BnXTH1 regulates cadmium tolerance by modulating vacuolar compartmentalization and the cadmium binding capacity of cell walls in ramie (Boehmeria nivea).

Xyloglucan endotransglucosylase/hydrolases (XTH) are cell wall-modifying enzymes important in plant response to abiotic stress. However, the role of XTH in cadmium (Cd) tolerance in ramie remains largely unknown. Here, we identified and cloned BnXTH1, a member of the XTH family, in response to Cd stress in ramie. The BnXTH1 promoter (BnXTH1p) demonstrated that MeJA induces the response of BnXTH1p to Cd stress. Moreover, overexpressing BnXTH1 in Boehmeria nivea increased Cd tolerance by significantly increasing the Cd content in the cell wall and decreasing Cd inside ramie cells. Cadmium stress induced BnXTH1-expression and consequently increased xyloglucan endotransglucosylase (XET) activity, leading to high xyloglucan contents and increased hemicellulose contents in ramie. The elevated hemicellulose content increased Cd chelation onto the cell walls and reduced the level of intracellular Cd. Interestingly, overexpressing BnXTH1 significantly increased the content of Cd in vacuoles of ramie and vacuolar compartmentalization genes. Altogether, these results evidence that Cd stress induced MeJA accumulation in ramie, thus, activating BnXTH1 expression and increasing the content of xyloglucan to enhance the hemicellulose binding capacity and increase Cd chelation onto cell walls. BnXTH1 also enhances the vacuolar Cd compartmentalization and reduces the level of Cd entering the organelles and soluble solution.

Exogenous plant growth regulator and foliar fertilizers for phytoextraction of cadmium with Boehmeria nivea [L.] Gaudich from contaminated field soil.

As a enrichment plant, ramie can be used for the phytoremediation of cadmium (Cd)-contaminated soil. However, it is worth exploring the role of plant growth regulators and foliar fertilizers in the process of plant growth and development and Cd adsorption. By measuring the agronomic traits, Cd content of aboveground and underground ramie, calculating the Cd transfer coefficient (TF) and Cd bioconcentration factors (BCF), and the correlation between various indicators. This study examined the effects of plant growth regulators and foliar fertilizers on ramie's capacity for Cd accumulation and transportation. Plant growth regulators and foliar fertilizers increased the Cd content of the aboveground ramie, reduced the Cd content of the underground ramie, and increased the TF. Among them, GA-1 increased the Cd content of the aboveground ramie to 3 times more than that of the control and reduced the Cd content of the underground ramie by 54.76%. Salicylic acid (SA) increased the Cd content of the aboveground ramie to three times more than that of the control. The combination of GA and foliar fertilizer reduced the Cd content of the aboveground and underground ramie and the TF and BCF of the underground ramie. After the hormones were sprayed, the TF of ramie had a significant positive correlation with the Cd content of the aboveground ramie; the BCF of the aboveground ramie had a significant positive correlation with the Cd content and TF of the aboveground ramie. The results indicate that Brassinolide (BR), gibberellin (GA), ethephon (ETH), polyamines (PAs), and salicylic acid (SA) have different effects on the enrichment and transport of Cd in ramie. This study provided an effective method to improve the capacity for ramie to adsorb heavy metals during cultivation.

Biochar facilitated the phytoremediation of cadmium contaminated sediments: Metal behavior, plant toxicity, and microbial activity.

Cadmium (Cd) contamination in river sediments becomes increasingly serious, and phytoremediation has been used to remediate Cd contaminated sediments, but the remediation efficiency needs to be improved. In this study, tea waste derived biochar (TB) was used to facilitate the phytoremediation of Cd contaminated sediments. Results showed that TB at 100, 500 and 1000 mg kg-1 increased Cd accumulation and translocation in ramie seedlings by changing Cd speciation in sediments and altering the subcellular distribution of Cd in plant cells. TB at low contents alleviated Cd induced toxicity in ramie seedlings by promoting plant growth and mitigating the oxidative stress. In addition, the activities of urease-, phosphatase-, and catalase-producing microbes in the Cd contaminated sediments were promoted by the application of TB. These findings demonstrated that biochar at low concentrations could improve the phytoremediation efficiency and mitigating Cd-induced toxicity to plants and microbes in Cd contaminated sediments. This study herein provides a novel technological application of waste biomass in controlling and mitigating risks of heavy metals.

Effects of exogenous calcium and spermidine on cadmium stress moderation and metal accumulation in Boehmeria nivea (L.) Gaudich.

Cadmium (Cd) is a detrimental metal in the environment and it is easily taken up by plants, thus entering the food chain and posing a severe threat to human health. Phytoremediation being low cost, highly stable, and environmentally friendly has been considered as a promising green technology for Cd remediation. The addition of exogenous substances to the culture media has been recognized as an efficient strategy to improve plant phytoremediation capability. Pot trials were conducted to investigate the combined effects of exogenous calcium (Ca) and spermidine (Spd) on Cd-induced toxicity in Boehmeria nivea (L.) Gaudich. (ramie). Results showed that the application of 5-mM exogenous Ca significantly alleviated Cd toxicity in ramie by reducing Cd accumulation, depressing H2O2 and malondialdehyde contents, increasing plants dry weights and chlorophyll concentrations, as well as altering the activities of total superoxide dismutase and guaiacol peroxidase. Furthermore, as a non-Cd hyperaccumulator plant, ramie hyperaccumulated Cd and suffered more severe toxic effects of Cd by the treatment of 1 mM Ca/Cd. The aggravated Cd toxicity could be compensated by the addition of exogenous Spd via the promotion of plant growth and the reduction of the oxidative stress. Overall, the combination effects of 1 mM Ca and Spd appeared to be more superior compared to other treatments in the plants under Cd stress with a higher Cd accumulation ability and the evaluated Cd stress tolerance.

Expression profiling of cadmium response genes in ramie (Boehmeria nivea L.) root.

Ramie (Boehmeria nivea), a perennial herb belongs to Urticaceae family, is a rapid growth and high biomass crop with highly tolerant and accumulative to heavy metals. However, the gene expression and regulation caused by cadmium (Cd) in ramie has not been well studied. In the present study, a gene expression database of ramie root in the absence (control) or presence of 100 µM Cd was established. Solexa high-throughput sequencing technology showed that 3,654,395 and 3,572,333 tags have been obtained from control and Cd treatment respectively. In total, 3887 genes were detected with significant differential expression levels, in which 2883 genes were up-regulated and 1004 genes were down-regulated. Gene ontology and pathway-based analyses were performed to determine and further to understand the biological functions of those differentially expressed genes. Fifteen genes were selected and their expression levels were confirmed by quantitative RT-PCR, and twelve of them showed consistent expression patterns with the digital gene expression data. Results on these expression profiling of genes lay the basis for biotechnological modification of new transgenic plants with improved phytoremediation capacity.

Genome-wide transcriptomic profiling of ramie (Boehmeria nivea L. Gaud) in response to cadmium stress.

Cadmium (Cd) contamination in agricultural soils has become a major environmental problem in China. Ramie, a fiber crop, has frequently been proposed for use as a phytoremediation crop for the restoration of Cd-contaminated farmlands. However, high levels of Cd can greatly inhibit stem growth in ramie, which reduces its economic value as a crop. To understand the potential mechanisms behind this phenomenon, the ramie genes involved in the Cd stress response were identified using Illumina pair-end sequencing on two Cd-stressed plants (CdS1 and CdS2) and two control plants (CO1 and CO2). Approximately 48.7, 51.6, 41.2, and 47.1 million clean sequence reads were generated from the libraries of CO1, CO2, CdS1, and CdS2, respectively, and de novo assembled to yield 56,932 non-redundant unigenes. A total of 26,686 (46.9%) genes were annotated for their function. Comparison of gene expression levels in CO and CdS ramie revealed 155 differentially expressed genes (DEGs) between treatment and control conditions. Sixteen DEGs were further analyzed for expression differences by using real-time quantitative PCR (qRT-PCR). Among these 16 DEGs, 2 genes encoding GA2-oxidase (a major enzyme for deactivating bioactive gibberellins [GAs]) showed markedly up-regulated expression in Cd stressed ramie. This might be responsible for the growth inhibition of Cd-stressed ramie. Pathway enrichment analysis revealed that the cutin, suberine and wax biosynthesis pathway was markedly enriched by DEGs. The discovery of these Cd stress-responsive genes and pathways will be helpful in further understanding the mechanism of Cd-stress response and improving Cd stress tolerance in ramie.

Effect of exogenous nitric oxide on antioxidative system and S-nitrosylation in leaves of Boehmeria nivea (L.) Gaud under cadmium stress.

Cadmium (Cd)-induced growth inhibition is one of the primary factors limiting phytoremediation effect of Boehmeria nivea (L.) Gaud in contaminated soil. Sodium nitroprusside (SNP), a donor of nitric oxide (NO), has been evidenced to alleviate Cd toxicity in many plants. However, as an important mechanism of NO in orchestrating cellular functions, S-nitrosylation is still poorly understood in its relation with Cd tolerance of plants. In this study, higher exogenous NO levels were found to coincide with higher S-nitrosylation level expressed as content of S-nitrosothiols (SNO). The addition of low concentration (100 µM) SNP increased the SNO content, and it simultaneously induced an alleviating effect against Cd toxicity by enhancing the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR) and reduced the accumulation of H2O2 as compared with Cd alone. Application of S-nitrosoglutathione reductase (GSNOR) inhibitors dodecanoic acid (DA) in 100 µM SNP group brought in an extra elevation in S-nitrosylation level and further reinforced the effect of SNP. While the additions of 400 µM SNP and 400 µM SNP + 50 µM DA further elevated the S-nitrosylation level, it markedly weakened the alleviating effect against Cd toxicity as compared with the addition of 100 µM SNP. This phenomenon could be owing to excess consumption of glutathione (GSH) to form SNO under high S-nitrosylation level. Therefore, the present study indicates that S-nitrosylation is involved in the ameliorating effect of SNP against Cd toxicity. This involvement exhibited a concentration-dependent property.

Enhanced efficiency of cadmium removal by Boehmeria nivea (L.) Gaud. in the presence of exogenous citric and oxalic acids.

Boehmeria nivea (L.) Gaud. is a potential candidate for the remediation of Cd contaminated sites. The present investigation aims to explore Cd tolerance threshold and to quickly identify the role of exogenous organic acids in Cd uptake and abiotic metal stress damage. Elevated Cd levels (0-10mg/L) resulted in an obvious rise in Cd accumulation, ranging from 268.0 to 374.4 in root and 25.2 to 41.2mg/kg dry weight in shoot, respectively. Citric acid at 1.5 mmol/L significantly facilitated Cd uptake by 26.7% in root and by 1-fold in shoot, respectively. Cd translocation efficiency from root to shoot was improved by a maximum of 66.4% under 3 mmol/L of oxalic acid. Citric acid exhibited more prominent mitigating effect than oxalic acid due to its stronger ligand affinity for chelating with metal and avoiding the toxicity injury of free Cd ions more efficiently. The present work provides a potential strategy for efficient Cd remediation with B. nivea.

Comparative proteome analysis of the response of ramie under N, P and K deficiency.

Ramie is an important natural fiber. There has been little research on the molecular mechanisms of ramie related to the absorption, utilization and metabolism of nitrogen (N), phosphorus (P) and potassium (K). One approach to reveal the mechanisms of N, P and K (NPK) utilization and metabolism in ramie is comparative proteome analysis. The differentially expressed proteins in the leaves of ramie were analyzed by proteome analysis after 6 days of N- and K-deficient treatments and 3 days of P-deficient treatment using MALDI-TOF/TOF mass spectrometry and 32, 27 and 51 differential proteins were obtained, respectively. These proteins were involved in photosynthesis, protein destination and storage, energy metabolism, primary metabolism, disease/defense, signal transduction, cell structure, transcription, secondary metabolism and protein synthesis. Ramie responded to NPK stress by enhancing secondary metabolism and reducing photosynthesis and energy metabolism to increase endurance. Specifically, ramie adapted to NPK deficiency by increasing signal transduction pathways, enhancing the connection between glycolysis and photosynthesis, promoting the intracellular flow of carbon and N; promoting the synthesis cysteine and related hormones and upregulating actin protein to promote growth of the root system. The experimental results provide important information for further study on the high-efficiency NPK utilization mechanism of ramie.