The effects of exogenous organic acids on the growth, photosynthesis and cellular ultrastructure of Salix variegata Franch. Under Cd stress

Exogenously Applied Trehalose Augments Cadmium Stress Tolerance and Yield of Mung Bean (Vigna radiata L.) Grown in Soil and Hydroponic Systems through Reducing Cd Uptake and Enhancing Photosynthetic Efficiency and Antioxidant Defense Systems

Cadmium (Cd) toxicity is a serious environmental issue causing a significant reduction in crop growth and productivity globally. Trehalose (Tre) has emerged as an important reducing sugar that can reduce the adverse impacts of different abiotic stresses. Therefore, the present investigation was performed to determine the key role of Tre in alleviating Cd stress in the mung bean (Vigna radiata L.) crop. The study was comprised of different treatments of cadmium (0, 10, 20 mg kg^-1 soil) and Tre (0, 15 and 30 mM). Cd stress significantly restricted the growth and yield of mung bean. However, Tre supplementation markedly improved growth and yield due to pronounced reductions in Cd uptake and Cd-induced oxidative stress as shown by the lower production of hydrogen peroxide (H₂O₂), electrolyte leakage (EL) and malondialdehyde (MDA) in Cd-stressed plants as well as by the enhanced activities of antioxidant enzymes (CAT, POD, APX and AsA). Moreover, the ameliorative role of Tre to Cd toxicity was also demonstrated by its ability to enhance chlorophyll contents, total soluble protein (TSP) and free amino acids (FAA). Taken together, Tre supplementation played a key beneficial role in improving Cd stress tolerance and yield traits of mung bean through restricting Cd uptake and enhancing photosynthetic capacity, osmolytes biosynthesis and antioxidant activities.

Pyramiding ascorbate-glutathione pathway in Lycopersicum esculentum confers tolerance to drought and salinity stress

Stacking Glutathione-Ascorbate pathway genes (PgSOD, PgAPX, PgGR, PgDHAR and PgMDHAR) under stress inducible promoter RD29A imparts significant tolerance to drought and salinity stress in Solanum lycopersicum. Although the exposure of plants to different environmental stresses results in overproduction of reactive oxygen species (ROS), many plants have developed some unique systems to alleviate the ROS production and mitigate its deleterious effect. One of the key pathways that gets activated in plants is ascorbate glutathione (AsA-GSH) pathway. To demonstrate the effect of this pathway in tomato, we developed the AsA-GSH overexpression lines by stacking the genes of the AsA-GSH pathway genes isolated from Pennisetum glaucoma (Pg) including PgSOD, PgAPX, PgGR, PgDHAR and PgMDHAR under stress inducible promoter RD29A. The overexpression lines have an improved germination and seedling growth with concomitant elevation in the survival rate. The exposure of transgenic seedlings to varying stress regiments exhibited escalation in the antioxidant enzyme activity and lesser membrane damage as reflected by decreased electrolytic leakage and little accumulation of malondialdehyde and H₂O₂. Furthermore, the transgenic lines accumulated high levels of osmoprotectants with increase in the relative water content. The increased photosynthetic activity and enhanced gaseous exchange parameters further confirmed the enhanced tolerance of AsA-GSH overexpression lines. We concluded that pyramiding of AsA-GSH pathway genes is an effective strategy for developing stress resistant crops.

Comparative cytology combined with transcriptomic and metabolomic analyses of Solanum nigrum L. in response to Cd toxicity

Cadmium (Cd) triggers molecular alterations in plants, perturbs metabolites and damages plant growth. Therefore, understanding the molecular mechanism underlying the Cd tolerance in plants is necessary for assessing the persistent environmental impact of Cd. In this study, Solanum nigrum was selected as the test plant to investigate changes in biomass, Cd translocation, cell ultrastructure, metabolites and genes under hydroponic conditions. The results showed that the plant biomass was significantly decreased under Cd stress, and the plant has a stronger Cd transport capability. Transmission electron microscopy revealed that increased Cd concentration gradually damaged the plant organs (roots, stems and leaves) cell ultrastructure, as evidenced by swollen chloroplasts and deformed cell walls. Additionally, metabolomics analyses revealed that Cd stress mainly affected seven metabolism pathways, including 19 differentially expressed metabolites (DEMs). Moreover, 3908 common differentially expressed genes (DEGs, 1049 upregulated and 2859 downregulated) were identified via RNA-seq among five Cd treatments. Meanwhile, conjoint analysis found several DEGs and DEMs, including laccase, peroxidase, D-fructose, and cellobiose etc., are associated with cell wall biosynthesis, implying the cell wall biosynthesis pathway plays a critical role in Cd detoxification. Our comprehensive investigation using multiple approaches provides a molecular-scale perspective on plant response to Cd stress.

Metabolomics analysis of cucumber fruit in response to foliar fertilizer and pesticides using UHPLC-Q-Orbitrap-HRMS

Pesticides and fertilizers are often used to improve the yield and quality of cucumber fruit. In this study, the effect of pesticide applied with or without foliar fertilizer on the cucumber fruit metabolism was investigated. The results showed that the mixed use of pesticides and foliar fertilizer could significantly increase the contents of organic acids and the antioxidant level. When pesticide was used without foliar fertilizer, cucumber fruit up-regulated (1.3 times) shikimate-phenylpropanoid pathway and improved the antioxidant capacity to deal with the pesticide stress. However, the tricarboxylic acid cycle was up-regulated 1.1 times and the antioxidant capacity was improved to promote the pesticide dissipation when pesticide was applied with foliar fertilizer. These observations indicate that the mixed application of foliar fertilizer and pesticides can regulate related metabolites and metabolic pathways, improve the quality and antioxidant capacity of cucumber fruit, and promote the dissipation of pesticides.

Accumulation Mechanism and Risk Assessment of Artemisia selengensis Seedling In Vitro with the Hydroponic Culture under Cadmium Pressure

Artemisia selengensis is a perennial herb of the Compositae with therapeutic and economic value in China. The cadmium (Cd) accumulation mechanism and healthy risk evaluation of A. selengensis were investigated in this study. Tissue culture seedlings were obtained by plant tissue culture in vitro, and the effect of Cd stress (Cd concentration of 0.5, 1, 5, 10, 25, 50 and 100 μM) on A. selengensis was studied under hydroponic conditions. The results showed that low-Cd (0.5–1 μM) stress caused a rare effect on the growth of A. selengensis seedlings, which regularly grew below the 10 μM Cd treatment concentration. The biomass growth rate of the 0.5, 1, and 5 μM treatment groups reached 105.8%, 96.6%, and 84.8% after 40 days of cultivation, respectively. In addition, when the concentration of Cd was greater than 10 μM, the plant growth was obviously inhibited, i.e., chlorosis of leaves, blackening roots, destroyed cell ultrastructure, and increased malondialdehyde (MDA) content. The root could be the main location of metal uptake, 57.8–70.8% of the Cd was concentrated in the root after 40 days of cultivation. Furthermore, the root cell wall was involved in the fixation of 49–71% Cd by subcellular extraction, and the involvement of the participating functional groups of the cell wall, such as -COOH, -OH, and -NH₂, in metal uptake was assessed by FTIR analysis. Target hazard quotient (THQ) was used to assess the health risk of A. selengensis, and it was found that the edible part had no health risk only under low-Cd stress (0.5 to 1 μM) and short-term treatment (less than 20 days).

A fed-batch feeding with succinic acid strategy for astaxanthin and lipid hyper-production in Haematococcus pluviualis

Effects of succinic acid (SA) in fed-batch feeding mode on astaxanthin and lipids biopoduction of Haematococcus pluvialis against abiotic stresses were explored. By comparison with the control, the initial addition of SA on day 0 increased the production of astaxanthin by 71.61%. More importantly, the maximum values of astaxanthin (35.88 mg g^-1) and lipid (54.79%) contents were obtained after supplementation of SA on day 7. Meanwhile, under SA treatment, the chlorophyll, carbohydrate, and protein levels were reduced, but the intracellular levels of SA and reactive oxygen species (ROS), the transcription levels of astaxanthin and fatty acids biosynthesis-, and antioxidant system-related genes were increased. Furthermore, scaling-up cultivation in bioreactor further enhanced the astaxanthin productivity from H. pluvialis. Generally, this study proved the intermittent SA feeding method in fed-batch culture as a potent strategy that facilitated massive astaxanthin and lipids production in algae.

Citric Acid-Mediated Abiotic Stress Tolerance in Plants

Several recent studies have shown that citric acid/citrate (CA) can confer abiotic stress tolerance to plants. Exogenous CA application leads to improved growth and yield in crop plants under various abiotic stress conditions. Improved physiological outcomes are associated with higher photosynthetic rates, reduced reactive oxygen species, and better osmoregulation. Application of CA also induces antioxidant defense systems, promotes increased chlorophyll content, and affects secondary metabolism to limit plant growth restrictions under stress. In particular, CA has a major impact on relieving heavy metal stress by promoting precipitation, chelation, and sequestration of metal ions. This review summarizes the mechanisms that mediate CA-regulated changes in plants, primarily CA’s involvement in the control of physiological and molecular processes in plants under abiotic stress conditions. We also review genetic engineering strategies for CA-mediated abiotic stress tolerance. Finally, we propose a model to explain how CA’s position in complex metabolic networks involving the biosynthesis of phytohormones, amino acids, signaling molecules, and other secondary metabolites could explain some of its abiotic stress-ameliorating properties. This review summarizes our current understanding of CA-mediated abiotic stress tolerance and highlights areas where additional research is needed.

Effects of cadmium stress on growth and physiological characteristics of sassafras seedlings

The effects of cadmium stress on the growth and physiological characteristics of Sassafras tzumu Hemsl. were studied in pot experiments. Five Cd levels were tested [CT(Control Treatment) : 0 mg/kg, Cd5: 5 mg/kg, Cd20: 20 mg/kg, Cd50: 50 mg/kg, and Cd100: 100 mg/kg]. The growth and physiological characteristics of the sassafras seedlings in each level were measured. The results showed that soil Cd had negative influences on sassafras growth and reduced the net growth of plant height and the biomass of leaf, branch and root. Significant reductions were recorded in root biomass by 18.18%(Cd5), 27.35%(Cd20), 27.57%(Cd50) and 28.95%(Cd100). The contents of hydrogen peroxide decreased first then increased while malondialdehyde showed the opposite trend with increasing cadmium concentration. Decreases were found in hydrogen peroxide contents by 10.96%(Cd5), 11.82%(Cd20) and 7.02%(Cd50); increases were found in malondialdehyde contents by 15.47%(Cd5), 16.07%(Cd20) and 7.85%(Cd50), indicating that cadmium stress had a certain effect on the peroxidation of the inner cell membranes in the seedlings that resulted in damage to the cell membrane structure. Superoxide dismutase activity decreased among treatments by 17.05%(Cd5), 10,68%(Cd20), 20.85%(Cd50) and 8.91%(Cd100), while peroxidase activity increased steadily with increasing cadmium concentration; these results suggest that peroxidase is likely the main protective enzyme involved in the reactive oxygen removal system in sassafras seedlings. Upward trends were observed in proline content by 90.76%(Cd5), 74.36%(Cd20), 99.73%(Cd50) and 126.01%(Cd100). The increase in proline content with increasing cadmium concentration indicated that cadmium stress induced proline synthesis to resist osmotic stress in the seedlings. Compared to that in CT, the soluble sugar content declined under the different treatments by 32.84%(Cd5), 5.85%(Cd20), 25.55%(Cd50) and 38.69%(Cd100). Increases were observed in the soluble protein content by 2.34%(Cd5), 21.36%(Cd20), 53.15%(Cd50) and 24.22%(Cd100). At different levels of cadmium stress, the chlorophyll content in the seedlings first increased and then decreased, and it was higher in the Cd5 and Cd20 treatments than that in the CT treatment. These results reflected that cadmium had photosynthesis-promoting effects at low concentrations and photosynthesis-suppressing effects at high concentrations. The photosynthetic gas exchange parameters and photosynthetic light-response parameters showed downward trends with increasing cadmium concentration compared with those in CT; these results reflected the negative effects of cadmium stress on photosynthesis in sassafras seedlings.

Enantioselective effects of imazethapyr on the secondary metabolites and nutritional value of wheat seedlings

The secondary metabolism of plants is key for mediating responses to environmental stress, but few studies have examined how the relationship between secondary metabolism and the stress response of plants is affected by exposure to chiral herbicides. Here, we studied the enantioselective disturbance of the chiral herbicide imazethapyr (IM) on the secondary metabolism and nutrient levels of wheat seedlings. The bioactive enantiomer R-IM significantly increased the contents of major secondary metabolites, including phenolic acids, flavonoids, and carotenoids but greatly inhibited the production of benzoxazine. The antioxidant system also responded strongly to R-IM; specifically, the activities of SOD, CAT, and GPX enzymes were all significantly induced, and the GSH content initially increased but then decreased. Furthermore, the nutrient levels of wheat seedlings were also affected; dietary fiber content decreased, while the contents of the microelements Fe, Mn, and Zn increased. In sum, this study provides new insight into the phytotoxic effects of IM and raises new questions on the role of secondary metabolites and nutrients in mediating enantioselective effects.

Impact of Foliar Application of Chitosan Dissolved in Different Organic Acids on Isozymes, Protein Patterns and Physio-Biochemical Characteristics of Tomato Grown under Salinity Stress

In this study, the anti-stress capabilities of the foliar application of chitosan, dissolved in four different organic acids (acetic acid, ascorbic acid, citric acid and malic acid) have been investigated on tomato (Solanum lycopersicum L.) plants under salinity stress (100 mM NaCl). Morphological traits, photosynthetic pigments, osmolytes, secondary metabolites, oxidative stress, minerals, antioxidant enzymes activity, isozymes and protein patterns were tested for potential tolerance of tomato plants growing under salinity stress. Salinity stress was caused a reduction in growth parameters, photosynthetic pigments, soluble sugars, soluble proteins and potassium (K⁺) content. However, the contents of proline, ascorbic acid, total phenol, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), sodium (Na⁺) and antioxidant enzyme activity were increased in tomato plants grown under saline conditions. Chitosan treatments in any of the non-stressed plants showed improvements in morphological traits, photosynthetic pigments, osmolytes, total phenol and antioxidant enzymes activity. Besides, the harmful impacts of salinity on tomato plants have also been reduced by lowering MDA, H₂O₂ and Na⁺ levels. Chitosan treatments in either non-stressed or stressed plants showed different responses in number and density of peroxidase (POD), polyphenol oxidase (PPO) and superoxide dismutase (SOD) isozymes. NaCl stress led to the diminishing of protein bands with different molecular weights, while they were produced again in response to chitosan foliar application. These responses were varied according to the type of solvent acid. It could be suggested that foliar application of chitosan, especially that dissolved in ascorbic or citric acid, could be commercially used for the stimulation of tomato plants grown under salinity stress.

Biochar for cadmium pollution mitigation and stress resistance in tobacco growth

Pot experiments were conducted to investigate the influence of biochar addition and the mechanisms that alleviate Cd stress in the growth of tobacco plant. Cadmium showed an inhibitory effect on tobacco growth at different post-transplantation times, and this increased with the increase in soil Cd concentration. The growth index decreased by more than 10%, and the photosynthetic pigment and photosynthetic characteristics of the tobacco leaf were significantly reduced, and the antioxidant enzyme activity was enhanced. Application of biochar effectively alleviated the inhibitory effect of Cd on tobacco growth, and the alleviation effect of treatments is more significant to the plants with a higher Cd concentration. The contents of chlorophyll a, chlorophyll b, and carotenoids in the leaves of tobacco plants treated with biochar increased by 9.99%, 12.58%, and 10.32%, respectively, after 60 days of transplantation. The photosynthetic characteristics index of the net photosynthetic rate increased by 11.48%, stomatal conductance increased by 11.44%, and intercellular carbon dioxide concentration decreased to 0.92. Based on the treatments, during the growth period, the antioxidant enzyme activities of tobacco leaves comprising catalase, peroxidase, superoxide dismutase, and malondialdehyde increased by 7.62%, 10.41%, 10.58%, and 12.57%, respectively, after the application of biochar. Our results show that biochar containing functional groups can effectively reduce the effect of Cd stress by intensifying the adsorption or passivation of Cd in the soil, thereby, significantly reducing the Cd content in plant leaves, and providing a theoretical basis and method to alleviate soil Cd pollution and effect soil remediation.

Isolation, identification and characterization of phenolic acid-degrading bacteria from soil

AIMS

To isolate, identify and characterize phenolic acid-degrading bacteria and reduce plant growth inhibition caused by phenolic acids.

METHODS AND RESULTS

A total of 11 bacterial isolates with high phthalic acid (PA)-degrading ability were obtained using mineral salt medium (MSM) medium containing PA as sole carbon source. These isolates were identified as Arthrobacter globiformis, Pseudomonas putida and Pseudomonas hunanensis by sequence analyses of the 16S rRNA gene. Among them, five Pseudomonas strains could also effectively degrade ferulic acid (FA), p-hydroxybenzoic acid (PHBA) and syringic acid (SA) in MSM solution. P. putida strain 7 and P. hunanensis strain 10 showed highly efficient degradation of PA, SA, FA and PHBA, and could reduce their inhibition of lily, watermelon, poplar and strawberry seedling growth in soils respectively. These two strains could promote plant growth in soil with phenolic acids.

CONCLUSIONS

In this study, bacterial strains with highly efficient phenolic acid-degrading abilities could not only effectively reduce the autotoxicity of phenolic acids on plants but also were able to promote plant growth in soil with phenolic acids.

SIGNIFICANCE AND IMPACT OF THE STUDY

In this study, Pseudomonas can promote plant growth while degrading phenolic acids. Our results provide new choices for the biological removal of autotoxins.

Effects of mineral-solubilizing microbial strains on the mechanical responses of roots and root-reinforced soil in external-soil spray seeding substrate

There are a large number of abandoned mining areas in China, where external-soil spray seeding is a common technique used to assist with the restoration of these areas. However, the soil component of external-soil spray seeding is deficient, and they are prone to collapse, which complicates ecological restoration. In this study, we added a mineral-solubilizing microbial strain to an external-soil spray seeding substrate in Robinia pseudoacacia and Lespedeza bicolor pots, which were monitored from December 2018 to November 2019. We investigated their root growth and root tensile properties, as well as root-reinforced soil shear strength. The results revealed that the addition of the microbial strain in the substrate improved root growth of Robinia pseudoacacia. The root-reinforced soil shear strength, tensile force and strength were also strengthened by the added microbial strain. Although the growth rate of Robinia pseudoacacia was faster than that of Lespedeza bicolor, the shear strength of the root-reinforced Robinia pseudoacacia soil was lower than that of the Lespedeza bicolor root-reinforced soil of the same diameter. Finally, compared with the cohesion, the change in the friction angle is relatively small, and differences in cohesion resulted in shear strength changes under the same treatment. Our results suggested that the addition of a mineral-solubilizing microbial strain to the external-soil spray seeding substrate could help plants strengthen the soil and positively enhance its effects. These results might also enrich the existing data on the effects of mineral-solubilizing microbial strains on plant roots, while guiding further studies toward improving the efficacy of external-soil spray seeding technologies.

Effects of Exogenous Organic Acids on Cd Tolerance Mechanism of Salix variegata Franch. Under Cd Stress

Chelate induction of organic acids has been recognized to enhance metal uptake and translocation in plants, but the underlying mechanism remains unclear. In this study, seedlings of Salix variegata were hydroponically exposed to the combinations of Cd (0 and 50 μM) and three exogenous organic acids (100 μM of citric, tartaric, or malic acid). Plant biomass, antioxidant enzymes, non-protein thiol compounds (NPT) content, and the expression of candidate genes associated with Cd accumulation and tolerance were determined. Results showed that Cd significantly inhibited plant biomass but stimulated the activity of antioxidant enzymes in the roots and leaves, while the lipid peroxidation increased as well. Respective addition of three organic acids greatly enhanced plant resistance to oxidative stress and reduced the lipid peroxidation induced by Cd, with the effect of malic acid showing greatest. The addition of organic acids also significantly increased the content of glutathione in the root, further improving the antioxidant capacity and potential of phytochelatin biosynthesis. Moreover, Cd induced the expression level of candidate genes in roots of S. variegata. The addition of three organic acids not only promoted the expression of candidate genes but also drastically increased Cd accumulation in S. variegata. In summary, application of citric, tartaric, or malic acid alleviated Cd-imposed toxicity through the boost of enzymatic and non-enzymatic antioxidants and candidate gene expression, while their effects on Cd tolerance and accumulation of S. variegata differed.