Effect of Cd stress on the bioavailability of Cd and other mineral nutrition elements in broad bean grown in a loess subsoil amended with municipal sludge compost

Comparative transcriptomic analysis provides key genetic resources in clove basil (Ocimum gratissimum) under cadmium stress

Planting aromatic plant might be a promising strategy for safely utilizing heavy metal (HM)-contaminated soils, as HMs in essential oil could be completely excluded using some special technologies with ease. Clove basil (Ocimum gratissimum L.) is an important aromatic plant used in essential oil production. Improving cadmium (Cd) tolerance in clove basil can increase its production and improve the utilization efficiency of Cd-contaminated soils. However, the lack of genomic information on clove basil greatly restricts molecular studies and applications in phytoremediation. In this study, we demonstrated that high levels of Cd treatments (0.8, 1.6 and 6.5 mg/L) significantly impacted the growth and physiological attributes of clove basil. Cd contents in clove basil tissues increased with treatment concentrations. To identify Cd stress-responsive genes, we conducted a comparative transcriptomic analysis using seedlings cultured in the Hoagland's solution without Cd ion (control) or containing 1.6 mg/L CdCl2 (a moderate concentration of Cd stress for clove basil seedlings). A total of 104.38 Gb clean data with high-quality were generated in clove basil under Cd stress through Illumina sequencing. More than 1,800 differential expressed genes (DEGs) were identified after Cd treatment. The reliability and reproducibility of the transcriptomic data were validated through qRT-PCR analysis and Sanger sequencing. KEGG classification analysis identified the "MAPK signaling pathway," "plant hormone signal transduction" and "plant-pathogen interaction" as the top three pathways. DEGs were divided into five clusters based on their expression patterns during Cd stress. The functional annotation of DEGs indicated that downregulated DEGs were mainly involved in the "photosynthesis system," whereas upregulated DEGs were significantly assigned to the "MAPK signaling pathway" and "plant-pathogen interaction pathway." Furthermore, we identified a total of 78 transcription factors (TFs), including members of bHLH, WRKY, AP2/ERF, and MYB family. The expression of six bHLH genes, one WRKY and one ERF genes were significantly induced by Cd stress, suggesting that these TFs might play essential roles in regulating Cd stress responses. Overall, our study provides key genetic resources and new insights into Cd adaption mechanisms in clove basil.

Effect of humic acid on seedling growth and trace metal accumulation of pak choi (Brassica chinensis L.) cultivated on molybdenum slag-spiked soil

The growth performance and trace metal accumulation of pak choi (Brassica chinensis L.) were investigated to evaluate the ameliorative effect of humic acid on molybdenum (Mo) slag-spiked calcareous soil. Calcareous soil spiked with 5.0% (w/w) slag was amended with humic acid derived from leonardite from 0 to 5.0% (w/w). With increasing application rate, humic acid enhanced the antioxidative capacity of pak choi seedling, as indicated by increases in the activities of antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) and a decrease in malondialdehyde content; humic acid application also increased total chlorophyll content, leaf area, seedling height, and fresh biomass of pak choi. These stimulation effects started to decrease above 2.5-5.0% application of humic acid. The contents of trace metals (Cu, Mn, Zn, As, Cd, and Pb) in the aboveground part of pak choi seedling generally decreased at low rates (0.5% and 1.0%), and then increased with higher rates (2.5% and 5.0%) of humic acid application. Health risk assessment of trace metals based on target hazard quotient (THQ) suggested that consuming pak choi grown on these soils is safe. Low rate (0.5%) of humic acid reduced the potential health risk, while high rates (2.5% and 5.0%) accumulated trace metals and increased health risk. Humic acid could be added to Mo slag-spiked calcareous soil for the yield and food safety of pak choi, but the overuse of humic acid should be avoided.

Morphological and Physiological Changes of Broussonetia papyrifera Seedlings in Cadmium Contaminated Soil

Broussonetia papyrifera is a widely distributed economic tree species, and it is also a pioneer species in adverse environments. In order to investigate the growth and adaptation mechanism of B. papyrifera under cadmium (Cd) contaminated soil, potted experiments were used with six-month treatments to study Cd enrichment and the transportation, morphological and physiological characteristics of B. papyrifera tissues. The results showed that Cd mainly accumulated in the root when the Cd concentration was high (14.71 mg/kg), and the root biomass was significantly reduced by Cd stress although Cd promoted the growth of seedlings. The bioconcentration factors (BCF) increased with the increase in Cd concentration, and reached the maximum value of 0.21 at 14.71 mg/kg. On the contrary, translocation factor (TF) decreased significantly at 8.28–14.71 mg/kg Cd concentration. Cd not only led to the loose arrangement of the xylem vessels of leaves, but also changed the chlorophyll content. However, B. papyrifera could synthesize organic solutes such as soluble protein, soluble sugar and proline to reduce the intracellular osmotic potential. Our study proved that B. papyrifera has good tolerance to Cd stress and is a pioneer tree species for soil and ecological environment restoration.

Insights into factors driving the transmission of antibiotic resistance from sludge compost-amended soil to vegetables under cadmium stress

Sludge compost is often used as a fertilizer for crops, although it might be enriched with antibiotic resistance genes (ARGs) and heavy metals that cannot be removed through composting. A robust understanding of the factors affecting the transmission of ARGs to vegetables grown in soils treated with sludge products is lacking. In this study, target ARGs in the bulk and rhizosphere soils and endophytes of shallots under heavy metal stress (i.e., Cd) were assessed, and the factors driving the transmission of ARGs were identified. Cd stress resulted in an increase in the relative abundances of target ARGs in the bulk and rhizosphere soils and endophytes. The driving factors were different in soils and plants under different degrees of Cd stress. The fungal community composition was the main driving factor of ARG variation in both bulk and rhizosphere soils. Moreover, endophytic bacteria played a crucial role in transferring ARGs to plants. Higher Cd stress promoted the transfer of most target ARGs from the below-ground plant parts to the above-ground parts. These findings indicate that application of sludge contaminated with heavy metals, such as Cd, can facilitate the dissemination of ARGs into vegetables, which must be considered while assessing the risks to public health.

Physiological stress responses, mineral element uptake and phytoremediation potential of Morus alba L. in cadmium-contaminated soil

Fast growing woody plants are proposed for potential application for phytoremediation of contaminated soil. In this study, the plant growth, physiological responses, mineral element uptake, and phytoremediation potential of the woody plant Morus alba L. were studied in different levels of Cd-contaminated soil through dynamic sampling (30, 60, 120, and 180 d). The results indicated that M. alba L. had strong physiological coordination, tolerance and detoxification capacity in response to Cd in contaminated soil. Compared with the control, the photosynthetic pigment content in M. alba L. leaves was significantly suppressed during initial cultivation (30-60 d) and the malonaldehyde (MDA) content and electrolyte leakage (EL) were increased from 30 to 120 d of cultivation. Furthermore, the uptake of Cu, Mn, and Zn in plant tissues was imbalanced throughout cultivation (30-180 d) under 55 mg·kg^-1 Cd stress. However, the chlorophyll a, chlorophyll b, carotenoid, soluble protein, and soluble sugar contents and the peroxidase (POD) and ascorbate peroxidase (APX) activities in plant leaves, as well as the uptake of macronutrients (K, Ca, and Mg) in plant stems and leaves were maintained at normal levels. Furthermore, the catalase (CAT) activities in plant leaves and the Ca and Mg contents in plant roots were significantly (p < 0.05) enhanced in response to Cd stress after 180 d of cultivation. Furthermore, the biomass of M. alba L. was significantly increased with cultivation time in Cd-contaminated soil. Therefore, normal photosynthesis, antioxidant protection, and macronutrient regulation contribute to M. alba L. with high tolerance to Cd. Moreover, the uptake and total extraction amount of Cd in aboveground M. alba L. were significantly (p < 0.05) increased with both the plant growth period and soil Cd level, and the maximum amount of Cd reached up to 340.5 μg·plant^-1. Thus, M. alba L. can be regarded as a potential candidate for phytoremediation in Cd-contaminated sites.

Interactive effect of 24-epibrassinolide and silicon alleviates cadmium stress via the modulation of antioxidant defense and glyoxalase systems and macronutrient content in Pisum sativum L. seedlings

BACKGROUND

This study assessed the effects of 24-epibrassinolide (EBL, 10-7M) and silicon (2 mM) on the alleviation of cadmium (Cd, 150 mg L-1) toxicity in Pisum sativum L. seedlings via the modulation of growth, antioxidant defense, glyoxalase system, and nutrient uptake.

RESULTS

Shoot and root lengths declined by 46.43% and 52.78%, respectively, following Cd stress. Shoot and root dry weights also declined with Cd toxicity. Biochemical and physiological aspects exhibit significant decline including total chlorophyll (33.09%), carotenoid (51.51%), photosynthetic efficiency (32.60%), photochemical quenching (19.04%), leaf relative water content (40.18%), and gas exchange parameters (80.65%). However, EBL or Si supplementation alone or in combination modulates the previously mentioned parameters. Cadmium stress increased proline and glycine betaine (GB) contents by 4.37 and 2.41-fold, respectively. Exposure of plants to Cd stress increased the accumulation of H2O2, malondialdehyde content, electrolyte leakage, and methylglyoxal, which declined significantly with EBL and Si supplementation, both individually and in combination. Similarly, Cd stress adversely affected enzymatic and non-enzymatic antioxidants, but EBL and/or Si supplementation maintained antioxidant levels. Glyoxalase I (GlyI) accumulated after Cd stress and increased further with the application of EBL and Si. However, GlyII content declined after Cd stress but increased with supplementation of EBL and Si. Cadmium accumulation occurred in the following order: roots > shoots>leaves. Supplementation with EBL and Si, individually and in combination reduced Cd accumulation and enhanced the uptake of macronutrients and micronutrients in shoots and roots, which declined with Cd toxicity.

CONCLUSION

The application of 24-EBL and Si, individually and in combination, alleviated the adverse effects of Cd by improving growth, biochemical parameters, nutrient uptake, osmolyte accumulation, and the anti-oxidative defense and glyoxalase systems in Pisum sativum seedlings.