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Trifolium pratense L. (red clover) is an important leguminous crop with great potential for Ag-contaminated environment remediation. Whereas, the molecular mechanisms of Ag tolerance in red clover are largely unknown. Red clover seedlings were used for physiological and transcriptomic investigation under 0, 20, 50, and 100 mg/L Ag+ stress in our research to reveal potential molecular resistance mechanism. Research showed that red clover possessed fairly strong Ag absorbance capacity, the Ag level reached 0.14 and 2.35 mg/g·FW in the leaves and roots under 100 mg/L AgNO3 stress condition. Root fresh weight, root dry weight, root water content, and photosynthetic pigments contents were significantly decreased with elevating AgNO3 concentration. Obvious withered plant tissue, microstructure disorder, and disrupted organelles were observed. In vitro evaluations (e.g., PI and DCFH-DA staining) represented that AgNO3 at high concentration (100 mg/L) exhibited obvious inhibition on cell viability, which was due possibly to the induction of reactive oxygen species (ROS) accumulation. A total of 44643 differentially expressed genes (DEGs) were identified under Ag stress, covering 27155 upregulated and 17488 downregulated genes. 12 stress-responsive DEGs was authenticated utilizing real-time quantitative PCR (qRT-PCR). Gene ontology (GO) analysis revealed that the DEGs were mostly related to metal ion binding (molecular function), nucleus (cellular component), and defense response (biological process). Involved DEGs in sequence-specific DNA binding transcription factor activity, response to various hormones (e.g., abscisic acid, IAA/Auxin, salicylic acid, and etc), calcium signal transduction, and protein ubiquitination were concluded to play crucial roles in Ag tolerance of red clover. On the other hand, Kyoto Encyclopedia of Genes and Genomes (KEGG) database annotated several stress responsive pathways such as plant-pathogen interaction, phenylpropanoid biosynthesis, ubiquitin mediated proteolysis, hormone signal transduction, and autophagy. Several down-regulated genes (e.g., RSF2, RCD1, DOX1, and etc) were identified indicating possible metabolic disturbance. Besides, protein-protein interaction network (PPI) identified several pivotal genes such as ribosomal proteins, TIR, and ZAT.

This present study investigated detoxification mechanisms of leguminous forage Trifolium pratense L. (red clover) seedlings upon exposure to Ag ions (Ag+) on an atomic level. Depressed plant growth (maximum inhibition rate: 46.57%) and significantly altered antioxidase/antioxidant substances levels (maximum inhibition rate: 65.45%/55.41%) revealed that the physiological metabolism was disturbed. Notable lesions were observed in both leaf and root cells at 588 μM Ag+ treatment. All differentially expressed genes (DEGs) were remarkably mapped to biological metabolism related pathways. Red clover seedlings were speculated to initially transform and immobilize Ag+ in the culture medium, then transporting and fixing them inside the cell, mainly as unreduced Ag+ bound to oxygen-, nitrogen-, sulfur-, chloride-containing biological molecules. A portion of Ag+ was reduced to Ag0 and aggregated to form crystalline argentiferous nanoparticles. Effective reducing agents such as alcohols, carboxylic acid, and etc, which are capable of coordinating heavy metals to reduce and stabilize them, were assumed to play a role in Ag+ reduction. The research results are of great value to understand the defense and tolerance mechanisms of red clover to Ag+ and explore the main existing forms of Ag+ in vivo and in vitro, which could indicate contamination condition in regional ecological environment such as mining area and its potential effects.

The toxicity of silver nanoparticles (AgNPs) with single morphology exposed to aquatic organisms had been well revealed in the past decade, but few studies have been carried out to evaluate the toxicity differences between AgNPs with various microstructure morphologies, especially to algae. In this work, Chlorella vulgaris was used as the tested organism to illustrate the differences of toxic effects between silver nanospheres (AgNSs), silver nanocubes (AgNCs) and silver nanoplates (AgPLs) with concentration of 0.5, 1.0, 2.0, 5.0 mg⸳L-1, based on the algae’s growth (72h), chlorophyll-a content, antioxidant enzyme activity, lipid peroxidation and cell apoptosis (48h). The results showed that the toxicity level exposed to Chlorella vulgaris was in the order of AgPLs> AgNCs> AgNSs. The difference shown indicated that the potential toxicity of AgNPs is primarily depended on their microstructure morphologies. This current study initially revealed the structure-effects of AgNPs on Chlorella vulgaris, provided a scientific basis for aquatic environmental risk assessment.

Numerous studies have shown that there are complex interactions between silver nanoparticles (AgNPs) and humic acid (HA) in the natural ecosystem. However, little is known about the interaction between AgNPs and HA in terms of molecular mechanisms. It is well-accepted that aromatics substituted by phenolic hydroxyl (Ar−OH) and carboxyl (−COOH) groups are primary constituents of HA. In this research, we proved that five benzene series (phenol, BA, OHBA, MHBA and PHBA) have different adsorption capacity sequences with AgNPs by experimental adsorption coefficient in UV-vis. Subsequently, FTIR analysis indicated the weak interaction in molecules is the main adsorption mechanism. Based on these facts, the weak interaction of Ar−OH and −COOH with silver nanoparticles (AgNPs) was systematically investigated by the first-principles calculations using density functional theory (DFT). The structure optimization and energy calculation demonstrated that the Ar−OH and −COOH groups on the benzene ring had the greatest influence on the AgNPs adsorption site, energy and deformation when they are in adjacent positions. Furthermore, the calculations of electronic structure, Density of states (DOS) and crystal orbital Hamilton population (COHP) provided evidence that the charges in C and H atoms were transferred to O atoms after the adsorption. Whereas, there was little charge transfer from AgNPs to benzene series. This study is hoped to provide a useful theoretical reference for the evaluation of HA and AgNPs in the water environment.