Proteomic Response of Alfalfa Subjected to Aluminum (Al) Stress at Low pH Soil

Proteome insights of citric acid-mediated cadmium toxicity tolerance in Brassica napus L

Cadmium (Cd) is a toxic substance that is uptake by plants from soils, Cd easily transfers into the food chain. Considering global food security, eco-friendly, cost-effective, and metal detoxification strategies are highly demandable for sustainable food crop production. The purpose of this study was to investigate how citric acid (CA) alleviates or tolerates Cd toxicity in Brassica using a proteome approach. In this study, the global proteome level was significantly altered under Cd toxicity with or without CA supplementation in Brassica. A total of 4947 proteins were identified using the gel-free proteome approach. Out of these, 476 proteins showed differential abundance between the treatment groups, wherein 316 were upregulated and 160 were downregulated. The gene ontology analysis reveals that differentially abundant proteins were involved in different biological processes including energy and carbohydrate metabolism, CO2 assimilation and photosynthesis, signal transduction and protein metabolism, antioxidant defense, heavy metal detoxification, plant development, and cytoskeleton and cell wall structure in Brassica leaves. Interestingly, several candidate proteins such as superoxide dismutase (A0A078GZ68) L-ascorbate peroxidase 3 (A0A078HSG4), glutamine synthetase (A0A078HLB2), glutathione S-transferase DHAR1 (A0A078HPN8), glutamine synthetase (A0A078HLB2), cysteine synthase (A0A078GAD3), S-adenosylmethionine synthase 2 (A0A078JDL6), and thiosulfate/3-mercaptopyruvate sulfur transferase 2 (A0A078H905) were involved in antioxidant defense system and sulfur assimilation-involving Cd-detoxification process in Brassica. These findings provide new proteome insights into CA-mediated Cd-toxicity alleviation in Brassica, which might be useful to oilseed crop breeders for enhancing heavy metal tolerance in Brassica using the breeding program, with sustainable and smart Brassica production in a metal-toxic environment.

Regulation of Na+/H+ exchangers, Na+/K+ transporters, and lignin biosynthesis genes, along with lignin accumulation, sodium extrusion, and antioxidant defense, confers salt tolerance in alfalfa

Accumulation of high sodium (Na⁺) leads to disruption of metabolic processes and decline in plant growth and productivity. Therefore, this study was undertaken to clarify how Na⁺/H⁺ exchangers and Na⁺/K⁺ transporter genes contribute to Na⁺ homeostasis and the substantial involvement of lignin biosynthesis genes in salt tolerance in alfalfa (Medicago sativa L.), which is poorly understood. In this study, high Na⁺ exhibited a substantial reduction of morphophysiological indices and induced oxidative stress indicators in Xingjiang Daye (XJD; sensitive genotype), while Zhongmu (ZM; tolerant genotype) remained unaffected. The higher accumulation of Na⁺ and the lower accumulation of K⁺ and K⁺/(Na⁺ + K⁺) ratio were found in roots and shoots of XJD compared with ZM under salt stress. The ZM genotype showed a high expression of SOS1 (salt overly sensitive 1), NHX1 (sodium/hydrogen exchanger 1), and HKT1 (high-affinity potassium transporter 1), which were involved in K⁺ accumulation and excess Na⁺ extrusion from the cells compared with XJD. The lignin accumulation was higher in the salt-adapted ZM genotype than the sensitive XJD genotype. Consequently, several lignin biosynthesis-related genes including 4CL2, CCoAOMT, COMT, CCR, C4H, PAL1, and PRX1 exhibited higher mRNA expression in salt-tolerant ZM compared with XJD. Moreover, antioxidant enzyme (catalase, superoxide dismutase, ascorbate peroxidase, and glutathione reductase) activity was higher in ZM relative to XJD. This result suggests that high antioxidant provided the defense against oxidative damages in ZM, whereas low enzyme activity with high Na⁺ triggered the oxidative damage in XJD. These findings together illustrate the ion exchanger, antiporter, and lignin biosysthetic genes involving mechanistic insights into differential salt tolerance in alfalfa.

Heat Shock Proteins and Antioxidant Genes Involved in Heat Combined with Drought Stress Responses in Perennial Rye Grass

The frequent occurrence of heat and drought stress can severely reduce agricultural production of field crops. In comparison to a single stress, the combination of both heat (H) and drought (D) further reduce plant growth, survival and yield. This study aimed to explore the transcriptional responses of heat shock protein (HSP) and antioxidant genes under H combined D stress in perennial rye grass (PRG). The results demonstrated that oxidative stress indicators (hydrogen peroxide, lipid peroxidation) significantly increased, particularly in the case of combined H and D treatment, suggesting that oxidative stress-induced damage occurred in plants under the combined stresses. Transcriptional responses of heat shock protein 70 (HSP70), heat shock protein 90-6 (HSP90-6), and the mitochondrial small heat shock protein HSP26.2 (HSP26.2) occurred rapidly, and showed high level of expression particularly under H and D stress. Antioxidant genes including ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), catalase (CAT), copper–zinc superoxide dismutase (Cu/ZnSOD), peroxidase (POD), ferredoxin–thioredoxin (FTR), thioredoxin (Trx), 2-cysteine peroxiredoxin (2-Cys Prx) showed response to combined H and D, followed by either D or H stress alone in rye grass. An interactome map revealed the close partnership of these heat shock protein genes and antioxidant genes, respectively. These candidate genes were predominantly linked to stress responses and antioxidant defense in plants. These findings may advance our understanding about the HSP and the antioxidant genes underlying combined abiotic stress response and tolerance in perennial rye grass.

Foliar application of putrescine alleviates terminal drought stress by modulating water status, membrane stability, and yield- related traits in wheat (Triticum aestivum L.)

Drought stress is one of the major limitations to the growth and yield productivity of cereal crops. It severely impairs the early growing and grain -filling stages of wheat. Therefore, cost- effective and eco-friendly approaches for alleviating drought stress in cereal crops are in high demand. Polyamines, such as putrescine, have a significant effect on improving crop yield under drought- stress conditions. Therefore, the current study was executed with the aim of exploring the significance of putrescine in alleviating drought stress and improving yield- related traits in wheat. Two distinct wheat cultivars (Fakhar-e-Bhakkar and Anaj-2017) were treated with the foliar application of different concentrations (control, 0.5, 1.0, and 1.5 PPM) of putrescine (put) under two moisture conditions (well- watered and terminal drought stress). The results demonstrate that the imposition of terminal drought stress significantly reduces different physiological and yield- related traits of both wheat cultivars. The reduction of relative water content (RWC%), membrane stability index (MSI), leaf area, tillers per plant, biomass yield, number of spikelets per spike, 100-grain weight, grain yield per plant, and straw yield was greater in Anaj-2017 than in Fakhar-e-Bhakkar cultivar. The results further explain that the foliar application of increased concentrations of putrescine from 0.0 to 1.0 PPM gradually improved physiological and yield traits, whereas these traits declined with the application of putrescine at the highest dose (1.5 PPM). The exogenous application of 1.0 PPM putrescine improved the relative water content (19.76%), specific leaf area (41.47%), and leaf area ratio (35.84%) compared with the controlled treatment. A higher grain yield (28.0 g plant^-1) and 100-grain weight (3.8 g) were obtained with the foliar application of 1.0 PPM putrescine compared with controlled treatments. The findings of this study confirm the protective role of putrescine against terminal drought stress. It is therefore recommended to use putrescine at a concentration of 1.0 PPM, which could help alleviate terminal drought stress and attain better wheat yield.

Transcriptome Analyses Reveal Candidate Genes Potentially Involved in Al Stress Response in Alfalfa

Alfalfa is the most extensively cultivated forage legume, yet most alfalfa cultivars are not aluminum tolerant, and the molecular mechanisms underlying alfalfa responses to Al stress are largely unknown. In this study, we aimed to understand how alfalfa responds to Al stress by identifying and analyzing Al-stress-responsive genes in alfalfa roots at the whole-genome scale. The transcriptome changes in alfalfa roots under Al stress for 4, 8, or 24 h were analyzed using Illumina high-throughput sequencing platforms. A total of 2464 differentially expressed genes (DEGs) were identified, and most were up-regulated at early (4 h) and/or late (24 h) Al exposure time points rather than at the middle exposure time point (8 h). Metabolic pathway enrichment analysis demonstrated that the DEGs involved in ribosome, protein biosynthesis, and process, the citrate cycle, membrane transport, and hormonal regulation were preferentially enriched and regulated. Biosynthesis inhibition and signal transduction downstream of auxin- and ethylene-mediated signals occur during alfalfa responses to root growth inhibition. The internal Al detoxification mechanisms play important roles in alfalfa roots under Al stress. These findings provide valuable information for identifying and characterizing important components in the Al signaling network in alfalfa and enhance understanding of the molecular mechanisms underlying alfalfa responses to Al stress.

Aluminum Toxicity-Induced Alterations of Leaf Proteome in Two Citrus Species Differing in Aluminum Tolerance

Seedlings of aluminum-tolerant ‘Xuegan’ (Citrus sinensis) and Al-intolerant ‘sour pummelo’ (Citrus grandis) were fertigated for 18 weeks with nutrient solution containing 0 and 1.2 mM AlCl₃·6H₂O. Al toxicity-induced inhibition of photosynthesis and the decrease of total soluble protein only occurred in C. grandis leaves, demonstrating that C. sinensis had higher Al tolerance than C. grandis. Using isobaric tags for relative and absolute quantification (iTRAQ), we obtained more Al toxicity-responsive proteins from C. sinensis than from C. grandis leaves, which might be responsible for the higher Al tolerance of C. sinensis. The following aspects might contribute to the Al tolerance of C. sinensis: (a) better maintenance of photosynthesis and energy balance via inducing photosynthesis and energy-related proteins; (b) less increased requirement for the detoxification of reactive oxygen species and other toxic compounds, such as aldehydes, and great improvement of the total ability of detoxification; and (c) upregulation of low-phosphorus-responsive proteins. Al toxicity-responsive proteins related to RNA regulation, protein metabolism, cellular transport and signal transduction might also play key roles in the higher Al tolerance of C. sinensis. We present the global picture of Al toxicity-induced alterations of protein profiles in citrus leaves, and identify some new Al toxicity-responsive proteins related to various biological processes. Our results provide some novel clues about plant Al tolerance.