Sulfur is an essential macronutrient required for plant growth and development. Sulfur is a constituent of the amino acids cysteine (Cys) and methionine (Met) which are necessary for the synthesis of proteins and serve as precursors of important cofactors and sulfur containing secondary metabolites (Thomas et al., 2000; Mugford et al., 2011). The metabolites of sulfate assimilation and metabolism have important effects on plant growth, development, environmental response, resistance to biological and abiotic stress, crop quality and yield (Zhao et al., 1999; Hell et al., 2010; Gironde et al., 2014). Recent studies have suggested that sulfur containing compounds may also play a role in regulation of stomatal movement. Stomata are micro pores mainly found on leaf surface of terrestrial plants, controlled by two guard cells. CO₂ and H₂O exchanging through stomatal pores makes stomatal movement a key process for photosynthesis and drought resistance. In the present review, we focused on the transporters and summarized different sulfur compounds on regulating stomatal movement in details.

There are two sulfur uptake pathways in plants, including the sulfate uptake in roots and atmospheric sulfur entry pathway through stomata. Plants take up sulfur from soils by roots mainly in form of sulfate through sulfate transporters (SULTRs). In plants, genes encoding sulfate transporters are divided into four distinct subfamilies (SULTR1 to SULTR4) according to the similarity of the protein sequences (Vatansever et al., 2016). In the genome of Arabidopsis thaliana, there are eleven SULTR members and the functions of most of SULTRs have been comprehensively studied. In Arabidopsis, AtSULTR1;1 and AtSULTR1;2 are two high-affinity sulfate transporters responsible for sulfate uptake in roots (Figure 1A). AtSULTR1;2 is involved in the sulfate uptake in roots under sulfate sufficient condition, while AtSULTR1;1 is responsible for the absorption of sulfate under sulfate limitation condition (Hideki et al., 2000; Naoko et al., 2002; Rouached et al., 2008). AtSULTR1;2 is expressed in the root epidermal and cortical plasma membranes, and is co-localized with AtSULTR1;1 (Hideki et al., 2000; Naoko et al., 2002). Although sulfate deficiency induced the expressions of both AtSULTR1;1 and AtSULTR1;2, the induction of AtSULTR1;1 is much stronger than that of AtSULTR1;2 (Barberon et al., 2008; Rouached et al., 2008).

After entering root cells, sulfate could be transported into the vacuole for storage or translocated to shoots for assimilation in plastids. The flux and allocation of sulfate in plants depends on the demand of plants organs and cells for sulfate, as well as sulfur status of the whole plant (Rennenberg and Herschbach, 2014; Spicer, 2014). The root-to-shoot or shoot-to-root long-distance transport of sulfur in high plants takes place in the vascular system which consists of xylem and phloem (Rennenberg et al., 1979). The existence of sulfate in phloem sap and phloem exudate has long been demonstrated (Toshiyuki et al., 1990). In rice, sulfate deprivation for 7 days results in a decrease in sulfate concentration in the phloem sap, which suggests that sulfate can be transferred between shoots and roots (Kuzuhara et al., 2000). In Arabidopsis, the loading sulfate into sieve tube of phloem is mediated by AtSULTR1;3 which is mainly expressed in the sieve element-companion cell complexes in the cotyledons and roots (Kuzuhara et al., 2000). In addition to phloem transport, xylem transport is also an important role of sulfate translocation (Yoshimoto et al., 2003). AtSULTR3;5 and AtSULTR2;1 are involved in loading sulfate to xylem parenchyma cell and facilitate the root-to-shoot translocation (Figure 1A; Kataoka et al., 2004a).

Remobilization of sulfate from vacuole is critical for maintenance of sulfur homeostasis in plants, particularly under sulfate limited condition. In Arabidopsis, it has been demonstrated that tonoplast-localized AtSULTR4;1 and AtSULTR4;2 facilitate the sulfate efflux from the vacuoles (Figure 1A) evidenced by the increase of vacuolar sulfate concentration in the atsultr4;1 atsultr4;2 double knockout mutant (Kataoka et al., 2004b). Interestingly, although both AtSULTR4;1 and AtSULTR4;2 are induced by sulfur deficiency, they display differential sulfur-dependent expression pattern in roots. AtSULTR4;2 can increase accumulation in respond to sulfate limitation while AtSULTR4;1 constitutively expresses under different sulfate conditions. Expression of AtSULTR4;1 in the sultr4;1 sultr4;2 double knockout mutant can rescue its phenotype indicating that AtSULTR4;1 plays a primary role in maintaining intracellular sulfate redistribution and homeostasis (Kataoka et al., 2004b; Martinoia et al., 2007). However, whether AtSULTR4;1 and AtSULTR 4;2 participate in the sulfate efflux from vacuole in guard cells is not clear.

Once translocation from roots to shoots, sulfate is transported into plastids for assimilation. In Arabidopsis, AtSULTR3 family proteins are involved in transport of sulfate into chloroplasts (Figure 1A). Simultaneous knockout of all five members of AtSULTR3 reduced sulfate accumulation in chloroplasts by more than 50% compared to the wild type (Chen et al., 2019). In plastids, sulfate is first converted to adenosine-5′-phosphosulfate (APS) by ATP sulfurylase (ATPS) (Logan et al., 1996; Thomas et al., 2000), and further reduced to sulfide in a two-step reduction reaction catalyzed by APS reductase (APR) and sulfite reductase (SiR) (Olivier et al., 2007; Khan et al., 2010a). In the last step of sulfate primary metabolism, cysteine (Cys) is produced by condensation of sulfide and O-acetylserine (OAS) catalyzed by OAS (thiol)lyase (OASTL) (Heeg et al., 2008). Cys serves as a precursor for the biosynthesis of Met, GSH and other sulfur containing compounds (Ravilious and Jez, 2012). In parallel, APS also can be phosphorylated to PAPS, which is involved into sulfation reaction in secondary metabolism as a precursor of active sulfate (Mugford et al., 2009, 2011).

In addition to absorb sulfate via roots, plants are also able to utilize foliary-absorbed sulfur gases as directly sulfur source, such as hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) (Figure 1B; Herschbach et al., 1994; Sue et al., 2002; Aghajanzadeh et al., 2016). Atmospheric sulfur gases are derived from natural source and anthropogenic source. H₂S and SO₂ emitting by volcanic and geothermic activity are the main natural sulfur source while SO₂ is the main anthropogenic sulfur source releasing from industrial processes and human life (Garrec, 1984; Stern, 2005). The kinetics of SO₂ entry into plant leaves is correlated with stomatal conductance and SO₂ level in the atmosphere (Noland and Kozlowski, 1979; By et al., 1996). SO₂ is soluble in water phase in mesophyll cells, and reacts with water to release hydrogen ion (H⁺) and generate hydrogen sulfite (HSO₃^–). HSO₃^– can be directly reduced and assimilated into organic sulfur compounds in chloroplasts, or further oxidized into sulfate before entering the sulfur assimilation pathway (Noland and Kozlowski, 1979; De Bruyn et al., 1995). Different from SO₂, the conductivity of mesophyll cells to H₂S is largely determined by its metabolic rate in plants and the H₂S level in atmosphere. Due to the poor solubility of H₂S in water, H₂S can be dissociated into H⁺ and hydrogen sulfide ion (HS^–) in the atmosphere (By et al., 1996; Stuiver and De Kok, 2001; Lee et al., 2011).

The inorganic SO₂ or H₂S entry into plant leaves could be further assimilated into organic sulfur through sulfate assimilation pathway. Plants exposed to SO₂ or H₂S gases significantly increase the thiol content and change thiol composition in shoots (Aghajanzadeh et al., 2016; Ausma et al., 2021). In Arabidopsis, a short-term fumigation with 0.25 μl l^–1 H₂S strongly increase the concentrations of cysteine and glutathione by 20 and 4 times, respectively (Riemenschneider et al., 2005). However, the content and composition of glucosinolate in Brassica juncea and Brassica rapa were not affected by SO₂ or H₂S exposure regardless of sulfur sufficiency or deprivation (Aghajanzadeh et al., 2015). While SO₂ and H₂S exposure as sulfur compensation, can actually make foliar absorb more sulfur gas than sulfate-sufficient condition, but the absorption of sulfur nutrition in shoot does not affect the accumulation of transcript caused by sulfate limitation in roots, while their exposure can alleviated the up-regulated of APR, rather SULTR1;1, SULTR1;2 or OASTL (Stuiver and De Kok, 2001; Aleksandra et al., 2008; Birke et al., 2015). Although SO₂ and H₂S exposure may affect sulfate uptake in roots, the expression levels of SULTRs are independent on their exposure (Aleksandra et al., 2008). To be sure, SO₂ and H₂S is also a well-known toxic gas that can cause harm to plants at deleterious concentrations which may vary from plant species and environmental conditions (Thompson and Kats, 1978; Malhotra and Khan, 1984). It is controversial for determining what degree of foliar absorption contributes its toxification or helpfulness on account of variability of the growing environment and nutrient needs of different plants (Amaral et al., 2006; Yang et al., 2006; Lisjak et al., 2013).

Several organic sulfur compounds could also induce stomatal closure. Batool et al. (2018) found that the application of Cys could reduce stomatal aperture in Arabidopsis, and the sulfate-induced stomatal closure is dependent on the biosynthesis of Cys. GSH is involved in ABA-induced stomata closure (Okuma et al., 2011). Decreasing GSH level in guard cells in the GSH biosynthesis deficient mutant cad2-1 or inhibition of GSH biosynthesis enhanced ABA-induced stomatal closure (Figure 1A). Overexpression of gamma-glutamylcysteine synthetase (γ-ECS), a rate-limiting enzyme in GSH biosynthesis, significantly reduced the stomatal aperture and density (Lu et al., 2021). Moreover, exogenous application of L-methionine (L-Met) has been shown to enhance stomatal closure by activating Ca²⁺ channels and generation of reactive oxygen species (ROS) (Kong et al., 2016).

Whether from soil or atmosphere, sulfur compounds mainly cause stomatal closure, like stress signal, in an ABA dependent or independent pathway. Except for sulfate, the transporters of H₂S and SO₂ are still unrevealed. The underlying mechanisms of the relationship between drought stress and sulfate accumulation in guard cells need to be investigated in the future.

ZR and R-YW wrote the manuscript. X-YH and YW reviewed and edited the manuscript. All authors have discussed and approved the submitted version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer YX declared a shared affiliation with several of the authors, R-YW and X-YH, to the handling editor at the time of the review.

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