Research articleSulfate nutrition improves short-term Al3+-stress tolerance in roots of Lolium perenne L
Introduction
Acid soils, high in soluble Al3+, comprise ~3.95 billion ha of the global ice-free land or ~40% of the world's arable land (Kopittke et al., 2016). Moreover, up to 60% of acidic soils are in underdeveloped countries, where crop production for feeding is critical (Kochian et al., 2015). The presence of trivalent aluminum ions (Al3+) of these soils are a limiting factor for crop production and also decreases the quality of pastures consequently, dairy and meat production (Mora et al., 2006).
Al3+-toxicity has adverse biochemical and physiological effects on plant roots growth (Kochian et al., 2015). The first symptoms of Al3+-toxicity are the injury of root tissues with a subsequent reduction in shoot tissue development. These alterations result in physiological dysfunctions of root nutrient uptake, among them phosphate, calcium (Ca) and magnesium (Mg) (Rengel, 1992; Keltjens, 1995; Wulff-Zottele et al., 2014). Al3+-toxicity is a rapid process that disrupt many cellular functions, such as: cellular division, cell wall organization, damage of root cellular membranes (Rengel, 1992; Tice et al., 1992; Cartes et al., 2012). Although Al3+ does not participates in redox reactions, numerous studies have demonstrated that Al3+-toxicity can elicit reactive oxygen species (ROS) in plant tissues (Yamamoto et al., 2002; Achary et al., 2012). Al3+-toxicity causes dysfunction of mitochondrial respiration in plant cells resulting in an increase of ROS, leading to the disequilibrium of the cellular redox balance of metabolites involved in ROS scavenging mechanisms, the oxidative damage of biological membranes and the up-regulation of antioxidant enzymes related to internal Al-tolerance mechanisms, such as superoxide dismutase (SOD) isoforms (Cartes et al., 2012, Maron et al., 2008, Wulff-Zottele et al., 2014).
Sulfur is an essential macronutrient assimilated from the soil as a sulfate oxyanion by plant roots (Takahashi et al., 2011). Sulfate is used to synthetize different molecules: amino acids (i.e. cysteine and methionine), antioxidants metabolites (glutathione), phytochelatins, vitamins and enzyme cofactors (Takahashi et al., 2011). Soil improvement is achieved by the addition of gypsum (CaSO4) and lime (CaCO3) that decreases soil acidity and reduces Al3+-toxicity, improving uptake of soil nutrients and plants growth (Meriño-Gergichevich et al., 2010). The addition of sulfate in hydroponic cultures has a long-term effect on the reduction of long-term Al3+-toxicity, mainly reducing lipid peroxidation and increasing physiological mechanisms that reduce the effects of long-term toxicity of this metal in L. perenne roots (Wulff-Zottele et al., 2014). However, the interplay between Al3+-toxicity and sulfate nutrition in plants under acidic conditions is not totally understood (Alarcón-Poblete at al. 2018).
The gypsum amendment reduces Al3+-toxicity symptoms in ryegrass, suggesting that AlSO4+ complex is involved in Al3+ detoxification (Kinraide, 1991; Mora et al., 2005). The effects of sulfate addition reducing the long-term Al3+-toxicity on perennial ryegrass has been associated to biochemical and physiological mechanisms related to the scavenging of ROS, reducing lipid peroxidation, increase in the biosynthesis of glutathione and modifications in the expression of proteins implicated in restoring the cellular energy balance (Wulff-Zottele et al., 2014).
Superoxide dismutase (SOD, 1.15.1.1) is the first defense against superoxide anions (O2−) transforming them into hydrogen peroxide, which has been suggested as a main contributor to Al3+-toxicity (Sanchez-Parra et al., 2014). Plants have different SOD isoforms classified according to their prosthetic group (Fe, Cu/Zn, and Mn) and their subcellular localization (Perry et al., 2010). The expression of SOD isoforms is regulated by environmental conditions (i.e. Sanchez- Parra et al., 2015). Short-term Al3+-toxicity exposure induce differential transcription patterns for Cu/Zn-SOD and Fe-SOD isoforms in roots of Al-sensitive perennial ryegrass, compared to an Al-tolerant cultivar of the same specie (Cartes et al., 2012). Furthermore, Wulff-Zottele et al. (2014) detected the inhibition of total SOD activity in roots of an Al-sensitive perennial ryegrass (Lolium perenne L) due to increased sulfate supply, after long-term Al3+-exposure. Moreover, the addition of sulfate in nutrient solutions caused the ROS scavenging enzyme gene repression, such as SOD and ascorbate peroxidase, along with reduction of lipid damage by ROS derived of long-term Al3+-toxicity. Although the long-term effect of sulfate on Al3+-toxicity are well known, the short-term effects are not documented.
Here, we analyzed the interplay of sulfate and Al3+-toxicity in Al-sensitive Lolium perenne (cv Jumbo) and its short-term effects, estimating root nutrient uptake, lipid peroxidation, total SOD activity and the expression levels of Cu/Zn and Fe-SOD genes.
Section snippets
Plant tissue and growth conditions
Seeds of Al-sensitive perennial ryegrass cv Jumbo (Lolium perenne) were surface sterilized in 4% of sodium hypochlorite solution for 10 min and washed 3 times with sterilized distilled water, according to the protocol described by Cartes et al. (2012). A total of 50 seeds were germinated in plastic plates were inserted into 3.5 L plastic pots filled with distilled water and cultivated under controlled climatic conditions (23 °C, 100 μmol of photons m−2 s−1, 60% of relative humidity) in the
Optimization of nutrient solution composition for factorial testing with different sulfate supply conditions and short-term Al3+ toxicity
To evaluate the effects of sulfate fertilization conditions in root tissues of an Al-sensitive perennial ryegrass (cv. Jumbo) subjected to short-term Al3+ toxicity we used a modified TF hydroponic solution previously designed by Wulff-Zottele et al. (2014). Conventional TF nutrient solution contains 120 μM sulfate, in these modified TF solutions the total amount of sulfate was double and triple, respectively (240 μM–360 μM sulfate). Detailed composition of nutrient solutions is explained in
Nutrient solution composition for factorial testing with different sulfate supply conditions and short-term Al3+ toxicity
Studies related to the combined effects of N and S fertilization reported that optimal plant growth for Poaceae species require a N/S ratio of approximately 15/1 of these nutrients, such as perennial ryegrass (Goh and Kee, 1978) and wheat (Howarth et al., 2008; Zhao et al., 1999). In this study, the data obtained through the calculation of N/S ratio allows to conclude that plants grown in 120 μM sulfate suffered a suboptimal supply with N/S ratio of 33.1. In contrast, increased conditions of
Conclusions
In summary, these results demonstrated that sulfate nutrition have a significant effect on the short-term Al3+-toxicity in root homeostasis, linked to physiological, and biochemical response in L. perenne plants (Fig. 8). The decrease in lipid peroxidation suggest that these homeostatic mechanisms are fully established by the first 48 h of Al3+-treatment. The first Al3+ symptoms were amended after 4 h by the reduction of lipid peroxidation, reestablishment of total SOD activity and
Author contributions
LL-D, HV-V, DP-R, ASG, AM-S, DR, JCL and CW-Z, were mostly implicated in the experimental design, logistics, and development of the experimental work. HV-V, ASG, BCJ, PZM and CW-Z, substantially contributed to the conception of the work, data analysis and manuscript revision, approved the final version, and agree to be accountable for the whole work. The Biochemist thesis of DP-R is partially based on this scientific report. The M.Sc. of Biotechnology thesis of LL-D at the Universidad de
Funding
This study was supported by FONDECYT Chile (Regular project N° 1130655).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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