecular markers. Both neural networks (71.8%) and genetic programming (75.3%) presented much higher global classification success rates than k-nearest neighbors (58.5%) or decision trees (51.6%), genetic programming performing slightly better than neural networks. However, compared with a random classifier (success rate = 14%), even the less successful algorithms were good at the task of classifying. The use of rapid fluorescence transients, handled by genetic programming, for rapid preliminary classification of Vitis genotypes is foreseen as feasible.

Drought and Salinity Stress Responses and Microbe-Induced Tolerance in Plants

Drought and salinity are among the most important environmental factors that hampered agricultural productivity worldwide. Both stresses can induce several morphological, physiological, biochemical, and metabolic alterations through various mechanisms, eventually influencing plant growth, development, and productivity. The responses of plants to these stress conditions are highly complex and depend on other factors, such as the species and genotype, plant age and size, the rate of progression as well as the intensity and duration of the stresses. These factors have a strong effect on plant response and define whether mitigation processes related to acclimation will occur or not. In this review, we summarize how drought and salinity extensively affect plant growth in agriculture ecosystems. In particular, we focus on the morphological, physiological, biochemical, and metabolic responses of plants to these stresses. Moreover, we discuss mechanisms underlying plant-microbe interactions that confer abiotic stress tolerance.

Salinity Stress Responses and Adaptation Mechanisms in Eukaryotic Green Microalgae

High salinity is a challenging environmental stress for organisms to overcome. Unicellular photosynthetic microalgae are especially vulnerable as they have to grapple not only with ionic imbalance and osmotic stress but also with the generated reactive oxygen species (ROS) interfering with photosynthesis. This review attempts to compare and contrast mechanisms that algae, particularly the eukaryotic Chlamydomonas microalgae, exhibit in order to immediately respond to harsh conditions caused by high salinity. The review also collates adaptation mechanisms of freshwater algae strains under persistent high salt conditions. Understanding both short-term and long-term algal responses to high salinity is integral to further fundamental research in algal biology and biotechnology.

Salt Tolerance and Desalination Abilities of Nine Common Green Microalgae Isolates

In recent years, decline of freshwater resources has been recognized as one of the main environmental problems on global level. In addition to the increasing extent of primary salinization due to climate change, secondary salinization caused by human interventions is also a significantly increasing problem, therefore, the development of various chemical-free, biological desalination and removal procedures will become increasingly important. In the present study, the salinity tolerance, salinity, and nutrient reducing ability of nine common freshwater microalgae species from the genera Chlorella, Chlorococcum, Desmodesmus, Scenedesmus, and Monoraphidium were investigated. Our results proved that the studied green microalgae species are halotolerant ones, which are able to proliferate in environments with high salt concentrations. Furthermore, most of the species were able to reduce conductivity and remove significant amounts of chloride (up to 39%) and nutrients (more than 90% nitrate). The results proved that nitrate removal of the studied species was not influenced by salt concentration, only indirectly via growth inhibition. However, the results also highlighted that N:P ratio of the medium has primarily importance in satisfactory phosphorous removal. It can be concluded that assemblages of the studied microalgae species could be able to adapt to changing conditions even of salt-rich wastewaters and improve water quality during bioremediation processes.

Exogenous putrescine alleviates photoinhibition caused by salt stress through cooperation with cyclic electron flow in cucumber

When plants suffer from abiotic stresses, cyclic electron flow (CEF) is induced for photo-protection. Putrescine (Put), a primary polyamine in chloroplasts, plays a critical role in stress tolerance. However, the relationship between CEF and Put in chloroplasts for photo-protection is unknown. In this study, we investigated the role of Put-induced CEF for salt tolerance in cucumber plants (Cucumis sativus L). Treatment with 90 mM NaCl and/or Put did not influence the maximum photochemical efficiency of PSII (Fv/Fm), but the photoactivity of PSI was severely inhibited by NaCl. Salt stress induced a high level of CEF; moreover, plants treated with both NaCl and Put exhibited much higher CEF activity and ATP accumulation than those exhibited by single-salt-treated plants to provide an adequate ATP/NADPH ratio for plant growth. Furthermore, Put decreased the trans-membrane proton gradient (ΔpH), which was accompanied by reduced pH-dependent non-photochemical quenching (NPQ) and an increased the effective quantum yield of PSII (Y(II)). The ratio of NADP⁺/NADPH increased significantly with Put in salt-stressed leaves compared with the ratio in leaves treated with NaCl, indicating that Put relieved over-reduction pressure at the PSI acceptor side caused by salt stress. Collectively, our results suggest that exogenous Put creates an excellent condition for CEF promotion: a large amount of pmf is predominantly stored as Δψ, resulting in moderate lumen pH and low NPQ, while maintaining high rates of ATP synthesis (high pmf).