Exploration of defense and tolerance mechanisms in dominant species of mining area - Trifolium pratense L. upon exposure to silver

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.