Evaluation of heavy metal-induced responses in Silene vulgaris ecotypes

Abstract

Silene vulgaris is a pseudometallophyte that spontaneously occurs in various ecological niches. Therefore, three ecotypes of this species representing calamine (CAL), serpentine (SER), and non-metallicolous (NM) populations were investigated in this study. Owing to the presence of Pb or Ni ions in natural habitats from metallicolous populations originated, we used these metals as model stressors to determine the survival strategy of tested ecotypes and analyze metal distribution at various levels of organism organization. We focused on growth tolerance, non-enzymatic antioxidants, and photosynthetic apparatus efficiency as well as anatomical and ultrastructural changes occurred in contrasting ecotypes exposed in vitro to excess amounts of Pb 2+ and Ni 2+ . Although Ni application contributed to shoot culture death, the study revealed that the mechanisms of Pb detoxification differed between ecotypes. The unspecific reaction of both metallicolous specimens relied on the formation of effective mechanical barrier against toxic ion penetration, while the Pb appearance in the protoplasts led to the activation of ecotype-specific intracellular defense mechanisms. Hence, the response of CAL and SER ecotypes was almost unchanged under Pb treatment, whereas the reaction of NM one resulted in growth disturbances and physiological alternations. Moreover, both metallicolous ecotypes exhibited increase generation of reactive oxygen species (ROS) in leaves, even before the harmful ions got into these parts of plants. It may implicate the potential role of ROS in CAL and SER adaptation to heavy metals and, for the first time, indicate on integral function of ROS as signaling molecules in metal-tolerant species.

Ecotype-Specific Pathways of Reactive Oxygen Species Deactivation in Facultative Metallophyte Silene vulgaris (Moench) Garcke Treated with Heavy Metals

“…Plant material constituted the in vitro shoot culture of three ecotypes of Silene vulgaris (Moench) Garcke (Caryophyllaceae) that were propagated on the optimized proliferation medium according to the protocol proposed by Muszyńska et al [22]. Two HM-tolerant cultures were obtained earlier from seeds of specimens representing calamine (described further as CAL) and serpentine (SER) populations, while control non-metallicolous (NM) culture originated from seeds collected from an unpolluted stand [9,22]. In the experimental period, apical fragments of shoots (explants) were placed on the proliferation medium enriched simultaneously with zinc, lead, and cadmium salts.…”

“…To sum up, the ROS scavenging pathways in S. vulgaris relied on antioxidants specific to the respective ecotypes, however the synthesis of polyphenols was proved to be a universal reaction to HMs.Antioxidants 2020, 9, 102 2 of 30 become toxic if their concentration exceeds the genotype-dependent thresholds. What is more, all metals are non-biodegradable, and therefore they persist inside the protoplast, where they may cause cellular damage and interfere with various physiological processes [7][8][9].The primary effect of HM exposition is the overproduction of reactive oxygen species (ROS). Although ROS play an integral role in the regulation of plant metabolism, their enhanced formation induces oxidative damage, and leads to the oxidation of lipids, proteins, or nucleic acids and the depletion of enzymatic activities [10][11][12][13].…”

“…All these compounds possess the ability to inactivate ROS in a complex way by their decomposition or transformation into less harmful molecules [15]. What is more, phenolic compounds show an additional tendency to chelate metallic ions through hydroxyl and carboxyl groups, as well as participate in pathways of cell wall lignification, causing the formation of a thicker mechanical barrier that restricts the penetration of HMs responsible for oxidative damage [7,9,16]. Therefore, modulation of the particular antioxidants production and accumulation depending on the stressor presence can be used to anticipate the direction of metabolism remodeling under HM treatment.The toxicity of HMs applied separately has been intensively studied in taxonomically diversified organisms [8,17,18].…”

“…Our recent study showed that contrasting ecotypes of this species from two metal-tolerant populations growing on waste heaps deposited after the mining and processing of calamine and serpentine rocks differed significantly in anatomical, physiological, and genetic features, not only from reference specimens occurring in unpolluted areas, but to some extent also to each other [22]. What is more, under fully controlled in vitro conditions, we have ascertained the survival strategy resulting from the ecotype-dependent activation of antioxidant machinery in these three ecotypes treated separately with Pb or Ni ions, i.e., metals which are characteristic for natural habitats of metallicolous populations [9]. Taken together, these experiments revealed that ROS may play an essential function as signaling molecules in response of metallicolous specimens to HMs.…”

“…Antioxidants 2020, 9, 102 2 of 30 become toxic if their concentration exceeds the genotype-dependent thresholds. What is more, all metals are non-biodegradable, and therefore they persist inside the protoplast, where they may cause cellular damage and interfere with various physiological processes [7][8][9].…”