A differential tolerance to mild salt stress conditions among six Italian rice genotypes does not rely on Na+ exclusion from shoots

A complex array of factors regulate the activity of Arabidopsis thaliana δ1 -pyrroline-5-carboxylate synthetase isoenzymes to ensure their specific role in plant cell metabolism

The first and committed step in proline synthesis from glutamate is catalyzed by δ1 -pyrroline-5-carboxylate synthetase (P5CS). Two P5CS genes have been found in most angiosperms, one constitutively expressed to satisfy proline demand for protein synthesis, the other stress-induced. Despite the number of papers to investigate regulation at the transcriptional level, to date, the properties of the enzymes have been subjected to limited study. The isolation of Arabidopsis thaliana P5CS isoenzymes was achieved through heterologous expression and affinity purification. The two proteins were characterized with respect to kinetic and biochemical properties. AtP5CS2 showed KM values in the micro- to millimolar range, and its activity was inhibited by NADP+ , ADP and proline, and by glutamine and arginine at high levels. Mg2+ ions were required for activity, which was further stimulated by K+ and other cations. AtP5CS1 displayed positive cooperativity with glutamate and was almost insensitive to inhibition by proline. In the presence of physiological, nonsaturating concentrations of glutamate, proline was slightly stimulatory, and glutamine strongly increased the catalytic rate. Data suggest that the activity of AtP5CS isoenzymes is differentially regulated by a complex array of factors including the concentrations of proline, glutamate, glutamine, monovalent cations and pyridine dinucleotides.

Physiological Responses to Salt Stress at the Seedling Stage in Wild (Oryza rufipogon Griff.) and Cultivated (Oryza sativa L.) Rice

Domesticated rice Oryza sativa L. is a major staple food worldwide, and the cereal most sensitive to salinity. It originated from the wild ancestor Oryza rufipogon Griff., which was reported to possess superior salinity tolerance. Here, we examined the morpho-physiological responses to salinity stress (80 mM NaCl for 7 days) in seedlings of an O. rufipogon accession and two Italian O. sativa genotypes, Baldo (mildly tolerant) and Vialone Nano (sensitive). Under salt treatment, O. rufipogon showed the highest percentage of plants with no to moderate stress symptoms, displaying an unchanged shoot/root biomass ratio, the highest Na+ accumulation in roots, the lowest root and leaf Na+/K+ ratio, and highest leaf relative water content, leading to a better preservation of the plant architecture, ion homeostasis, and water status. Moreover, O. rufipogon preserved the overall leaf carbon to nitrogen balance and photosynthetic apparatus integrity. Conversely, Vialone Nano showed the lowest percentage of plants surviving after treatment, and displayed a higher reduction in the growth of shoots rather than roots, with leaves compromised in water and ionic balance, negatively affecting the photosynthetic performance (lowest performance index by JIP-test) and apparatus integrity. Baldo showed intermediate salt tolerance. Being O. rufipogon interfertile with O. sativa, it resulted a good candidate for pre-breeding towards salt-tolerant lines.

Effects of the salinity-temperature interaction on seed germination and early seedling development: a comparative study of crop and weed species

BACKGROUND

Weeds represent a great constraint for agricultural production due to their remarkable adaptability and their ability to compete with crops. Climate change exacerbates the abiotic stresses that plants encounter. Therefore, studying plant responses to adverse conditions is extremely important. Here, the response to saline stress at different temperatures of three weed species (Chenopodium album, Echinochloa crus-galli and Portulaca oleracea) and three crops (maize, soybean and rice) was investigated.

RESULTS

The germination percentage of soybean notably decreased as salinity and low temperatures increased. In contrast, maize and rice consistently maintained a high germination percentage, particularly when subjected to low salinity levels. Regarding weed species, the germination percentage of C. album was not significantly affected by salinity, but it decreased in E. crus-galli and P. oleracea with increasing salinity. The mean germination time for all species increased with salinity, especially at lower temperatures. This effect was most pronounced for soybean and E. crus-galli. C. album exhibited significant reduction in stem growth with high salinity and high temperatures, while in E. crus-galli stem growth was less reduced under similar conditions.

CONCLUSION

This study showed that successful germination under saline stress did not ensure successful early development and emphasizes the species-specific nature of the temperature-salinity interaction, perhaps influenced by intraspecific variability. Increasing salinity levels negatively impacted germination and seedling growth in most species, yet higher temperatures partially alleviated these effects.

Biofilm and Rivers: The Natural Association to Reduce Metals in Waters

This article focuses on a very peculiar habitat, the thin biofilm that covers the surface of rocks, cobbles, sediment grains, leaf litter, and vegetation on a riverbed. Species composition changes over time and depends on environmental conditions and perturbation of water quality. It provides several ecosystem services, contributing to the biogeochemical fluxes and reducing contamination by absorbing the pollutants. Biofilm into the Toce River (Ossola Valley, Piedmont, Italy) was investigated to assess its capacity to accumulate the metals and macroions from the water column. In this preliminary work, we investigated three sample points, in two different seasons. The community composition of biofilm was determined via morphological analysis (diatoms and non-diatoms algal community). We characterize the biofilm, a community of different organisms, from different perspectives. In the biofilm, Hg was analyzed with an automated mercury analyzer, other metals and macroions with inductively coupled plasma mass spectrometry (ICP-MS) (Al, As, Ba, Ca, Cr, Cu, Fe, K, Mg, Mn, Ni, P, Pb, and Zn), and the carotenoid and chlorophyll composition of the photosynthetic organism with HPLC analysis for the primary producers. The results evidence a seasonal pattern in metals and macroions levels in the biofilm, and a significant difference in the biofilm community and in carotenoid composition, suggesting the utility of using the biofilm as an additional bioindicator to monitor the water quality of the river.

Short-term effects of combined freeze–thaw and saline–alkali stresses on the physiological response in highland barley (Hordeum vulgare)

Highland barley (Hordeum vulgare L.), as the dominant crop on the Qinghai-Tibetan Plateau, is a typical representative of plants adapted to extreme environmental conditions. However, the harsh environment, severe salinisation and frequent freezing and thawing in the Qinghai-Tibetan Plateau are main limiting factor for crop growth in this region. The physiological response of highland barley to salinisation and freeze-thaw stresses was studied in this paper. Under the combined stresses of 60mmol/LNaCl·60mmol/LNaHCO3 and freeze-thaw cycles (10, -5, and 10°C), the changes in the relative moisture content, relative electrical conductivity, soluble protein, malondialdehyde (MDA) and photosynthetic indices Pn and E in seedling leaves of eight groups of treatments (CK, S, A, S-A, CK (FT), S (FT), A (FT), and S-A (FT)) were analysed. Results showed that a single stress did not cause a change in the MDA content. All of the combined stresses in S-A, CK (FT), S (FT), A (FT), and S-A (FT) treatments increased the MDA content of barley seedlings, and the MDA content of S-A (FT) reached 28.438 at T2 (-5°C) μmol/g. During the freeze-thaw cycle, the cell membrane of seedlings was damaged more seriously by alkali stress, which showed a significant increase in relative conductivity. The relative moisture content value of seedlings was more than 100% because the seedlings could absorb more moisture due to mechanical injury. The protein content of osmoregulatory substances in highland barley seedlings increased with increasing stress, indicating resistance to stress. Moreover, the effect of freeze-thaw stress on photosynthesis was more significant. The changes in indices proved that an appropriate amount of salt stress could improve the resistance of the plant cell membrane. Alkali stress had a significant effect on the growth of highland barley seedlings. Freezing and thawing can aggravate the damage of saline-alkali stress to highland barley seedlings, resulting in changes in the biological membrane permeability and photosynthesis of seedlings. The fluctuation of osmoregulation substance content confirmed that highland barley seedlings had a certain degree of stress resistance. Freeze-thaw cycles will aggravate the damage of land salinisation to highland barley seedlings. To better reduce the impact and loss of land salinisation and freeze-thaw disasters on agriculture in the Qinghai-Tibetan Plateau, priority should be given to solving freeze-thaw stress in the process of grain production.

Physiological response of barley seedlings to salinity and artemisinin combined stresses under freeze-thaw environment

In the Qinghai-Tibet Plateau, both the large daily temperature difference and soil salinization make plants susceptible to abiotic stresses such as freeze-thaw and salinity. Meanwhile, crops in this area can be affected by artemisinin, an antimalarial secondary metabolite produced in Artemisia. Under freeze-thaw and salinity stresses, artemisinin was induced as an allelopathy stress factor to explore the physiological response of highland barley, including the relative electrical conductivity (RC), soluble protein (SP) content, malondialdehyde (MDA) content, antioxidant enzyme activity, and water use efficiency (WUE). Compared with the control group, the contents of RC and MDA in seedling leaves under stress were significantly increased by 24.74-402.37% and 20.18-77.95%, indicating that cell membrane permeability was greatly damaged, and WUE was significantly decreased by 15.77-238.59%. The activity of enzymes increased under single stress and decreased under combined stress. Salinity, artemisinin, and freeze-thaw stress show a synergistic relationship; that is, compound stresses were more serious than single stress. In summary, the results of this study revealed the physiological and ecological responses of barley seedlings under different habitat stresses and the interactions among different stress factors.

B2, an abscisic acid mimic, improves salinity tolerance in winter wheat seedlings via improving activity of antioxidant enzymes

Salinity severely inhibits growth and reduces yield of salt-sensitive plants like wheat, and this effect can be alleviated by plant growth regulators and phytohormones, among which abscisic acid (ABA) plays a central role in response to various stressful environments. ABA is highly photosensitive to light disruption, which this limits its application. Here, based on pyrabactin (a synthetic ABA agonist), we designed and synthesized a functional analog of ABA and named B2, then evaluated its role in salt resistance using winter wheat seedlings. The phenotypes showed that B2 significantly improved the salt tolerance of winter wheat seedlings by elevating the biomass. The physiological analysis found that B2 treatment reduced the generation rate of O₂ ^-, electrolyte leakage, the content of proline, and the accumulation of malonaldehyde (MDA) and H₂O₂ and also significantly increased the contents of endogenous hormones zeatin riboside (ZA) and gibberellic acid (GA). Further biochemical analysis revealed that the activities of various antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX), were enhanced by B2, and the activities of antioxidase isozymes SOD3, POD1/2, and APX1/2 were particularly increased, largely resembling ABA treatment. The abiotic stress response-related gene TaSOS1 was significantly upregulated by B2, while the TaTIP2;2 gene was suppressed. In conclusion, an ABA analog B2 was capable to enhance salt stress tolerance in winter wheat seedlings by stimulating the antioxidant system, providing a novel regulator for better survival of crops in saline soils and improving crop yield.

A Multifactorial Regulation of Glutathione Metabolism behind Salt Tolerance in Rice

Knowledge of the stress-induced metabolic alterations in tolerant and sensitive plants is pivotal for identifying interesting traits that improve plant resilience toward unfavorable environmental conditions. This represents a hot topic area of plant science, particularly for crops, due to its implication in food security. Two rice varieties showing dissimilar resistance to salt, Baldo and Vialone Nano, have been studied to investigate the mechanisms underpinning tolerance toward salinity, and these studies have focused on the root system. A detailed analysis of the salt stress-dependent modulation of the redox network is here presented. The different phenotype observed after salt exposure in the two rice varieties is coherent with a differential regulation of cell-cycle progression and cell-death patterns observed at root level. Baldo, the tolerant variety, already showed a highly responsive antioxidative capacity in control conditions. Consistently, stressed Baldo plants showed a different pattern of H2O2 accumulation compared to Vialone Nano. Moreover, glutathione metabolism was finely modulated at transcriptional, post-transcriptional, and post-translational levels in Baldo. These results contribute to highlight the role of ROS and antioxidative pathways as a part of a complex redox network activated in rice toward salt stress.

Physiological evaluation for salt tolerance in green and purple leaf color rice cultivars at seedling stage

Anthocyanin, a water-soluble pigment found in plants, has been reported to be associated with abiotic stress tolerance including salt stress. For a better understanding of the role of anthocyanin in response to salt stress, two salt-tolerant rice genotypes having different leaf anthocyanin content, one having green ('Pokkali'; PK) and the other purple leaves ('Niew Dam 019'; ND 019), were used in this study. After being subjected to salt stress (150 mM NaCl) for 5 d, the 3-week-old rice genotypes PK and ND 019 exhibited significant physiological responses (water content, Na⁺/K⁺ ratio, osmolyte accumulation, osmotic adjustment, antioxidant capacity, membrane damage and chlorophyll) and expression of ion transporter genes, indicating overall salt tolerance ability. However, the green-leaved rice variety, PK, had better root-to-shoot Na⁺ exclusion mechanism than the purple-leaved variety, ND 019 as evidenced by lower Na⁺ accumulation in leaves compared to ND 019 despite the fact that they accumulated the similar level of Na⁺ in roots. On the other hand, ND 019 accumulated higher concentration of osmolytes leading to more enhanced osmotic adjustment. These results revealed that Na⁺ ion exclusion was the prominent salt tolerance mechanism in the green-leaved PK whereas in the purple-leaved ND 019 osmotic adjustment was the more significant strategy. Under salt stress, there was no remarkable change in anthocyanin in PK while a reduction was found in ND 019. Thus, it could be proposed that anthocyanin did not play a vital role in protecting the purple-leaved rice, ND 019 from salt stress during seedling stage.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01114-y.

Varietal differences in physiological and biochemical responses to salinity stress in six finger millet plants

Finger millet is an important cereal that is grown in semi-arid and arid regions of East-Africa. Salinity stress is a major environmental impediment for the crop growth and production. This study aimed to understand the physiological and biochemical responses to salinity stress of six Kenyan finger millet varieties (GBK043137, GBK043128, GBK043124, GBK043122, GBK043094, GBK043050) grown across different agroecological zones under NaCl-induced salinity stress (100, 200 and 300 mM NaCl). Seeds were germinated on the sterile soil and treated using various concentrations of NaCl for 2 weeks. Early-seedling stage of germinated plants were irrigated with the same salt concentrations for 60 days. The results indicated depression in germination percentage, shoot and root growth rate, leaf relative water content, chlorophyll content, leaf K⁺ concentration, and leaf K⁺/Na⁺ ratios with increased salt levels and the degree of increment differed among the varieties. On the contrary, the content of proline, malonaldehyde, leaf total proteins, and reduced sugar increased with increasing salinity. At the same time, the leaf Na⁺ and Cl^- amounts of all plants increased substantially with increasing stress levels. Clustering analysis placed GBK043094 and GBK043137 together and these varieties were identified as salt-tolerant based on their performance. Taken together, our findings indicated a significant varietal variability for most of the parameters analysed. The superior varieties identified could be used as promising genetic resources in future breeding programmes directed towards development of salt-tolerant finger millet hybrids. Further analysis at genomic level needs to be undertaken to better understand the genetic factors that promote salinity tolerance in finger millet.

What is the Difference between the Response of Grass Pea (Lathyrus sativus L.) to Salinity and Drought Stress?—A Physiological Study

Understanding the mechanisms of plant tolerance to osmotic and chemical stress is fundamental to maintaining high crop productivity. Soil drought often occurs in combination with physiological drought, which causes chemical stress due to high concentrations of ions. Hence, it is often assumed that the acclimatization of plants to salinity and drought follows the same mechanisms. Grass pea (Lathyrus sativus L.) is a legume plant with extraordinary tolerance to severe drought and moderate salinity. The aim of the presented study was to compare acclimatization strategies of grass pea seedlings to osmotic (PEG) and chemical (NaCl) stress on a physiological level. Concentrations of NaCl and PEG were adjusted to create an osmotic potential of a medium at the level of 0.0, −0.45 and −0.65 MPa. The seedlings on the media with PEG were much smaller than those growing in the presence of NaCl, but had a significantly higher content percentage of dry weight. Moreover, the stressors triggered different accumulation patterns of phenolic compounds, soluble and insoluble sugars, proline and β-N-oxalyl-L-α,β-diamino propionic acid, as well as peroxidase and catalase activity. Our results showed that drought stress induced a resistance mechanism consisting of growth rate limitation in favor of osmotic adjustment, while salinity stress induced primarily the mechanisms of efficient compartmentation of harmful ions in the roots and shoots. Furthermore, our results indicated that grass pea plants differed in their response to drought and salinity from the very beginning of stress occurrence.

Physiological effects of the combined stresses of freezing-thawing, acid precipitation and deicing salt on alfalfa seedlings

BACKGROUND

Frequent freeze-thaw phenomena, together with widely used deicing salt and intense acid precipitation, often occur in northeastern China, causing damage to various aspects of plants, such as the permeability of biological membranes, osmotic adjustment, and photosystems. Aiming to explore the resistance of alfalfa to freezing-thawing (F), acid precipitation (A) and deicing salt (D), this study used Medicago sativa cv. Dongmu-70 as the experimental material, and the contents of malondialdehyde (MDA), soluble protein, soluble sugars, proline and chlorophyll were evaluated.

RESULTS

As the temperature decreased, the MDA content in the seedlings of the group under combined stress (A-D-F) increased and was significantly higher than that of group F (by 69.48 ~ 136.40%). Compared with those in the control (CK) group, osmotic substances such as soluble sugars and proline in the treatment groups were higher, while the soluble protein content was lower. The chlorophyll contents in the seedlings of the treatment groups were lower than those of the CK group; however, the chlorophyll content displayed a non-significant change during the free-thaw cycle.

CONCLUSION

Injury to the permeability of the biological membranes and photosystems of alfalfa results from stress. Moreover, alfalfa maintains osmotic balance by adaptively increasing the potential of osmotic substances such as soluble sugars and proline. Furthermore, the influence of stress from freezing-thawing and deicing salt is highly substantial, but the combined stresses of acid precipitation with the two factors mentioned above had little effect on the plants.

Expression levels of the Na+/K+ transporter OsHKT2;1 and vacuolar Na+/H+ exchanger OsNHX1, Na enrichment, maintaining the photosynthetic abilities and growth performances of indica rice seedlings under salt stress

Salt affected soil inhibits plant growth, development and productivity, especially in case of rice crop. Ion homeostasis is a candidate defense mechanism in the salt tolerant plants or halophyte species, where the salt toxic ions are stored in the vacuoles. The aim of this investigation was to determine the OsNHX1 (a vacuolar Na⁺/H⁺ exchanger) and OsHKT2;1 (Na⁺/K⁺ transporter) regulation by salt stress (200 mM NaCl) in two rice cultivars, i.e. Pokkali (salt tolerant) and IR29 (salt susceptible), the accumulation of Na⁺ in the root and leaf tissues using CoroNa Green^® staining dye and the associated physiological changes in test plants. Na⁺ content was largely increased in the root tissues of rice seedlings cv. Pokkali (15 min after salt stress) due to the higher expression of OsHKT2;1 gene (by 2.5 folds) in the root tissues. The expression of OsNHX1 gene in the leaf tissues was evidently increased in salt stressed seedlings of Pokkali, whereas it was unchanged in salt stressed seedlings of IR29. Na⁺ in the root tissues of both Pokkali and IR29 was enriched, when subjected to 200 mM NaCl for 12 h and easily detected in the leaf tissues of salt stressed plants exposed for 24 h, especially in cv. Pokkali. Moreover, the overexpression of OsNHX1 gene regulated the translocation of Na⁺ from root to leaf tissues, and compartmentation of Na⁺ into vacuoles, thereby maintaining the photosynthetic abilities in cv. Pokkali. Overall growth performance, maximum quantum yield (F_v/F_m), photon yield of PSII (Φ_PSII) and net photosynthetic rate (P_n) was improved in salt stressed leaves of Pokkali than those in salt stressed IR29.

Stress-driven increase in proline levels, and not proline levels themselves, correlates with the ability to withstand excess salt in a group of 17 Italian rice genotypes

In most plant species, a rapid increase in free proline content occurs following exposure to hyperosmotic stress conditions. However, inconsistent results were reported concerning the role of such an increase on the plant response to water shortage or excess salt. Therefore, the possibility that proline accumulation may help the cell to withstand stress conditions, or that it simply represents a stress marker, is still a matter of debate. A possible relationship between proline accumulation and salt tolerance was investigated in a set of 17 Italian rice varieties. Rice seedlings were exposed to increasing salt concentrations during germination and early growth. The resulting levels of free proline were measured separately in shoots and roots and compared to those in untreated controls. Results were related to the corresponding ability of a given genotype to tolerate stress conditions. Neither absolute proline levels in untreated or in salt-stressed seedlings showed a straightforward relationship to the relative tolerance to salt, estimated as conductivity values able to reduce growth by 10 or 50%. Conversely, a highly significant correlation was found between the increase in proline levels in shoots and the ability to withstand stress. The results strengthen a recent hypothesis suggesting than an increase in proline metabolic rates, more than the resulting proline content, may help the cell to counteract the effects of abiotic stress conditions.