The emerging key role of reactive sulfur species in abiotic stress tolerance in plants.

In plants, sulfur plays a critical role in the formation of important biomolecules such as cysteine, methionine, and tripeptide glutathione. Thiol groups, composed of sulfur, are essential to numerous metabolic processes. The easy and reversible oxidation and reduction of thiol groups have drawn attention to the redox regulation of cellular metabolism. Reactive sulfur species (RSS), including hydrogen sulfide (H S), persulfides, and polysulfides, are synthetized in all living organisms, mainly from cysteine, and have been recognized in the last two decades as very important molecules in redox regulation. RSS are considered potent signaling molecules, being involved in the regulation of virtually all aspects of cell function. With regard to stress, reactive species and the antioxidant machinery maintain a delicate balance that gets disturbed under stress conditions, wherein reactive species biosynthesis, transportation, scavenging, and overall metabolism become decisive for plant survival. While reactive oxygen and nitrogen species have been much discussed over recent years, research into RSS biosynthesis, signaling, and relation to abiotic stresses is still nascent. RSS evolved long before reactive oxygen species, and because both are metabolized by catalase, it has been suggested that "antioxidant" enzymes originally evolved to regulate RSS and may still do so today. In this review, we have tried to summarize the generation, signaling, and interaction of RSS in plant systems and to discuss in detail the roles under various abiotic stresses.