Abstract
Selenium (Se) shows antioxidant properties that can be exploited in plants to combat abiotic stresses caused by reactive oxygen species produced in excess (ROS). Here, we show that the Se-fertilization of olive trees with sodium selenate effectively protects the pollen from oxidative stress. Pollen isolated from plants treated with Se or from untreated controls was incubated in vitro with H2O2 to produce an oxidative challenge. Given the impact of ROS on Ca2+ homeostasis and Ca2+-dependent signaling, cytosolic Ca2+ was measured to monitor cellular perturbations. We found that H2O2 interrupted Ca2+ homeostasis only in untreated pollen, while in samples treated in vitro with sodium selenate or selenium methionine, Ca2+ homeostasis was preserved. Furthermore, germination rates were considerably better maintained in Se-fertilized pollen compared to non-fertilized pollen (30% vs. 15%, respectively) after exposure to 1 mM H2O2. The same was observed with pollen treated in vitro with Se-methionine, which is the organic form of Se, in which part of the fertigated sodium selenate is converted in the plant. Combined, our results show a close correlation between ROS, Ca2+ homeostasis, and pollen fertility and provide clear evidence that Se-fertilization is a potential approach to preserve or improve agricultural productivity.
Highlights
Selenium (Se), a microelement, has been used for many years in disease prevention (Boosalis, 2008; Rayman, 2008; Rees et al, 2013; D’Amato et al, 2014)
The SeO42- distributed with the treatment was absorbed by olive trees and organicated for ≈ 50% as Se-methionine (Se-Met) in the pollen grains (Table 1)
We found that H2O2-induced oxidative stress caused a significant alteration of Ca2+ homeostasis, evidenced by an increase in cytosolic Ca2+ and a marked enhanced Ca2+ entry when exogenous 1 mM CaCl2 was added to the pollen suspension (Figure 1)
Summary
Selenium (Se), a microelement, has been used for many years in disease prevention (Boosalis, 2008; Rayman, 2008; Rees et al, 2013; D’Amato et al, 2014). ROS are normally produced by cells at low concentrations and participate in membrane signals and cell events, such as sexual plant reproduction (recognition between pollen and stigma), their accumulation leads to oxidative stress due to the loss of cell scavenging capacity (Hancock et al, 2001; Laloi et al, 2004; Kwak et al, 2006). When ROS are produced at high concentrations, as under biotic stress, they become toxic by altering the molecular signals of the cell, including cytosolic Ca2+. A rich literature describes the correlation between Ca2+ and ROS, but few studies have investigated the effects of selenium on cytosolic Ca2+ and pollen germination (Steinhorst and Kudla, 2013; Görlach et al, 2015). The ROS behave like agonists, stimulating the mobilization of the ion from the "internal stores" and activating the entry of Ca2+ from the extracellular medium (Ca2+-entry) (Carafoli, 1987; Yan et al, 2006; Clapham, 2007; Brini et al, 2013; Görlach et al, 2015; Orrenius et al, 2015)
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