Exogenous silicon induces aluminum tolerance in white clover (Trifolium repens) by reducing aluminum uptake and enhancing organic acid secretion.

Excessive aluminum (Al) in acidic soils is a primary factor that hinders plant growth. The objective of the present study was to investigate the effect and physiological mechanism of exogenous silicon (Si) in alleviating aluminum toxicity. Under hydroponic conditions, 4 mM Al significantly impeded the growth of white clover; however, pretreatments with 1 mM Si mitigated this inhibition, as evidenced by notable changes in growth indicators and physiological parameters. Exogenous silicon notably increased both shoot and root length of white clover and significantly decreased electrolyte leakage (EL) and malondialdehyde (MDA) content compared to aluminum treatments. This positive effect was particularly evident in the roots. Further analysis involving hematoxylin staining, scanning electron microscopy (SEM), and examination of organic acids (OAs) demonstrated that silicon relieved the accumulation of bioactive aluminum and ameliorated damage to root tissues in aluminum-stressed plants. Additionally, energy-dispersive X-ray (EDX) analysis revealed that additional silicon was primarily distributed in the root epidermal and cortical layers, effectively reducing the transport of aluminum and maintaining the balance of exchangeable cations absorption. These findings suggest that gradual silicon deposition in root tissues effectively prevents the absorption of biologically active aluminum, thereby reducing the risk of mineral nutrient deficiencies induced by aluminum stress, promoting organic acids exudation, and compartmentalizing aluminum in the outer layer of root tissues. This mechanism helps white clover alleviate the damage caused by aluminum toxicity.

γ-Aminobutyric acid (GABA) priming alleviates acid-aluminum toxicity to roots of creeping bentgrass via enhancements in antioxidant defense and organic metabolites remodeling.

MAIN CONCLUSION: γ-Aminobutyric acid alleviates acid-aluminum toxicity to roots associated with enhanced antioxidant metabolism as well as accumulation and transportation of citric and malic acids. Aluminum (Al) toxicity has become the main limiting factor for crop growth and development in acidic soils and is further being aggravated worldwide due to continuous industrial pollution. The current study was designed to examine effects of GABA priming on alleviating acid-Al toxicity in terms of root growth, antioxidant defense, citrate and malate metabolisms, and extensive metabolites remodeling in roots under acidic conditions. Thirty-seven-day-old creeping bentgrass (Agrostis stolonifera) plants were used as test materials. Roots priming with or without 0.5 mM GABA for 3 days were cultivated in standard nutrient solution for 15 days as control or subjected to nutrient solution containing 5 mM AlCl3·6H2O for 15 days as acid-Al stress treatment. Roots were sampled for determinations of root characteristics, physiological and biochemical parameters, and metabolomics. GABA priming significantly alleviated acid-Al-induced root growth inhibition and oxidative damage, despite it promoted the accumulation of Al in roots. Analysis of metabolomics showed that GABA priming significantly increased accumulations of organic acids, amino acids, carbohydrates, and other metabolites in roots under acid-Al stress. In addition, GABA priming also significantly up-regulated key genes related to accumulation and transportation of malic and citric acids in roots under acid-Al stress. GABA-regulated metabolites participated in tricarboxylic acid cycle, GABA shunt, antioxidant defense system, and lipid metabolism, which played positive roles in reactive oxygen species scavenging, energy conversion, osmotic adjustment, and Al ion chelation in roots.

γ-Aminobutyric Acid Priming Alleviates Acid-Aluminum Toxicity to Creeping Bentgrass by Regulating Metabolic Homeostasis.

Aluminum (Al) toxicity is a major limiting factor for plant growth and crop production in acidic soils. This study aims to investigate the effects of γ-aminobutyric acid (GABA) priming on mitigating acid-Al toxicity to creeping bentgrass (Agrostis stolonifera) associated with changes in plant growth, photosynthetic parameters, antioxidant defense, key metabolites, and genes related to organic acids metabolism. Thirty-seven-old plants were primed with or without 0.5 mM GABA for three days and then subjected to acid-Al stress (5 mmol/L AlCl3·6H2O, pH 4.35) for fifteen days. The results showed that acid-Al stress significantly increased the accumulation of Al and also restricted aboveground and underground growths, photosynthesis, photochemical efficiency, and osmotic balance, which could be effectively alleviated by GABA priming. The application of GABA significantly activated antioxidant enzymes, including superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase, to reduce oxidative damage to cells under acid-Al stress. Metabolomics analysis demonstrated that the GABA pretreatment significantly induced the accumulation of many metabolites such as quinic acid, pyruvic acid, shikimic acid, glycine, threonine, erythrose, glucose-6-phosphate, galactose, kestose, threitol, ribitol, glycerol, putrescine, galactinol, and myo-inositol associated with osmotic, antioxidant, and metabolic homeostases under acid-Al stress. In addition, the GABA priming significantly up-regulated genes related to the transportation of malic acid and citric acid in leaves in response to acid-Al stress. Current findings indicated GABA-induced tolerance to acid-Al stress in relation to scavenging of reactive oxygen species, osmotic adjustment, and accumulation and transport of organic metabolites in leaves. Exogenous GABA priming could improve the phytoremediation potential of perennial creeping bentgrass for the restoration of Al-contaminated soils.

A Trifolium repens flavodoxin-like quinone reductase 1 (TrFQR1) improves plant adaptability to high temperature associated with oxidative homeostasis and lipids remodeling.

Maintenance of stable mitochondrial respiratory chains could enhance adaptability to high temperature, but the potential mechanism was not elucidated clearly in plants. In this study, we identified and isolated a TrFQR1 gene encoding the flavodoxin-like quinone reductase 1 (TrFQR1) located in mitochondria of leguminous white clover (Trifolium repens). Phylogenetic analysis indicated that amino acid sequences of FQR1 in various plant species showed a high degree of similarities. Ectopic expression of TrFQR1 protected yeast (Saccharomyces cerevisiae) from heat damage and toxic levels of benzoquinone, phenanthraquinone and hydroquinone. Transgenic Arabidopsis thaliana and white clover overexpressing TrFQR1 exhibited significantly lower oxidative damage and better photosynthetic capacity and growth than wild-type in response to high-temperature stress, whereas AtFQR1-RNAi A. thaliana showed more severe oxidative damage and growth retardation under heat stress. TrFQR1-transgenic white clover also maintained better respiratory electron transport chain than wild-type plants, as manifested by significantly higher mitochondrial complex II and III activities, alternative oxidase activity, NAD(P)H content, and coenzyme Q10 content in response to heat stress. In addition, overexpression of TrFQR1 enhanced the accumulation of lipids including phosphatidylglycerol, monogalactosyl diacylglycerol, sulfoquinovosyl diacylglycerol and cardiolipin as important compositions of bilayers involved in dynamic membrane assembly in mitochondria or chloroplasts positively associated with heat tolerance. TrFQR1-transgenic white clover also exhibited higher lipids saturation level and phosphatidylcholine:phosphatidylethanolamine ratio, which could be beneficial to membrane stability and integrity during a prolonged period of heat stress. The current study proves that TrFQR1 is essential for heat tolerance associated with mitochondrial respiratory chain, cellular reactive oxygen species homeostasis, and lipids remodeling in plants. TrFQR1 could be selected as a key candidate marker gene to screen heat-tolerant genotypes or develop heat-tolerant crops via molecular-based breeding.

The enhanced effect of activated sludge attached to the roots of Pistia stratiotes on nutrient removal for secondary effluent.

Aquatic plants are widely used for treating wastewater treatment plant secondary effluent. During this process, some residual activated sludge in the secondary effluent is intercepted and attaches to the plant roots. However, the effect of the attached activated sludge on nutrient removal in secondary effluent has up to now been unknown. Aiming at this problem, this investigation was conducted to compare the nutrient removal rates in secondary effluent by washed Pistia stratiotes (washed batch) and Pistia stratiotes with activated sludge attached to the roots (study batch). Extracellular polymeric substances (EPS) from the activated sludge attached to the roots were extracted and characterized by three-dimensional excitation emission matrix (3D-EEM) fluorescence spectroscopy. The results showed that the nutrient removal rates in the study batch were better than that in the washed batch. The 3D-EEM results showed that the protein content of EPS increased during the experiment, indicating the growth of microorganisms in the attached activated sludge. Our work demonstrated the enhanced effect of activated sludge attached to the roots of Pistia stratiotes on the removal of pollutants in secondary effluent, which is useful to guide the practical engineering of secondary effluent treatment.