Hydrogen sulfide (H₂S) has always been considered a toxic molecule hazardous to the environment and life. For mammals, it can poison several different systems, although the nervous system is most affected. However, below a particular threshold, H₂S has been implicated in normal cellular events and therefore are currently recognized to be important signaling molecules, functioning as physiological gasotransmitters in both mammals and plants.
H₂S is endogenously produced and metabolized by cells in a precise and regulated manner. In animal systems, it is already recognized as an important signaling molecule, comparable to carbon monoxide and nitric oxide. Emerging data in plant systems have recently changed the concept of sulfide from a toxic molecule to a signaling molecule of the same importance as nitric oxide and hydrogen peroxide. The role of the sulfide has been deeply studied in several essential processes for plant performance. Thus, it has been increasingly accepted that H₂S mediates tolerance and protection against many different plant stresses, enabling important aspects of development, such as seed germination, root elongation, and plant viability. H₂S also regulates processes that are critical for adequate plant function, such as progression of autophagy, stomatal movement, photosynthesis, flower and leaf senescence, and fruit maturation.

The endogenous production of H₂S occurs through the action of enzymes involved in the metabolism of cysteine. In plants, the chloroplast is the main source of sulfide; however, at the basic pH found within the stroma, sulfide cannot be transported outside the chloroplastic membrane. Thus, sulfide is metabolically generated in the cytosol; in Arabidopsis, the L-cysteine desulfhydrase, DES1, has been demonstrated to be responsible for sulfide production. Recent research has shown that sulfide behaves as a signaling molecule, acting as a repressor of autophagy. However, the underlying mechanism and the specific molecular targets are not well known, although two mechanisms have been proposed. The first mechanism relates to the nucleophilic properties of this molecule, in which sulfide acts as an antioxidant with reactive oxygen species as its molecular targets. The second mechanism, S-sulfhydration, consists of a posttranslational modification to thiol groups of cysteines within proteins to form persulfide groups. In plants, the action of sulfide through the endogenous S-sulfhydration of proteins has already been demonstrated, and different autophagic proteins can be hypothesized as targets for sulfide.

In this Research Topic, recent advances on plant H₂S metabolism and signaling will be covered, with the main focus on the role of H₂S as a signaling molecule in the regulation of multiple metabolic and cellular processes.

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Frontiers in Plant Science

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