Abstract
Silicon (Si) has been well documented to alleviate aluminum (Al) toxicity in vascular plants. However, the mechanisms underlying these responses remain poorly understood. Here, we assessed the effect of Si on the modulation of Si/Al uptake and the antioxidant performance of ryegrass plants hydroponically cultivated with Al (0 and 0.2 mM) in combination with Si (0, 0.5, and 2.0 mM). Exposure to Al significantly increased Al concentration, mainly in the roots, with a consequent reduction in root growth. However, Si applied to the culture media steadily diminished the Al concentration in ryegrass, which was accompanied by an enhancement in root dry matter production. A reduced concentration of Si in plant tissues was also observed when plants were simultaneously supplied with Al and Si. Interestingly, Si transporter genes (Lsi1 and Lsi2) were down-regulated in roots after Si or Al was applied alone; however, both Lsi1 and Lsi2 were up-regulated as a consequence of Si application to Al-treated plants, denoting that there is an increase in Si requirement in order to cope with Al stress in ryegrass. Whereas Al addition triggered lipid peroxidation, Si contributed to an attenuation of Al-induced oxidative stress by increasing phenols concentration and modulating the activities of superoxide dismutase (SOD), catalase, peroxidase, and ascorbate peroxidase antioxidant enzymes. Differential changes in gene expression of SOD isoforms (Mn-SOD, Cu/Zn-SOD, and Fe-SOD) and the profile of peroxide (H2O2) generation were also induced by Si in Al-stressed plants. This, to the best of our knowledge, is the first study to present biochemical and molecular evidence supporting the effect of Si on the alleviation of Al toxicity in ryegrass plants.
Highlights
Aluminum (Al) toxicity represents one of the main yield-limiting factors for crops in acid soils
A negative correlation between Si concentration and Al concentration was observed in shoots (r = 0.927, p ≤ 0.01) and roots (r = 0.935, p ≤ 0.01) of ryegrass grown with Al and Si (Table 3)
The exposure of plants to 0.2 mM Al significantly increased Al accumulation, mainly in the roots (Table 2), with a consequent reduction of approximately 28.5% in root dry matter production (Table 2)
Summary
Aluminum (Al) toxicity represents one of the main yield-limiting factors for crops in acid soils (von Uexküll and Mutert, 1995). Alterations in the structure and/or functions of Silicon-Mediated Alleviation of Aluminum Toxicity in Ryegrass cell wall components (Horst et al, 2010), plasma membrane properties (Yamamoto et al, 2001), nutrient homeostasis (Delhaize and Ryan, 1995; Gupta et al, 2013; Singh et al, 2017), and signal transduction pathways (Matsumoto, 2000; Ma et al, 2002; Sivaguru et al, 2003; Goodwin and Sutter, 2009) can be induced as a consequence of Al binding to numerous cell sites. Molecular approaches have revealed that Al resistance in several plant species is regulated by genes encoding membrane transporter proteins involved in the efflux of organic acid anions, including members of the ALMT (aluminum-activated malate transporters) and MATE (multidrug and toxic compound extrusion) families (Sasaki et al, 2004; Furukawa et al, 2007; Ryan et al, 2011). A bacterialtype ATP binding cassette (ABC) transporter (Huang et al, 2009) and antioxidant defense genes (e.g., Milla et al, 2002; Goodwin and Sutter, 2009; Du et al, 2010; Panda and Matsumoto, 2010) have been implicated in Al tolerance in plants
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