Comparative transcriptomic and metabolomic analyses provide insights into the responses to NaCl and Cd stress in Tamarix hispida

The ethylene response factor gene, ThDRE1A, is involved in abscisic acid- and ethylene-mediated cadmium accumulation in Tamarix hispida

Tamarix hispida is highly tolerant to salt, drought and heavy metal stress and is a potential material for the remediation of cadmium (Cd)-contaminated soil under harsh conditions. In this study, T. hispida growth and chlorophyll content decreased, whereas flavonoid and carotenoid contents increased under long-term Cd stress (25 d). The aboveground components of T. hispida were collected for RNA-seq to investigate the mechanism of Cd accumulation. GO and KEGG enrichment analyses revealed that the differentially expressed genes (DEGs) were significantly enriched in plant hormone-related pathways. Exogenous hormone treatment and determination of Cd2+ levels showed that ethylene (ETH) and abscisic acid (ABA) antagonists regulate Cd accumulation in T. hispida. Twenty-five transcription factors were identified as upstream regulators of hormone-related pathways. ThDRE1A, which was previously identified as an important regulatory factor, was selected for further analysis. The results indicated that ThABAH2.5 and ThACCO3.1 were direct target genes of ThDRE1A. The determination of Cd2+, ABA, and ETH levels indicated that ThDRE1A plays an important role in Cd accumulation through the antagonistic regulation of ABA and ETH. In conclusion, these results reveal the molecular mechanism underlying Cd accumulation in plants and identify candidate genes for further research.

The transcription factor ThDOF8 binds to a novel cis-element and mediates molecular responses to salt stress in Tamarix hispida

Salt stress is a common abiotic factor that restricts plant growth and development. As a halophyte, Tamarix hispida is a good model plant for exploring salt-tolerance genes and regulatory mechanisms. DNA-binding with one finger (DOF) is an important transcription factor (TF) that influences and controls various signaling substances involved in diverse biological processes related to plant growth and development, but the regulatory mechanisms of DOF TFs in response to salt stress are largely unknown in T. hispida. In the present study, a newly identified Dof gene, ThDOF8, was cloned from T. hispida, and its expression was found to be induced by salt stress. Transient overexpression of ThDOF8 enhanced T. hispida salt tolerance by enhancing proline levels, and increasing the activities of the antioxidant enzymes superoxide dismutase (SOD) and peroxidase (POD). These results were also verified in stably transformed Arabidopsis. Results from TF-centered yeast one-hybrid (Y1H) assays and EMSAs showed that ThDOF8 binds to a newly identified cis-element (TGCG). Expression profiling by gene chip analysis identified four potential direct targets of ThDOF8, namely the cysteine-rich receptor-like kinases genes, CRK10 and CRK26, and two glutamate decarboxylase genes, GAD41, and GAD42, and these were further verified by ChIP-quantitative-PCR, EMSAs, Y1H assays, and β-glucuronidase enzyme activity assays. ThDOF8 can bind to the TGCG element in the promoter regions of its target genes, and transient overexpression of ThCRK10 also enhanced T. hispida salt tolerance. On the basis of our results, we propose a new regulatory mechanism model, in which ThDOF8 binds to the TGCG cis-element in the promoter of the target gene CRK10 to regulate its expression and improve salt tolerance in T. hispida. This study provides a basis for furthering our understanding the role of DOF TFs and identifying other downstream candidate genes that have the potential for improving plant salt tolerance via molecular breeding.

Biochemical and multi-omics analyses of response mechanisms of rhizobacteria to long-term copper and salt stress: Effect on soil physicochemical properties and growth of Avicennia marina

Mangroves are of important economic and environmental value and research suggests that their carbon sequestration and climate change mitigation potential is significantly larger than other forests. However, increasing salinity and heavy metal pollution significantly affect mangrove ecosystem function and productivity. This study investigates the tolerance mechanisms of rhizobacteria in the rhizosphere of Avicennia marina under salinity and copper (Cu) stress during a 4-y stress period. The results exhibited significant differences in antioxidant levels, transcripts, and secondary metabolites. Under salt stress, the differentially expressed metabolites consisted of 30% organic acids, 26.78% nucleotides, 16.67% organic heterocyclic compounds, and 10% organic oxides as opposed to 27.27% organic acids, 24.24% nucleotides, 15.15% organic heterocyclic compounds, and 12.12% phenyl propane and polyketides under Cu stress. This resulted in differential regulation of metabolic pathways, with phenylpropanoid biosynthesis being unique to Cu stress and alanine/aspartate/glutamate metabolism and α-linolenic acid metabolism being unique to salt stress. The regulation of metabolic pathways enhanced antioxidant defenses, nutrient recycling, accumulation of osmoprotectants, stability of plasma membrane, and chelation of Cu, thereby improving the stress tolerance of rhizobacteria and A. marina. Even though the abundance and community structure of rhizobacteria were significantly changed, all the samples were dominated by Proteobacteria, Chloroflexi, Actinobacteriota, and Firmicutes. Since the response mechanisms were unbalanced between treatments, this led to differential growth trends for A. marina. Our study provides valuable inside on variations in diversity and composition of bacterial community structure from mangrove rhizosphere subjected to long-term salt and Cu stress. It also clarifies rhizobacterial adaptive mechanisms to these stresses and how they are important for mitigating abiotic stress and promoting plant growth. Therefore, this study can serve as a reference for future research aimed at developing long-term management practices for mangrove forests.

Integration of Physiological, Transcriptomic, and Metabolomic Analyses Reveal Molecular Mechanisms of Salt Stress in Maclura tricuspidata

Salt stress is a universal abiotic stress that severely affects plant growth and development. Understanding the mechanisms of Maclura tricuspidate's adaptation to salt stress is crucial for developing salt-tolerant plant varieties. This article discusses the integration of physiology, transcriptome, and metabolome to investigate the mechanism of salt adaptation in M. tricuspidata under salt stress conditions. Overall, the antioxidant enzyme system (SOD and POD) of M. tricuspidata exhibited higher activities compared with the control, while the content of soluble sugar and concentrations of chlorophyll a and b were maintained during salt stress. KEGG analysis revealed that deferentially expressed genes were primarily involved in plant hormone signal transduction, phenylpropanoid and flavonoid biosynthesis, alkaloids, and MAPK signaling pathways. Differential metabolites were enriched in amino acid metabolism, the biosynthesis of plant hormones, butanoate, and 2-oxocarboxylic acid metabolism. Interestingly, glycine, serine, and threonine metabolism were found to be important both in the metabolome and transcriptome-metabolome correlation analyses, suggesting their essential role in enhancing the salt tolerance of M. tricuspidata. Collectively, our study not only revealed the molecular mechanism of salt tolerance in M. tricuspidata, but also provided a new perspective for future salt-tolerant breeding and improvement in salt land for this species.

Combined microbe-plant remediation of cadmium in saline-alkali soil assisted by fungal mycelium-derived biochar

Cadmium contamination in saline-alkali soil is becoming a great concern. Combined microbe-plant remediation is an economic way to treat this contamination, but is compromised by its low cadmium-removing capacity. In this study, the novel fungus-derived biochar was prepared to enhance the salt-tolerant bacterium-plant remediation of cadmium-contaminated saline-alkali soil. This biochar was prepared by pre-incubation of living Trichoderma atroviride hyphae with imidazole and further heating at 500 °C for 1 h. The obtained fungus-derived nitrogen-doped biochar (FBioCN) exhibited the high affinity to bacterial cells, leading to efficient colonization of exogenous salt-tolerant bacteria (e.g., Rhizobacter sp. and Sphingomonas sp.) on Amaranthus hypochondriacus roots. During culturing of the plants in the cadmium-contaminated saline-alkali soil, FBioCN drastically remodeled the rhizosphere microbiome, leading to enhance colonization of the exogeneous salt-tolerant bacteria, and increase bacterial diversity. The combination of FBioCN and the exogeneous bacteria further improved the activity of rhizosphere functional enzymes, protected the plants from the multiple stress, and promoted cadmium transport from the soil to the plants. Consequently, FBioCN together with the salt-tolerant bacteria drastically improved cadmium removal from the saline-alkali soil, with the percent of cadmium removal at the rhizosphere region increasing from 35.1% to 95.1%. This study sheds a light on the application of fungus-derived biochar in combined microbe-plant remediation in saline-alkali soil.

Effect of forest planting patterns on the formation of soil organic carbon during litter lignocellulose degradation from a microbial perspective

Litter decomposition is an important source of soil organic carbon, and it plays a key role in maintaining the stability of forest ecosystems. The microbial mechanism of soil organic carbon (SOC) formation in different urban forest planting patterns during litter lignocellulose degradation is still unclear. The key genes, microbes, and metabolites in the process of lignocellulose degradation and SOC formation were determined by metagenomics and metabolomics in different litter decomposition layers and soil layers in different urban forest planting patterns, including three types of broadleaf forests (BP forests), three types of coniferous forests (CP forests), and two types of mixed coniferous and broadleaf forests (MCBP forests). The results indicated that the cellulose, hemicellulose, and lignin concentrations from the undecomposed layer to the totally decomposed layer decreased by 70.07, 86.83, and 73.04% for CP litter; 74.30, 93.80, and 77.55% for BP litter; and 62.51, 48.58, and 90.61% for MCBP litter, respectively. The soil organic carbon of the BP forests and MCBP forests was higher than that of the CP forests by 38.06 and 94.43% for the 0-10 cm soil layer and by 38.55 and 20.87% for the 10-20 cm soil layer, respectively. Additionally, the gene abundances of glycoside hydrolases (GHs) and polysaccharide lyases (PLs) in the BP forests were higher than those in the MCBP forests and CP forests. Amino acid metabolism, sugar metabolism, TCA metabolism, and cAMP signaling metabolism were mainly between the CP forests and BP forests, while the TCA cycle, pyruvate metabolism, phenylalanine metabolism, and tyrosine metabolism were mainly between the BP forests and MCBP forests during litter decomposition. Additionally, ammonia nitrogen and hemicellulose were key factors driving SOC formation in the CP forests, while ammonia nitrogen, hemicellulose, and lignocellulose-degrading genes were key factors driving SOC formation in the BP forests. For the MCBP forests, cellulose, pH, ammonia nitrogen, and lignin were key factors driving SOC formation. Our findings revealed that the BP forests and MCBP forests had stronger lignocellulose degradation performance in the formation of SOC. This study provided a theoretical basis for the flow and transformation of nutrients in different urban forest management patterns.

Physiological effects of combined NaCl and NaHCO3 stress on the seedlings of two maple species

Salt stress impacts growth and physiological processes in plants, and some plants exposed to salt stress will produce physiological mechanisms to adapt to the new environment. However, the effects of combined NaCl and NaHCO3 stress on the seedlings of Acer species are understudied. In this study, we designed an experiment to measure physiological characteristics by establishing a range of NaCl and NaHCO₃ concentrations (0, 25, 50, 75, and 100 mmol L^-1) to estimate the compound salt tolerance of Acer ginnala and Acer palmatum. When the concentrations of NaCl and NaHCO₃ were 25 mmol L^-1, the leaf water content, relative conductivity, malondialdehyde (MDA) content, proline content, soluble sugar content, and chlorophyll did not change (p > 0.05) in two maple seedlings. At concentrations greater than 50 mmol L^-1, the relative conductivity and MDA content increased, proline and soluble sugars accumulated, and the potential activity of PS II (F_v/F_o), potential photochemical efficiency of PS II (F_v/F_m), PS II actual photochemical efficiency (Yield), and photosynthetic electron transfer efficiency (ETR) decreased (p < 0.05). The superoxide dismutase (SOD) and catalase (CAT) activities showed the same trend of first increasing and then decreasing (p < 0.05). The peroxidase (POD) activity increased only when concentrations of NaCl and NaHCO₃ were 100 mmol L^-1, while there was no statistical difference between the other treatments and the control. Therefore, the two maple seedlings adjusted their osmotic balance and alleviated oxidative stress by accumulating proline, soluble sugars and increasing CAT and SOD activities. Further analysis showed that both species are salt tolerant and the salt tolerance of Acer ginnala is better than that of Acer palmatum.