Aluminium toxicity and phosphate deficiency activates antioxidant systems and up-regulates expression of phosphate transporters gene in ryegrass (Lolium perenne L.) plants

Comparative quantitative phosphoproteomic and parallel reaction monitoring analysis of soybean roots under aluminum stress identify candidate phosphoproteins involved in aluminum resistance capacity

Aluminum (Al) toxicity adversely impacts soybean (Glycine max) growth in acidic soil. Reversible protein phosphorylation plays an important role in adapting to adverse environmental conditions by regulating multiple physiological processes including signal transduction, energy coupling and metabolism adjustment in higher plant. This study aimed to reveal the Al-responsive phosphoproteins to understand their putative function and involvement in the regulation of Al resistance in soybean root. We used immobilized metal affinity chromatography to enrich the key phosphoproteins from soybean root apices at 0, 4, or 24 h Al exposure. These phosphoproteins were detected using liquid chromatography-tandem mass spectrometry measurement, verified by parallel reaction monitoring (PRM), and functionally characterized via overexpression in soybean hairy roots. A total of 638 and 686 phosphoproteins were identified as differentially enriched between the 4-h and 0-h, and the 24-h and 0-h Al treatment comparison groups, respectively. Typically, the phosphoproteins involved in biological processes including cell wall modification, and RNA and protein metabolic regulation displayed patterns of decreasing enrichment (clusters 3, 5 and 6), however, the phosphoproteins involved in the transport and metabolic processes of various substrates, and signal transduction pathways showed increased enrichment after 24 h of Al treatment. The enrichment of phosphoproteins in organelle organization bottomed after 4 h of Al treatment (cluster 1). Next, we selected 26 phosphoproteins from the phosphoproteomic profiles, assessed their enrichment status using PRM, and detected enrichment patterns similar to those observed via phosphoproteomic analysis. Among them, 15 phosphoproteins were found to reduce the accumulation of Al and callose in Al-stressed soybean root apices when their corresponding genes were individually overexpressed in soybean hairy roots. In summary, the findings of this study facilitated a comprehensive understanding of the protein phosphorylation events involved in Al resistance responses and revealed some critical phosphoproteins that enhance Al resistance in soybean roots.

Dissecting the Roles of Phosphorus Use Efficiency, Organic Acid Anions, and Aluminum-Responsive Genes under Aluminum Toxicity and Phosphorus Deficiency in Ryegrass Plants

Aluminum (Al) toxicity and phosphorus (P) deficiency are widely recognized as major constraints to agricultural productivity in acidic soils. Under this scenario, the development of ryegrass plants with enhanced P use efficiency and Al resistance is a promising approach by which to maintain pasture production. In this study, we assessed the contribution of growth traits, P efficiency, organic acid anion (OA) exudation, and the expression of Al-responsive genes in improving tolerance to concurrent low-P and Al stress in ryegrass (Lolium perenne L.). Ryegrass plants were hydroponically grown under optimal (0.1 mM) or low-P (0.01 mM) conditions for 21 days, and further supplied with Al (0 and 0.2 mM) for 3 h, 24 h and 7 days. Accordingly, higher Al accumulation in the roots and lower Al translocation to the shoots were found in ryegrass exposed to both stresses. Aluminum toxicity and P limitation did not change the OA exudation pattern exhibited by roots. However, an improvement in the root growth traits and P accumulation was found, suggesting an enhancement in Al tolerance and P efficiency under combined Al and low-P stress. Al-responsive genes were highly upregulated by Al stress and P limitation, and also closely related to P utilization efficiency. Overall, our results provide evidence of the specific strategies used by ryegrass to co-adapt to multiple stresses in acid soils.

Biochemical and Molecular Responses Underlying the Contrasting Phosphorus Use Efficiency in Ryegrass Cultivars

Improving plant ability to acquire and efficiently utilize phosphorus (P) is a promising approach for developing sustainable pasture production. This study aimed to identify ryegrass cultivars with contrasting P use efficiency, and to assess their associated biochemical and molecular responses. Nine ryegrass cultivars were hydroponically grown under optimal (0.1 mM) or P-deficient (0.01 mM) conditions, and P uptake, dry biomass, phosphorus acquisition efficiency (PAE) and phosphorus utilization efficiency (PUE) were evaluated. Accordingly, two cultivars with high PAE but low PUE (Ansa and Stellar), and two cultivars with low PAE and high PUE (24Seven and Extreme) were selected to analyze the activity and gene expression of acid phosphatases (APases), as well as the transcript levels of P transporters. Our results showed that ryegrass cultivars with high PAE were mainly influenced by root-related responses, including the expression of genes codifying for the P transporter LpPHT1;4, purple acid phosphatase LpPAP1 and APase activity. Moreover, the traits that contributed greatly to enhanced PUE were the expression of LpPHT1;1/4 and LpPHO1;2, and the APase activity in shoots. These outcomes could be useful to evaluate and develop cultivars with high P-use efficiency, thus contributing to improve the management of P in grassland systems.

Deciphering Interactions between Phosphorus Status and Toxic Metal Exposure in Plants and Rhizospheres to Improve Crops Reared on Acid Soil

Acid soils are characterized by deficiencies in essential nutrient elements, oftentimes phosphorus (P), along with toxicities of metal elements, such as aluminum (Al), manganese (Mn), and cadmium (Cd), each of which significantly limits crop production. In recent years, impressive progress has been made in revealing mechanisms underlying tolerance to high concentrations of Al, Mn, and Cd. Phosphorus is an essential nutrient element that can alleviate exposure to potentially toxic levels of Al, Mn, and Cd. In this review, recent advances in elucidating the genes responsible for the uptake, translocation, and redistribution of Al, Mn, and Cd in plants are first summarized, as are descriptions of the mechanisms conferring resistance to these toxicities. Then, literature highlights information on interactions of P nutrition with Al, Mn, and Cd toxicities, particularly possible mechanisms driving P alleviation of these toxicities, along with potential applications for crop improvement on acid soils. The roles of plant phosphate (Pi) signaling and associated gene regulatory networks relevant for coping with Al, Mn, and Cd toxicities, are also discussed. To develop varieties adapted to acid soils, future work needs to further decipher involved signaling pathways and key regulatory elements, including roles fulfilled by intracellular Pi signaling. The development of new strategies for remediation of acid soils should integrate the mechanisms of these interactions between limiting factors in acid soils.

Application of a newly recorded diazotrophic cyanobacterium in acidified and Cd contaminated paddy soil: Promotes rice yield and decreases Cd accumulation

Acidification caused by excessive fertilization and heavy metals contamination are two prominent problems of agricultural soils. Diazotrophic cyanobacteria play important role in nitrogen (N) input in agricultural ecosystem. However, the effects of diazotrophic cyanobacteria on the growth of rice and heavy metal uptake by rice grain in acidified and heavy metal contaminated paddy soil remain unknown. In this study, a newly recorded diazotrophic cyanobacterium Aliinostoc sp. YYLX235 was isolated from acidified paddy soil. The results of pot experiment and in situ field plot experiment demonstrated that Aliinostoc sp. YYLX235 could promote rice grain yield and decrease cadmium (Cd) accumulation in rice grain. Nitrogen input by N₂-fixation and increase of bio-available phosphorus (P) by promotion of activity of soil phosphatase may be the main mechanisms for growth-promoting effects of Aliinostoc sp. YYLX235 on rice. Binding and immobilization of Cd through hydroxyl, carboxyl, and amino groups may be the reason for decrease of Cd accumulation in rice grain by Aliinostoc sp. YYLX235 inoculation. The results presented in this study suggest that diazotrophic cyanobacteria have great potential in safe cropping in acidified and Cd contaminated paddy soils.

Research Advances in the Mutual Mechanisms Regulating Response of Plant Roots to Phosphate Deficiency and Aluminum Toxicity

Low phosphate (Pi) availability and high aluminum (Al) toxicity constitute two major plant mineral nutritional stressors that limit plant productivity on acidic soils. Advances toward the identification of genes and signaling networks that are involved in both stresses in model plants such as Arabidopsis thaliana and rice (Oryza sativa), and in other plants as well have revealed that some factors such as organic acids (OAs), cell wall properties, phytohormones, and iron (Fe) homeostasis are interconnected with each other. Moreover, OAs are involved in recruiting of many plant-growth-promoting bacteria that are able to secrete both OAs and phosphatases to increase Pi availability and decrease Al toxicity. In this review paper, we summarize these mutual mechanisms by which plants deal with both Al toxicity and P starvation, with emphasis on OA secretion regulation, plant-growth-promoting bacteria, transcription factors, transporters, hormones, and cell wall-related kinases in the context of root development and root system architecture remodeling that plays a determinant role in improving P use efficiency and Al resistance on acidic soils.

Effects of phosphorus deficiency on the absorption of mineral nutrients, photosynthetic system performance and antioxidant metabolism in Citrus grandis

Phosphorus (P) is an essential macronutrient for plant growth, development and production. However, little is known about the effects of P deficiency on nutrient absorption, photosynthetic apparatus performance and antioxidant metabolism in citrus. Seedlings of 'sour pummelo' (Citrus grandis) were irrigated with a nutrient solution containing 0.2 mM (Control) or 0 mM (P deficiency) KH2PO4 until saturated every other day for 16 weeks. P deficiency significantly decreased the dry weight (DW) of leaves and stems, and increased the root/shoot ratio in C. grandis but did not affect the DW of roots. The decreased DW of leaves and stems might be induced by the decreased chlorophyll (Chl) contents and CO2 assimilation in P deficient seedlings. P deficiency heterogeneously affected the nutrient contents of leaves, stems and roots. The analysis of Chl a fluorescence transients showed that P deficiency impaired electron transport from the donor side of photosystem II (PSII) to the end acceptor side of PSI, which showed a greater impact on the performance of the donor side of PSII than that of the acceptor side of PSII and photosystem I (PSI). P deficiency increased the contents of ascorbate (ASC), H2O2 and malondialdehyde (MDA) as well as the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) in leaves. In contrast, P deficiency increased the ASC content, reduced the glutathione (GSH) content and the activities of SOD, CAT, APX and monodehydroascorbate reductase (MDHAR), but did not increase H2O2 production, anthocyanins and MDA content in roots. Taking these results together, we conclude that P deficiency affects nutrient absorption and lowers photosynthetic performance, leading to ROS production, which might be a crucial cause of the inhibited growth of C. grandis.

Hydrogen sulfide mitigates cadmium induced toxicity in Brassica rapa by modulating physiochemical attributes, osmolyte metabolism and antioxidative machinery

Hydrogen sulfide (H₂S) is helpful for maintaining plant growth under abiotic stresses. The current study elucidated the physiological and biochemical strategies by which sodium hydrosulfide (NaHS), a donor of H₂S, alleviated cadmium (Cd) toxicity in Brassica rapa. B. rapa plants growing under 50 mgkg^-1 Cd stress showed reduced leaf relative water contents (LRWC), photosynthetic pigments, total soluble proteins, minerals uptake, antioxidants and growth. Furthermore, enhanced accumulation of Cd contents caused augmentation in levels of electrolyte leakage (EL) and methylglyoxal (MG). Nevertheless, improved physiochemical parameters in B. rapa seedlings obtained from seeds primed with 1.5 mM NaHS resulted better phenotype, growth and biomass production in Cd stressed plants. Protective stimulus of H₂S regulated minerals and Cd homeostasis besides increased activity of antioxidants which decreased level of reactive oxygen species (ROS), EL, malondialdehyde (MDA) and MG in Cd regimes. Furthermore, H₂S treated seedlings exhibited reduction in Cd content and revealed an active participation in the indole acetic acid (IAA) mediated pathway during stress. The findings of current study propose that H₂S improved stress tolerance and mitigated Cd stress in B. rapa by modulating growth biomarkers and antioxidative system.

Hydrogen sulfide mediated alleviation of cadmium toxicity in Phlox paniculata L. and establishment of a comprehensive evaluation model for corresponding strategy

A laboratory experiment was performed to evaluate the potential role of H₂S on cadmium (Cd) toxicity in Phlox paniculata L. Seeds pretreated with 0.3, 0.6, 0.9, and 1.2 mM NaHS as a donor of H₂S for 24 h and subsequently exposed to 100, 200, and 300 μM CdCl₂ for 26 days had significantly higher germination rate compared with Cd alone. Meanwhile, 2-year-old seedlings sprayed with 0.3, 0.6, and 0.9 μM NaHS were grown in soil with 0.3, 0.6, and 1.2 mg/kg CdCl₂, respectively. We observed that H₂S decreased Cd accumulation in leaves and elevated Cd concentration in roots. Cd toxicity in seedlings resulted in a substantial increase in Cd-induced overproduction of malondialdehyde (MDA), Cd accumulation, and electrolyte leakage. Meanwhile, addition of NaHS increased photosynthetic performance compared with Cd alone. Exogenous H₂S significantly elevated biomass, improved antioxidant enzyme activities, and reduced ABA content compared with Cd alone. H₂S also plays an important role in the ABA signaling pathway during stress. Notably, NaHS promoted Cd uptake by Phlox paniculate L. from soil. The prediction model of H₂S for increasing plant resistance and reducing soil Cd pollution was established by factor analysis method based on comprehensive evaluation of plant stress physiology.

Expression analysis and functional characterization of two PHT1 family phosphate transporters in ryegrass

The phosphate transporters LpPHT1;1 and LpPHT1;4 have different roles in phosphate uptake and translocation in ryegrass under P stress condition. The phosphate transporter 1 (PHT1) family are integral membrane proteins that operate in phosphate uptake, distribution and remobilization within plants. In this study, we report on the functional characterization and expression of two PHT1 family members from ryegrass plants (Lolium perenne L.) and determine their roles in the specificity of Pi transport. The expression level of LpPHT1;4 was strongly influenced by phosphorus (P) status, being higher under P-starvation condition. In contrast, the expression level of LpPHT1;1 was not correlated with P supply. Yeast mutant complementation assays showed that LpPHT1;4 can complement the growth defect of the yeast mutant Δpho84 under Pi-deficient conditions, whereas the yeast mutant expressing LpPHT1;1 was not able to restore growth. Phylogenetic and molecular analyses indicated high sequence similarity to previously identified PHT1s from other species in the Poaceae. These results suggest that LpPHT1;1 may function as a low-affinity Pi transporter, whereas LpPHT1;4 could acts as a high-affinity Pi transporter to maintain Pi homeostasis under stress conditions in ryegrass plants. This study will form the basis for the long-term goal of improving the phosphate use efficiency of ryegrass plants.

Manganese toxicity amelioration by phosphorus supply in contrasting Mn resistant genotypes of ryegrass

We evaluated whether phosphorus (P) ameliorates manganese (Mn) excess harmful effects on photosynthetic performance, growth, oxidative stress, and antioxidants in ryegrass. Two perennial ryegrass genotypes, Banquet-II as Mn-resistant and One-50 as Mn-sensitive genotype, were growth under hydroponic conditions subjected to increased P (25, 50, 100, 200 and 400 μM), excess (750 μM) and sufficient Mn (2.4 μM) for 15 days. Growth rate, lipid peroxidation (LP), enzymatic and non-enzymatic antioxidants, photosynthetic parameters, and pigments were determined. Significant reduction of photosynthesis and growth in One-50 was observed under Mn-excess combined with low and adequate P, recovering under greater P-doses. The P concentration of both genotypes was enhanced towards increased P-supply, regardless of Mn treatments. Shoots Mn-concentration remained constant in both genotypes under Mn-excess, independently of P-levels; meanwhile, Banquet-II roots Mn-concentration increased 23% by P-supply. Furthermore, Banquet-II roots showed higher superoxide dismutase (SOD) activity than One-50, which increased towards the highest P dose under sufficient and excess of Mn. A high dose of phosphorus amendment alleviated Mn-toxicity in Mn-sensitive genotype (One-50). Besides, in the Mn-resistant genotype, enhanced plant performance is highlighted, explained by a high Mn-accumulation in roots and increased SOD activity, decreasing Mn translocation to shoots and therefore protecting the photosynthetic apparatus.

Analysis of gene co-expression networks of phosphate starvation and aluminium toxicity responses in Populus spp

The adaptation of crops to acid soils is needed for the maintenance of food security in a sustainable way, as decreasing fertilizers use and mechanical interventions in the soil would favor the reduction of agricultural practices' environmental impact. Phosphate deficiency and the presence of reactive aluminum affect vital processes to the plant in this soil, mostly water and nutrient absorption. From this, the understanding of the molecular response to these stresses can foster strategies for genetic improvement, so the aim was to broadly analyze the transcriptional variations in Poupulus spp. in response to these abiotic stresses, as a plant model for woody crops. A co-expression network was constructed among 3,180 genes differentially expressed in aluminum-stressed plants with 34,988 connections. Of this total, 344 genes presented two-fold transcriptional variation and the group of genes associated with those regulated after 246 hours of stress had higher number of connections per gene, with some already characterized genes related to this stress as main hubs. Another co-expression network was made up of 8,380 connections between 550 genes regulated by aluminum stress and phosphate deficiency, in which 380 genes had similar profile in both stresses and only eight with transcriptional variation higher than 20%. All the transcriptomic data are presented here with functional enrichment and homology comparisons with already characterized genes in another species that are related to the explored stresses, in order to provide a broad analysis of the co-opted responses for both the stresses as well as some specificity. This approach improves our understanding regarding the plants adaptation to acid soils and may contribute to strategies of crop genetic improvement for this condition that is widely present in regions of high agricultural activity.

Changes in the Distribution of Pectin in Root Border Cells Under Aluminum Stress

Root border cells (RBCs) surround the root apices in most plant species and are involved in the production of root exudates. We tested the relationship between pectin content in root tips and aluminum (Al) tolerance by comparing these parameters in wild-type (WT) and sensitive-to-Al-rhizotoxicity (star1) mutant rice plants. Staining for demethylesterified pectin decreased after Al treatment in the WT. A high level of pectin was observed in RBCs of the root tips. The level of total pectin was increased by about 50% compared with the control. In the Al-sensitive star1 mutant, Al treatment decreased root elongation and pectin content, especially in RBCs. In addition, almost no Al accumulation was observed in the control, whereas more Al was accumulated in the RBCs of STAR1 roots. These results show that the amount of pectin influences Al tolerance; that Al accumulation in rice roots is reduced by the distribution of pectin in root-tip RBCs; and that these reactions occur in the field around the RBCs, including the surrounding mucilage. Al accumulation in rice roots is reduced by the distribution of pectin in root tips, and pectin in the root cell walls contributes to the acquisition of Al tolerance in rice by regulating its distribution. The release of Al-binding mucilage by RBCs could play a role in protecting root tips from Al-induced cellular damage.