The Role of Hydrogen Sulfide in Plants Exposed to Heavy Metal Stress

Abstract

Hydrogen sulfide (H₂S) with its characteristic odour has been known for centuries as a toxic gas for animals and human, especially at higher concentration. It has been also demonstrated that it can be phytototoxic resulting in disorders of the development but lower amounts of this gas may have beneficial effects on plants exposed to several abiotic stress factors such as salinity, drought, heat or heavy metal stress [1]. It seems that the activation of the antioxidant system triggerred by H₂S is responsible of the tolerance in H₂S-treated plants [2]. In this mini-review, we try to give an overview of the potentially positive and protective roles of H₂S in plants under metallic stress.

Keywords: Hydrogen sulfide; Gasotransmitter; Heavy metal stress; Nitrooxidative stress; Tolerance

Abbreviations

AOA: Aminooxyacetic Acid; APX: Ascorbate Peroxidase; AsA: Ascorbic Acid; CAS: Cyanoalanine Synthase; CAT: Catalase; Chl: Chlorophyll; CO: Carbon Monoxide; CS: Cysteine Synthase; Cys: Cysteine; D-DES: D-Cysteine Desulfhydrase; DHAR: Dehydroascorbate Reductase; GPX: Glutathione Peroxidase; H2O2: Hydrogen Peroxide; L-DES: L-Cysteine Desulfhydrase; DES: Cysteine Desulfhydrase; GR: Glutathione Reductase; GSH: Glutathione (reduced); GST: Glutathione-S-Transferase; GYY4137: Morpholin-4-ium 4-methoxyphenyl (morpholino) Phosphinodithioate; H₂S: Hydrogen Sulfide; HT: Hypotaurine; MDA: Malondialdehyde; MDHAR: Monodehydroascorbate Reductase; MT: Metallothionein; NaHS: Sodium hydrosulfide; NO: Nitric Oxide; PAG: Propargylglicine; PCs: Phytochelatins; POD: Peroxidase; Pro: Proline; ROS: Reactive Oxygen Species; RNS: Reactive Nitrogen Species; RSS: Reactive Sulphur Species; SOD: Superoxide dismutase; SNP: Sodium nitroprusside.

Introduction

Hydrogen sulfide (H₂S), a colourless gas with a small molecular size and a typical odour reminding to rotten egg, has been known to be toxic for animal and human at higher levels. At the same time, it was discovered that the presence of this gas has effects on plants’ physiological processes, such as photosynthetic activity, development or stress tolerance [3,4]. Lower concentrations of exogenous H₂S, mainly derived from sodium hydrosulfide (NaHS) as donor, might have beneficial influences on plant growth and development [1], while excess H₂S can be deleterious, as it was demonstrated in Arabidopsis treated with 200-800 μM NaHS [5].

It is well-known that certain thermal fountains and the typically sulphur emitting vulcanic vents (solfataras) are natural sources of hydrogen sulfide but it can be produced swamps or sewers due to sulfate- or sulfur-reducing microorganisms [6]. Besides, several industrial technologies e.g. in oil refineries produce this gas as a byproduct causing serious toxicity problems [7].

H₂S is the third gasotransmitter which was discovered in the 2000s and similar to Nitric Oxide (NO) and Carbon Monoxide (CO) it was firstly detected in animal and human [8]. Several studies confirmed that H₂S arises endogenously mainly in cerebral tissues and has numerous functions e.g. in neuromodulation, cardiovascular or neurodegenarative diseases [3,4]. Up to now many researchers exhibited that this gaseous molecule also has a remarkable cue in plant physiological processes such as seed germination, morphogenesis of root, photosynthesis, senescence or stomatal closure [9,10]. Moreover, it might have a key role in plant tolerance against nitrooxidative stress trigerred by biotic e.g. pathogens, [11] or abiotic factors like drought, excess salinity or Heavy Metals (HMs) [12,13]. Since the emission and circulation of HMs in food chain are still great challenges globally, it is useful to overview the results about the changes of H₂S level in plants exposed to HM stress and the potentially beneficial exogenous application of H₂S, as well.

The Role of H₂S in Plants at Physiological Conditions

In plants, cysteine Desulfhydrases (DES) are considered to be the key enzymes of H₂S synthesis catalyzing the production of H₂S from L- or D- Cysteine (L-Cys and D- Cys) [1]. L-Cys Desulfhydrases (L-DES, EC 4.4.1.28.) use L-Cys as substrate creating H₂S, pyruvate and ammonia and are mainly located in the cytosol, nucleus and mitochondria, whilst D-Cys Desulfhydrases (D-DES, EC 4.4.1.15.) degrade D-Cys and can be found in mitochondria [2,4]. On the contrary, earlier summary by Guo et al. [3] reported that the main location of D-DES enzymes is the cytoplasm and L-DESs are localized in the chloroplasts and mitochondria. The activity of L-DES enzymes have been already characterized in tobacco, pumpkin, cucumber, Arabidopsis and Brassica napus [14,3]. Cytosolic D-DES activity was exhibited in pumpkin, cucumber, spinach, tobacco and Arabidopsis [3,4]. Another difference between L- and D-DES enzymes is that the activity of L-DESs can be inhibited using Aminooxyacetic Acid (AOA) while it has no influence on D-DESs [3]. Not only DES enzymes ara capable of producing H2S in plant cells, but chloroplastic sulfite reductase (SiR, EC 1.8.7.1), cyanoalanine synthase (CAS, EC 4.4.1.9) localized in cytoplasm and mitochondria and cytosolic Cys synthase (CS, EC 2.5.1.47) also produce hydrogen sulfide [2,4]. Recently, Corpas et al. [15] demonstrated the presence of H₂S in plant peroxisomes, too.

Like the other gasotransmitters NO or CO, hydrogen sulfide has proved to be an essential participant of several physiological processes (e.g. germination, lateral root formation, stomatal closure) at „normal circumstances”. Li et al. [16] demonstrated that if Jatropha curcas seeds were pre-treated with Aminooxyacetic acid (AOA), an inhibitor of H₂S biosynthesis, endogenous H₂S content decreased and germination percentage was significantly lower than Control. The essential role of H₂S in lateral root emergence was also supported by a study [17] which showed an interaction among H₂S, NO and Ca²⁺ ions. Using the inhibitor of H₂S synthesis, Propargylglicine (PAG) and the scavanger of H₂S, Hypotaurine (HT) the number of lateral root primordia significantly reduced. It was also detected that the application of Sodium Nitroprusside (SNP) as NO donor enhanced the generation of endogenous H₂S, especially in lateral root primordia.

Some reports have already presented the signaling function of H₂S or the cooperation between H₂S and NO in regulating the stomatal apertures [18,19,9]. Nonetheless, H_2>S also displayed to be important in delayed flower opening [20] and postharvest senescence of fruits through mediating the level of ROS and activating antioxidant enzymes [21,6].

Besides, supra-optimal levels of H₂S, mainly due to excess application of H₂S donors, might be toxic for plants. The typical symptoms of H₂S toxicity are lesions in leaves, retarded growth of plants (both shoot and root), decrement of chlorophyll content and consequently reduced photosynthetic efficiency [6,5]. Zhang et al. presented that toxic level of endogenous H₂S, deriving from high NaHS concentration (200μM‹) may have inhibitory effect on primary root development via ROS overproduction.

Recent studies have confirmed the probable interaction among H₂S and Reactive Oxygen Species (ROS), Reactive Nitrogen Species (RNS) [9,22] resulting in modulation of nitro-oxidative status of the plant cells. H₂S itself is also regarded to be a member of Reactive Sulphur Species (RSS) [22] and beside Glutathione (GSH) and NO has an important role in posttranslational modification (namely persulfidation) of numerous proteins, as it was announced in case of Arabidopsis [23].

The Role of H₂S in Plants under HM Stress

Many researchers ascertained that the increased emission of HMs to environment originating from different sources like geological or anthropogenic activities (e.g. mining, industrial, excess application of fertilizers and pesticides or contaminated irrigation water) might have several negative effects on plants. The main symptoms of HM stress and/or toxicity in plants are generally disturbances in photosynthesis, reduced germination rate, abnormal development of vegetative parts (root and/or shoot) [24]. It is well-known that not only the toxic metals like Cd, Pb, Hg or As can cause nitro-oxidative stress in the plant cells but the essential HMs (e.g. Fe, Ni, Zn, or Cu) at supra-optimal concentration also cause ROS/RNS overproduction and consequently affect the activity of the antioxidant defense system [25,26].

There are some results demonstrating that HM stress irrespectively of being essential or non-essential can alter positively or negatively H₂S production in the plant organ investigated [27,28].

Beneficial Effects of Exogenous H₂S Treatment under HM Stress

Numerous studies have supported that the application of H₂S donors, like the most frequently used sodium hydrosulfide (NaHS) or the slow-releasing GYY4137 (morpholin-4-ium 4-methoxyphenyl(morpholino) phosphinodithioate) at a certain concentration may have significantly positive effects on seed germination [16,29, 14,30,31], lateral root formation [17], stomatal closure [18], as well as stress tolerance against salinity [32,33,12], heavy metals [34,35], drought [12,36], heat [37,38] and chilling [39,12] (Figure 1).

    

    

    Figure 1: The effects of exogenous H2S in HM stressed plants.