سمیت کروم (VI) در حبوبات: اثرات مدولاسیون Rhizobial Symbiosis Chromium(VI) Toxicity in Legume Plants: Modulation Effects of Rhizobial Symbiosis

1. Introduction Heavy metals are widespread environmental pollutants and their excessive levels in agricultural soils cause serious risks not only for normal plant growth and crop yield but also for the human health. Among heavy metals, chromium is a highly toxic metal to living organisms with many adverse effects reported in humans, animals, plants, and microorganisms [1–4]. Chromium belongs to transition metals and it occurs naturally in two predominant valence states: hexavalent chromium (Cr6+) and trivalent chromium (Cr3+). The hexavalent form of the metal, Cr6+, is reported to be more toxic than the relatively less reactive and mobile Cr3+ [1]. Hexavalent Cr compounds (mainly chromates and dichromates) are extensively used in diverse fields of industry leading to environmental pollution [1, 5]. Plants, including legumes, are able to uptake heavy metals like chromium from soils that result in many adverse effects, such as inhibition of seed germination and seedling development, reduction in root and shoot biomass, quality of flowers, and crop yield [6–8]. These effects of heavy metals are connected with inhibition of certain metabolic processes, including biosynthesis of chlorophylls and proteins [4, 9– 11]. As a result, progressive chlorosis, necrosis, and decreased protein content are typical signs of heavy metal toxicity to plants [1, 12–15]. The enhanced production of reactive oxygen species (ROS) is considered to be one of the most important hallmarks of Cr6+ toxicity (Figure 1) [3, 5]. ROS, such as superoxide anion radical (O2 ∙−), hydrogen peroxide (H2O2), and hydroxyl radical (OH∙ ), are highly reactive molecules, which can cause oxidative modification of proteins, lipids, and nucleic acids [3, 12]. In response to heavy metal exposure, plants upregulate various enzymatic or nonenzymatic defense mechanisms that help to support redox balance and prevent/repair oxidative damage under stress conditions (Table 1) [13, 14]. If the capacity of protective systems is not sufficient, modification of biomolecules can be significantly increased, leading to the development of oxidative stress with respective unfavorable effects for plants.