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Chemical Biology of Reactive Sulfur Species in Plants: History
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Contributor: Krishna Kumar Rai , Prashant Kaushik

Abiotic and biotic stresses negatively affect plant cellular and biological processes, limiting their growth and productivity. Plants respond to these environmental cues and biotrophic attackers by activating intricate metabolic-molecular signaling networks precisely and coordinately. One of the initial signaling networks activated is involved in the generation of reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS). RSS has been proclaimed to be inexorably interlinked with all life forms from its inception to the present day.

  • ROS
  • RNS
  • RSS

1. Introduction

In the 21st century agriculture and various climatic stresses, such as high temperature, drought, and salinity, have redundantly affected crop growth and productivity, prompting severe threats to global food security for ever-growing global populations [1]. In the Asian continent, a rainfed agriculture system is usually standard and is followed by most farmers. These climatic stresses have become a daunting challenge that has imposed severe repercussions on crop health, thereby ultimately affecting its productivity to a certain extent and leading to livestock death [2]. In addition to sessile, plants are constantly exposed to these climate extremes that instigate various cellular, physiological, biochemical, and molecular responses. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) act as integral components of signal transduction processes regulating vital functions in plants exposed to climate extremes [3]. Initially, ROS and RNS are considered toxic molecules where elevated levels provoke oxidative stress in plants leading to cellular damage and death [4]. However, several recent reports have highlighted that ROS and RNS also function as signaling molecules (when their generation is critically maintained below a threshold level by antioxidative systems), catalyzing several oxidation reactions, thereby modulating vital signaling cascades [5].
Redox chemistry is inextricably engaged in the generation, regulation, and sustainment of life on earth by exhilarating reduction–oxidation (redox) reactions essential for driving crucial cellular and metabolic processes such as photosynthesis, respiration, and other biochemical reactions in diverse life forms [6]. Interestingly, recent reports have restricted the involvement of ROS and RNS in stimulating thermodynamically favorable reactions that are essential for sustaining life, including (i) their ability to enhance metabolic reactions, (ii) regulate enzymatic and non-enzymatic antioxidants, (iii) cross barriers (membranes) to activate signaling cascades, and (iv) provide a source of energy (electrons) to defend against oxidative stress [7]. In the recent decade, a plethora of research has been conducted to assess the positive side of ROS/RNS signaling in plants’ growth, development, and defense response. A concomitantly large body of literature has pinpointed their exemplary role [8].
Reactive sulfur species (RSS) is a term still to be entered into the general scientific vocabulary due to its low expression and lack of consideration of its role in signal transduction [9][10]. Early prebiotic life forms, i.e., before photosynthesis, likely thrived under a sulfur-rich environment, and several reports claimed that when life originated, approx. 3.8 billion years ago, RSS was the first reactive molecule that influenced expansion [9]. Before photosynthesis, RSS, mainly hydrogen sulfide (H2S) released from volcanic eruptions and other geothermal activity, served as building blocks for nucleic acid biosynthesis and protein synthesis for early life forms, such as Beggiatoa, pupfish, giant tubeworms, and mollusks [10]. Researchers have also confirmed the involvement of RSS in providing reducing powers for fixing CO2 via the Calvin cycle in various green and purple sulfur bacteria [6][11]. Furthermore, a large body of literature has implicated the significant role of RSS in initiating oxidation reactions, thereby controlling redox homeostasis, cell signaling, and defense response in plants [12].
Recent studies have corroborated that ROS, RNS, and RSS have similar chemical structures, yet RSS is more versatile and reactive. In addition, ROS/RNS-mediated biosynthesis of RSS under an oxidative environment is its unprecedented source [9]. Due to its similar chemical nature, RSS triggers oxidation reactions by modifying cysteine sulfur and produces an identical effector response as ROS and RNS. Still, the reaction of RSS is more prominent and stable for a much longer duration [13]. Recent discoveries have spectated the growth stimulatory effect of RSS on plant growth and development under stress conditions [6][11][12]. The functional mechanism by which RSS exerts a change stimulatory effect in plants has been unearthed by a few researchers who identified that RSS signaling stimulates post-translational modification of cysteine residues (Cys-SSH) that regulate the expression of stress-responsive genes/proteins [13].

2. Chemical Biology of RSS in Plants

In recent years, RSS has been proclaimed to be inexorably interlinked with all life forms from its inception to the present day [14]. Several studies have confirmed that ROS and RSS are chemically similar and are often grouped under the category of chalcogens, i.e., both belong to group 16 of the periodic table [15]. Concomitantly, ROS and RNS mediate the production of RSS under oxidative environments, which is a potential route for RSS biosynthesis [9]. RSS, like ROS, is also categorized as free radicals and non-radicals. Non-radicals are comprised of thiol (RSH), disulfide (RSSR), sulfenic acid (RSOH), and thiosulfate (RSOSR), whereas free radicals are composed of thiyl-radical (RS˙) [14]. Various organisms such as plants, bacteria, fungi yeasts, nematodes, and humans employ different chemical reactions at intracellular and molecular levels, thereby affecting physiological and molecular processes in the concerned organisms [15].
During the prebiotic era, hydrogen sulfide (H2S) was the primary energy source catalyzing significant steps for incepting life that drove the evolution [16]. In a study, Wachtershauser’s group tried to mimic the conditions before photosynthesis, where organic carbon and nitrogen molecules in the presence of H2S catalyze the formation of thiosulfoxide, which is then converted to persulfide via one-electron oxidation reactions [17]. Correspondingly, upon its subsequent exposure to additional thiosulfoxide, persulfide produces polysulfides (H2Sn) capable of catalyzing oxidative metabolism in purple and green sulfur bacteria [17]. Thiosulfoxide, persulfide, and polysulfide can be stored and recycled/reduced back to H2S to accentuate future oxidation reactions, thus confirming their capability to induce oxidation/reduction reactions for stimulating various signaling pathways [18]. In contrast, ROS and RNS do not possess such ability; they cannot be stored or reused in signaling or related pathways because they function as “one and done”.
However, despite dissimilarity, RSS exhibits higher similarity with ROS than RNS because both are chalcogens with six electrons in their valence shell [19]. Nonetheless, RSS is considered the most versatile, promiscuous, and stable reactive oxidant of its counterparts, i.e., ROS/RNS. Due to higher electronegativity, outer shell electrons in oxygen are near the nucleus, as in sulfur, where electrons are farther from the nucleus [9]. Furthermore, the most stable oxidation state for oxygen is −2 and 0; however, it may also exist in a less stable form of −1, +1, and +2; on the other hand, the most stable oxidation state for sulfur ranges from −2 to +6 [14]. Additionally, sulfur contains more than 30 allotropes compared to oxygen, which has fewer than 10, further attesting to the flexibility of RSS under extreme environments [9]. Strikingly, ROS and RSS exist in various forms with similar chemical and functional properties, i.e., ROS is produced from the one-electron reduction of oxygen (Table 1). In contrast, RSS is produced from one-electron oxidation of H2S [10]. ROS, such as hydrogen peroxide (H2O2), superoxide anion (O2˙−), hydroxyl radical (OH˙), perhydroxy radical (HO2), and singlet oxygen (1O2), are exclusively involved in signaling under stress conditions [10].
Table 1. The enzymes involved in the generation of ROS, RNS, and RSS in plants.
  Enzymes Reaction Catalysed Cellular Location Uniprot/Gene References
ROS Cytochrome C oxidases and alternative oxidases O2 ↔+/_�_ �2˙−
(Mehler’s reaction)		 Thylakoid membrane A0A1P8AZ61 /AT2G43780, Q39219/AT3G22370 [20]
 
  Cytochrome C oxidases and alternative oxidases O2˙−+Fe3+→�2˙1+Fe2+ Thylakoid membrane/mitochondria A0A1P8AZ61 /AT2G43780, Q39219/AT3G22370 [20]
  Cytochrome C oxidases and superoxide dismutase 2O2˙−+2H+→O2+ �2�2Fe3+
(Haber- Weiss reaction)		 Thylakoid membrane/peroxisomes A0A1P8AZ61 /AT2G43780, P24704/AT1G08830 [20]
  Cytochrome C oxidases and alternative oxidases Fe2++H2O2→ Fe3++OH−+��˙
(Fenton reaction)		 Thylakoid membrane A0A1P8AZ61 /AT2G43780, Q39219/AT3G22370 [20]
RNS Nitrate reductase NO2˙+NADPH→��˙ Peroxisomes P11035 /AT1G37130, [8][21]
 
  NOS-like activity, aldehyde oxidase, sulfite oxidase, and xanthine dehydrogenase L−Arg+NOS cofactors (FAD, MOCO)→��˙ Chloroplast, mitochondria Q7G191/AT1G04580, Q8GUQ8 /AT4G34890 [8][21]
  Peroxidase Hydroxyurea+H2O2→��˙ Chloroplast, mitochondria Q9SMU8/AT3G49120 [8][21]
  Amidoxime reducing components NO2˙+Cyt C (red)→��˙ Chloroplast, mitochondria LOC9299625 [8][21]
RSS L-cysteine desulfhydrase L−Cys+ H2O→ �2�+NH3+pyruvate Cytosol Q9MIR1/At3g62130 [11]
 
  D-cysteine desulfhydrase D−Cys+ H2O→ �2�+NH3+pyruvate Mitochondria A1L4V7/At3g26115 [11]
  Sulfite reductase SO3−2+ e−→ �2�+3H2O Chloroplast Q9LZ66/At5g04590 [11]
  Cyanoalanine synthase L−Cys+hydrogen cyanide→ �2� Mitochondria Q9S757/At3g61440 [11]
  Cysteine synthase 1 L−Cys+acetate→ �2� cytosol P47998/At4g14880 [11]
Abbreviations: O2: molecular oxygen, O2˙−: superoxide anion, Fe3+: ferric ion, 1O2: singlet oxygen, Fe2+: ferrous ion, H2O2: hydrogen peroxide, OH−: hydroxyl anion, OH˙˙: hydroxyl radical, NO2˙: nitric dioxide, NADPH: nicotinamide adenine dinucleotide phosphate dehydrogenase, NO˙: nitric oxide radical, L-Arg: L-arginine, FAD: flavin adenine dinucleotide, MOCO: molybdenum cofactor, Cyt C: cytochrome C, L-Cys: L-cysteine, H2S: hydrogen sulfide, NH3: ammonia, SO3−2: sulfite ion.
While perhydroxy radical (HO2) and singlet oxygen (1O2) are less intense, signaling molecules often scavenged into peroxide and oxygen are relatively impermeable across membranes. They have a short half-life, are unstable, and are less reactive.
In contrast, `RSS, which are produced by single-electron oxidation of H2S, are often composed of thiyl radical (HS˙), hydrogen persulfide (H2S2), and persulfide radical (S2˙−) (Table 1). This biosynthetic reaction ends with the formation of elemental sulfur.

This entry is adapted from the peer-reviewed paper 10.3390/life13010204

References

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