Sulfur Homeostasis in Plants.

Sulfur (S) is an essential macronutrient for plant growth and development. S is majorly absorbed as sulfate from soil, and is then translocated to plastids in leaves, where it is assimilated into organic products. Cysteine (Cys) is the first organic product generated from S, and it is used as a precursor to synthesize many S-containing metabolites with important biological functions, such as glutathione (GSH) and methionine (Met). The reduction of sulfate takes place in a two-step reaction involving a variety of enzymes. Sulfate transporters (SULTRs) are responsible for the absorption of SO42- from the soil and the transport of SO42- in plants. There are 12-16 members in the S transporter family, which is divided into five categories based on coding sequence homology and biochemical functions. When exposed to S deficiency, plants will alter a series of morphological and physiological processes. Adaptive strategies, including cis-acting elements, transcription factors, non-coding microRNAs, and phytohormones, have evolved in plants to respond to S deficiency. In addition, there is crosstalk between S and other nutrients in plants. In this review, we summarize the recent progress in understanding the mechanisms underlying S homeostasis in plants.

1. Introduction

n class="Chemical">Sulfur (S) is oclass="Chemical">ne of the esseclass="Chemical">ntial elemeclass="Chemical">nts for growth aclass="Chemical">nd developmeclass="Chemical">nt, aclass="Chemical">nd is coclass="Chemical">nsidered to be the fourth most importaclass="Chemical">nt class="Chemical">nutrieclass="Chemical">nt elemeclass="Chemical">nt after class="Chemical">n class="Chemical">nitrogen (N), phosphorus (P), and potassium (K) in plants. S is a constituent of amino acids, chloroplasts, sulfatides, vitamins, coenzymes, and prosthetic groups (iron–S clusters, lipoic acid, thiamine, coenzyme A, etc.) [1,2,3]. Therefore, S plays an important role in photosynthesis, respiration, and the formation of cell membrane structures in plants. Because animals cannot synthesize S-containing amino acids, the circulation of S between plants and the environment is of great significance to the nutrition and health of humans and animals [1,2,4].

In recent years, with the gradual recognition of the importance of S, research on plant S nutrition has also received more and more attention. Indeed, there has been a systematic exploration of the absorption of S and the n class="Chemical">metabolic processes of S assimilatioclass="Chemical">n iclass="Chemical">n placlass="Chemical">nts [2,5,6,7]. Here, the receclass="Chemical">nt progress associated with studies of S homeostasis iclass="Chemical">n placlass="Chemical">nts is summarized.

2. The Physiological Functions of S in Plants

S plays an important role in the process of plant resistance to biotic and n class="Disease">abiotic stress [8,9]. There are maclass="Chemical">ny S-coclass="Chemical">ntaiclass="Chemical">niclass="Chemical">ng substaclass="Chemical">nces related to class="Chemical">n class="Disease">stress resistance in plants, including glutathione (GSH), S-containing proteins, phytochelatins, and glucosinolates. These S-containing compounds can enhance plant resistance under various stresses. For example, glutathione (GSH) is an important antioxidant substance. The redox system, composed of its reduced state and oxidized state, can eliminate the reactive oxygen species produced by oxidative stress, thereby improving the stress tolerance of plants [2,4,6,10,11]. Glucosinolate is an important secondary metabolite in plants, and its degradation products are important substances in plants for resistance to pests and herbivores [12,13,14].

S is closely related to crop yield and quality [11,15,16]. The content of S-containing amino acids in plants is an important index for evaluating crop quality. n class="Disease">S deficiency reduces the proportioclass="Chemical">n of S-coclass="Chemical">ntaiclass="Chemical">niclass="Chemical">ng amiclass="Chemical">no acids iclass="Chemical">n crop graiclass="Chemical">ns, whereas S applicatioclass="Chemical">n iclass="Chemical">ncreases the coclass="Chemical">nteclass="Chemical">nt of S-coclass="Chemical">ntaiclass="Chemical">niclass="Chemical">ng proteiclass="Chemical">ns, thereby iclass="Chemical">ncreasiclass="Chemical">ng the class="Chemical">nutritioclass="Chemical">nal value of graiclass="Chemical">ns [16,17]. Furthermore, S supply levels affect wheat flour extractioclass="Chemical">n rates, gluteclass="Chemical">n quality, aclass="Chemical">nd bakiclass="Chemical">ng quality [18]. S class="Chemical">nutritioclass="Chemical">n also affects the yield aclass="Chemical">nd quality of forage grass, aclass="Chemical">nd the applicatioclass="Chemical">n of S fertilizer caclass="Chemical">n improve these facclass="Chemical">n class="Chemical">tors, as well as the nutritional status of herbivorous livestock, increasing the yield and quality of wool and milk production and quality in dairy cows [19]. In addition, S nutrition also affects N fixation in legumes. For example, the application of S fertilizers could increase the capacity of N fixation in peas (Pisum sativum L.) and alfalfa (Medicago sativa L.) [20,21].

3. Response of Plants to S Deficiency

In recent years, due to the widespread application of high-purity S-free or low-S fertilizers, the control of industrial S-containing waste gas emissions, and intensive agriculture, S levels in the soil have been reduced, and the problem of soil n class="Disease">S deficiency has gradually emerged [22].

Due to the poor mobility of S in plants, the symptoms of n class="Disease">S deficiency first appear iclass="Chemical">n the youclass="Chemical">ng parts of the placlass="Chemical">nt [18]. Differeclass="Chemical">nt types of placlass="Chemical">nts have differeclass="Chemical">nt symptoms of class="Chemical">n class="Disease">S deficiency, but they are all characterized by the reduced height and chlorosis of leaves (especially new leaves) [18]. The responses of plant root morphology to S-deficient conditions have also been studied [23,24,25]. Under S-deficient conditions, the root development in Arabidopsis was enhanced, leading to more lateral roots and higher root hair density [26]. Dan et al. found that S deficiency had little effect on primary root elongation in Arabidopsis [27]. However, lateral root development in Arabidopsis was significantly inhibited, including a decrease in the number of lateral roots, as well as the density of lateral root primordia and lateral roots. In addition, S application can increase the root length and root surface area of alfalfa grown in soil with low available S [28].

Many studies have been carried out on the response of plants to S nutrition at the physiological level [29,30,31]. Since S is a component of proteins, chloroplasts, and some important enzymes and coenzymes, n class="Disease">S deficiency stress decreases S coclass="Chemical">nteclass="Chemical">nt aclass="Chemical">nd S-coclass="Chemical">ntaiclass="Chemical">niclass="Chemical">ng amiclass="Chemical">no acids, leadiclass="Chemical">ng to reduced class="Chemical">n class="Chemical">metabolic activity in plants [29,30,31]. S deficiency leads to a hindrance of the synthesis of key enzymes in the process of carbon © metabolism, slows the rate of photosynthesis, and results in accumulation of more reactive oxygen species in plants [32].

Plants might reuse S nutrients under S-deficient conditions. n class="Species">Oilseed rape (class="Chemical">n class="Species">Brassica napus L.) can transfer S from leaves to the root system during short-term S deficiency, thereby enhancing the recycling of S [33]. In Medicago truncatula plants, the distribution of photosynthetic products can be adjusted to avoid the impact of S deficiency on the next generation [34].

4. S Absorption in Plants

Although plant leaves can absorb gaseous n class="Chemical">SO2 aclass="Chemical">nd class="Chemical">n class="Chemical">H2S, the main source of available S for plants is sulfate (SO42−) in the soil [35]. Plant roots actively absorb SO42− and transport it to the aboveground parts via the xylem. Most SO42− is assimilated into reduced organic S in plastids (mainly in chloroplasts), while the excess SO42− is transported to vacuoles for storage [2,10,36]. Plant sulfate transporters (SULTRs) are responsible for the absorption of SO42− from the soil and the transport of SO42− in plants [37,38].

The plant S transporter family generally includes 12–16 genes, which are divided into five categories according to their coding sequence homology, biochemical properties, and physiological functions [39]. Mutant analysis proved that the two high-affinity S transporters, n class="Gene">SULTR1.1 aclass="Chemical">nd class="Chemical">n class="Gene">SULTR1.2, in Arabidopsis are mainly located in the root hairs, root epidermis, and cortex, and they are upregulated by S deficiency. Therefore, they are considered to be mainly responsible for the absorption of S from soil [40]. Another high-affinity S transporter, SULTR1.3 in Arabidopsis, is located in the phloem, and is responsible for the transport of S between source and sink tissues, with its expression in roots and leaves being enhanced under S-deficient conditions [41]. In addition, the high-affinity S transporter HVST1, cloned in barley, has been shown to be expressed in the pericycle and xylem parenchyma cells, and may be involved in S transport in vascular tissues [42].

The second category of S transporters is mainly distributed in the xylem and phloem, and is responsible for the transport of n class="Chemical">SO42− betweeclass="Chemical">n tissues. class="Chemical">n class="Gene">SULTR2.1, encoding a low-affinity S transporter in Arabidopsis, is expressed in the xylem parenchyma and pericycle cells, and is upregulated under S-deficient conditions [43]. SULTR2.1 is mainly responsible for the transport of S from the root system to the shoot. SULTR2.1 may mediate SO42− movement into the xylem parenchyma cells, and then increase the concentration of S in the symplast, thereby facilitating the unloading of S from the xylem [43]. SULTR2.1 has also been shown to be involved in the transfer of S to developing seeds [44]. A low-affinity transporter SULTR2.2 is expressed in the phloem, and is responsible for the transfer of SO42− into the companion cell, which may control the S concentration in the phloem sap [45].

At present, little is known about the function of the third category of S transporter. n class="Species">Arabidopsis SULTR3.5 has beeclass="Chemical">n showclass="Chemical">n to assist class="Chemical">n class="Gene">SULTR2.1 during the transport of SO42− from roots to the shoots, but it cannot function as an S transporter by itself [46]. Unlike in Arabidopsis, SULTR3.5 can function as an S transporter in yeast [46].

n class="Gene">SULTR4.1 aclass="Chemical">nd class="Chemical">n class="Gene">SULTR4.2 in Arabidopsis belong to the fourth category of S transporters. They are located in the vacuole membrane and are responsible for the transfer of SO42− from the vacuole. Under S-deficient conditions, the expression of SULTR4.1 and SULTR4.2 is upregulated, and the SO42− stored in the vacuole is transferred out to meet the plant’s demand for S [47]. In addition, the S transporter SULTR5.2 has been defined as a member of category 5, and is quite different from other members of the S transporter family. Nevertheless, Tomatsu et al. indicated that SULTR5.2 had been identified as a molybdenum transporter [48]. Finally, the important process of SO42− transportation to its assimilation site (i.e., plastids) has not yet been determined [49].

5. S Assimilation and Metabolism in Plants

n class="Chemical">Sulfate caclass="Chemical">nclass="Chemical">not be directly used by placlass="Chemical">nts. It class="Chemical">needs to be activated iclass="Chemical">nto class="Chemical">n class="Chemical">adenosine 5′-phosphosulfate (APS) and 3′-phosphoadenosine-5′-phosphosulfate (PAPS) [2,50]. ATP Sylase (ATPS) catalyzes SO42− to APS, with APS then phosphorylated to the storage form PAPS by APS kinase (APK). The reduction of sulfate takes place exclusively in the plastid in a two-step reaction. First, adenosine 5′-phosphosulfate reductase (APR) catalyzes the formation of sulfite from APS. Then, the sulfite reductase (SIR) catalyzes sulfite to divalent sulfide (S2−). Under the action of serine acetyltransferases (SAT) and O-acetylserine(thiol)lyase (OASTL), sulfide is integrated into the C scheleton of O-acetylserine (OAS), leading to the formation of the first important S-containing organic compound cysteine (Cys) [10]. More specifically, SAT catalyzes the acetylation reaction of serine and acetyl-CoA to generate OAS, while OASTL catalyzes OAS and S2− to generate Cys. It is generally believed that OASTL is not the rate-limiting step in the synthesis of Cys. As a limiting factor for the generation of OAS, SAT may indirectly affect the synthesis of Cys (Figure 1) [1,2,10].

Figure 1

The sulfate assimilation and metabolism pathway. Enzymes are indicated in orange letters. Abbreviations of metabolites: APS, adenosine 5′-phosphosulfate; PAPS, 3′-phosphoadenosine 5′-phosphosulfate; R-OH, hydroxylated precursor; Ser, serine; OAS, O-acetylserine; Cys, cysteine; OPH, O-phosphohomoserine; Thr, threonine; Cyst, cystathionine; Hcyst, homocysteine; Met, methionine; γ-EC, γ-glutamylcysteine; GSH, glutathione; GS-X, glutathione conjugate; Glu, glutamate; X-CysGly, cysteinylglycine conjugate. Abbreviations of enzymes: ATPS, ATP sufurylase; APK, APS kinase; SOT, sulfotransferase; APR, APS reductase; SiO, sulphite oxidase; SiR, sulfite reductase; SAT, serine acetyltransferase; OAS-TL, OAS(thiol)lyase; CGS, cystathionine γ-synthase; TS, threonine synthase; CBL, cystathionine β-lyase; MS, methionine synthase; γ-ECS, γ-glutamylcysteine synthetase; GSHS, glutathione synthetase; GST, glutathione-S-transferase; GGT, γ-glutamyltransferase.

The synthesis of n class="Chemical">Cys is a key liclass="Chemical">nk iclass="Chemical">n the process of S assimilatioclass="Chemical">n aclass="Chemical">nd class="Chemical">n class="Chemical">metabolism, and is also the main coordination link between S, C, and N nutrition. Unlike sulfate, which is specifically reduced in plastids, Cys synthesis can be performed in plastids, mitochondria, and the cytoplasmic matrix. Cys is used as a precursor to synthesize many S-containing metabolites with important biological functions in plants [2,10,51,52,53].

5.1. Glutathione

The content of n class="Chemical">GSH is highest amoclass="Chemical">ng the sulfhydryl compouclass="Chemical">nds iclass="Chemical">n placlass="Chemical">nts. It participates iclass="Chemical">n the regulatioclass="Chemical">n of redox aclass="Chemical">nd plays a role iclass="Chemical">n placlass="Chemical">nt resistaclass="Chemical">nce to adversity class="Chemical">n class="Disease">stress [54,55]. It is also the main form of storage and transportation of organic S in plants. Plants use Cys as a synthetic precursor to synthesize GSH through a two-step reaction. First, γ-glutamylcysteine (γ-EC) can be generated from Cys and glutamate under the action of γ-glutamylcysteine synthetase (γ-ECS), which is the rate-limiting step for the synthesis of GSH. Secondly, γ-EC interacts with glycine (or alanine) to produce GSH under the action of glutathione synthetase (GSHS) [56,57]. GSH plays a variety of important roles in maintaining the normal metabolism of plants, including being an important antioxidant, removing active oxygen and heterologous harmful substances, and protecting sulfhydryl groups in proteins and enzyme molecules [2,4,6,10,11]. GSH is also the synthetic precursor of phytochelatins, which play an important role in the process of plant resistance to heavy metal stress [58].

5.2. Methionine

n class="Chemical">Met is aclass="Chemical">nother S-coclass="Chemical">ntaiclass="Chemical">niclass="Chemical">ng amiclass="Chemical">no acid with importaclass="Chemical">nt biological fuclass="Chemical">nctioclass="Chemical">ns, aclass="Chemical">nd it plays a liclass="Chemical">nkiclass="Chemical">ng role iclass="Chemical">n the process of primary aclass="Chemical">nd secoclass="Chemical">ndary class="Chemical">n class="Chemical">metabolism in plants [59,60]. Met is synthesized by a multi-step enzymatic reaction, using Cys as the synthetic precursor. Met is synthesized to S-adenosyl methionine (SAM) under the catalysis of S-adenosylmethionine synthase (SAMS), and SAM is the precursor for the synthesis of ethylene, polyamines, vitamins, coenzymes, nicotianamine (NA), and mugineic acid (MA) [6,59]. Among these derivatives, ethylene is an important hormone in plants, and NA and MA play a key role in the absorption and transport of iron and other metal ions in plants [6,59].

5.3. PAPS

n class="Chemical">PAPS is the sclass="Chemical">n class="Chemical">torage form of APS and serves as a substrate for cytoplasmic sulfation pathways [53]. As the activated form of sulfate, PAPS participates in the synthesis and modification of many metabolites with important biological functions [4].

5.4. Glucosinolate

n class="Chemical">Glucosinolates are importaclass="Chemical">nt, S-coclass="Chemical">ntaiclass="Chemical">niclass="Chemical">ng, secoclass="Chemical">ndary class="Chemical">n class="Chemical">metabolites that are mostly found in cruciferous plants [12,13,61]. Their synthesis requires the participation of Cys, PAPS, and sulfotransferase (SOT) [62]. Glucosinolates themselves do not have physiological activity. When plants are damaged, glucosinolates are hydrolyzed by myrosinase in the cells to produce nitriles, thiocyanates, and isothiocyanates with different biological activities [63,64]. These hydrolysates have different physiological functions, including the production of peculiar aroma components in cruciferous vegetables, inhibition of microbial growth, resistance to insects and herbivores, and inactivation of cancer genes [12,13,14].

6. Molecular Regulatory Mechanisms of S Absorption and Assimilation in Plants

Compared with research into other macroelements, research on the regulation mechanisms of S assimilation and absorption in plants is lagging behind, and is still poorly understood [65,66]. In recent years, with the development of genomic, transcriptomic, and n class="Chemical">metabolomic research, uclass="Chemical">nderstaclass="Chemical">ndiclass="Chemical">ng of the physiological aclass="Chemical">nd molecular regulaclass="Chemical">n class="Chemical">tory mechanisms underlying responses to S-deficiency stress in plants has been enhanced [5,6,30,65,67,68,69].

A number of studies have shown that S transporters and the S assimilation enzyme n class="Gene">APR play a key role iclass="Chemical">n S class="Chemical">nutritioclass="Chemical">n regulatioclass="Chemical">n [37,70]. The regulatioclass="Chemical">n of S absorptioclass="Chemical">n, traclass="Chemical">nsport, aclass="Chemical">nd assimilatioclass="Chemical">n iclass="Chemical">n placlass="Chemical">nts always follows the class="Chemical">negative feedback mechaclass="Chemical">nism driveclass="Chemical">n by demaclass="Chemical">nd [71]. For example, the expressioclass="Chemical">n of geclass="Chemical">nes respoclass="Chemical">nsible for S absorptioclass="Chemical">n aclass="Chemical">nd assimilatioclass="Chemical">n is upregulated uclass="Chemical">nder S-deficieclass="Chemical">nt coclass="Chemical">nditioclass="Chemical">ns, aclass="Chemical">nd is reduced wheclass="Chemical">n S is supplied [72]. Iclass="Chemical">n additioclass="Chemical">n, S assimilatioclass="Chemical">n products are iclass="Chemical">nvolved iclass="Chemical">n regulaticlass="Chemical">ng S absorptioclass="Chemical">n aclass="Chemical">nd class="Chemical">n class="Chemical">metabolism in plants. Cys and GSH are considered as playing a negative regulation role in S absorption and assimilation, while OAS is believed to play a positive regulation role (Figure 2) [68,73].

Figure 2

Regulatory pathways of plant’s response to sulfur deficiency. Arrows denote positive effects, whereas lines ending with a short bar indicate negative effects.

6.1. The cis-Acting Element SURE (Sulfur-Responsive Element)

Maruyama-n class="Chemical">Nakashita et al. first revealed the regulaclass="Chemical">n class="Chemical">tory mechanism of S absorption and assimilation by plant at the molecular level [74]. By using various inhibitors, the regulation of SULTR1.1 was found to occur at the transcriptional level, and required the participation of protein phosphorylase under S-deficient conditions, which suggests that the promoter of SULTR1.1 may have a cis-acting element in response to S deficiency [74]. Later, a 16-base, cis-acting element was found in the promoter region of the S transporter SULTR1.1 in Arabidopsis. The core region of this element, GAGAC, plays a key role in the response to S deficiency, and was named SURE [75]. Homologous sequences of the core region of SURE have also been found in the promoters of other S-deficiency-inducible genes, including Arabidopsis NIT3 (NITRILASE3) and soybean seed storage protein b-conglycinin, which implies a general regulatory mechanism for S deficiency-induced gene expression [26,76]. However, the promoter region of another high-affinity S transporter SULTR1.2 does not contain this element, indicating that the regulatory pathways of S transporters are diverse [75]. Recent studies have also shown that SULTR1.1 is more specifically regulated by S deficiency, while SULTR1.2 seems to be affected by the plant’s overall metabolic requirements [77]. This result also implies that SURE has specificity and limitations in regulating the response to S deficiency. In addition, the SURE element contains an auxin response factor (ARF) binding sequence (GAGACA), but SURE only plays a role in the response to S deficiency and does not respond to auxin signals [75]. Recently, the sdi-1 gene, which is only induced by S deficiency, has been cloned in wheat. Its promoter region also contains a six-base, S deficiency response, cis-acting element. The position of this element in wheat sdi-1 is the same as that of SURE in SULTR1.1 in Arabidopsis [78].

6.2. SLIM1 Transcription Factor

n class="Gene">SLIM1 is the first traclass="Chemical">nscriptioclass="Chemical">n facclass="Chemical">n class="Chemical">tor known to be involved in the regulation of S absorption, and is responsible for the up-regulation of S transporters under S-deficient conditions [79,80]. These authors isolated the Arabidopsis mutant slim1 (sulfur limitation 1) by screening mutants with a green fluorescent reporter gene. The SLIM1 gene was identified as a member of the transcription factor EIL (ETHYLENE-INSENSITIVE3-LIKE) family, with a specific role in the regulation of S nutrition. SULTR1.2 in the slim1 mutant cannot be upregulated by S deficiency, indicating that SLIM1 is involved in the regulation of S transporters under S-deficient conditions. Transcriptome analysis of slim1 mutants has shown that SLIM1 regulates the S transporters SULTR1.2, SULTR1.1, SULTR3.4, and SULTR4.2, as well as a myrosinase gene. However, SLIM1 does not affect the expression of the key enzyme APR, which is induced by S deficiency [79]. In addition, the regulation of S transporters by SLIM1 does not require the participation of SURE elements. Therefore, the interaction between transcription factors and cis-acting elements in regulating the S deficiency response needs further study [79]. In addition, SLIM1 is located in the vascular tissue, but the nutrient absorption site is on the surface of the root system. Therefore, the regulation of S absorption by SLIM1 may involve long-distance signal transduction between tissues.

6.3. miR395

n class="Gene">miR395 is iclass="Chemical">nvolved iclass="Chemical">n the regulatioclass="Chemical">n of class="Chemical">n class="Disease">S deficiency signals in plants, and its target genes include the low-affinity S transporter SULTR2.1 and the three homologous genes APS1, APS3, and APS4 encoding the APS enzyme [2,81]. S deficiency induces the up-regulation of miR395, and this process depends on SLIM1 [2,81]. Strangely, S deficiency enhances the expression of miR395 and its target gene SULTR2.1 at the same time, but tissue localization studies have shown that the expression sites of the two genes are different. miR395 is located in the phloem companion cell, and SULTR2.1 is located in the xylem parenchyma. It seems that SULTR2.1 only transports S in the xylem. Similarly, due to constraints of spatial location, the expression of SULTR2.1 may not be fully regulated by miR395 [2,81]. In addition, miR395 is also involved in regulating the accumulation of Sin leaves and transporting it from old leaves to new leaves [81,82].

7. The Role of Hormones in the Regulation of S Nutrition in Plants

Phytohormones are involved in the regulation of S absorption, transport, and assimilation n class="Chemical">metabolism iclass="Chemical">n placlass="Chemical">nts.

7.1. Cytokinins

Cytokinins (CTKs) inhibit the expression of S transporters n class="Gene">SULTR1.1 aclass="Chemical">nd class="Chemical">n class="Gene">SULTR1.2, thereby negatively regulating S uptake in Arabidopsis. This process is dependent on cytokinin receptors CRE1/WOL/AHK4 [83]. However, CTKs do not affect the induction of S absorption under S-deficient conditions, indicating that the negative regulation of cytokinins and the signal pathway of S absorption under S-deficient conditions are two independent pathways [83]. In addition, studies have reported that exogenous treatment with cytokinin induces the expression of the S transporter SULTR2.2 and the key S assimilation enzyme APR [84]. These results suggest that the role of cytokinins in regulating the plant’s response to S deficiency is not very clearly understood.

7.2. Auxin

Some progress has been made on the role of n class="Chemical">auxin iclass="Chemical">n the regulatioclass="Chemical">n of root morphology uclass="Chemical">nder S-deficieclass="Chemical">nt coclass="Chemical">nditioclass="Chemical">ns [26,85]. As meclass="Chemical">ntioclass="Chemical">ned earlier, class="Chemical">n class="Chemical">glucosinolates are decomposed into SO42− and indole-3-acetonitrile (IAN) by myrosinase. IAN can be converted into indole acetic acid (IAA) under the action of nitrilase (NIT). There are four nitrilase genes in Arabidopsis, three of which (NIT1, NIT2, and NIT3) can convert IAN to IAA [86]. S deficiency enhances the expression of the nitrilase-encoding gene NIT3 in Arabidopsis, and then promotes the conversion of glucosinolates to IAN. Therefore, it has been speculated that the synthesis of auxin is enhanced to promote root development in Arabidopsis under S-deficient conditions [26]. However, there is no difference in auxin content in plants growing under S-deficient and S-sufficient conditions [26]. In addition, transcriptome studies have shown that S deficiency can induce IAA synthesis-related genes, which may increase the synthesis of auxin, thereby enhancing Arabidopsis root development under S-deficient conditions [85]. By using the DR5::GUS reporter strain, the inhibition of lateral root development in Arabidopsis by S deficiency was found to be due to reduced auxin synthesis or auxin sensitivity, which indicates that auxin negatively regulates the response of Arabidopsis to S deficiency [27]. Therefore, auxin may regulate root morphology under S-deficient conditions through both positive and negative feedback pathways [87,88]. First, S deficiency upregulates the NIT3 and myrosinase-related genes, thereby increasing the synthesis of auxin to enhance root growth [26,85]. On the other hand, the increased auxin contents change cell calcium ion concentrations and then upregulate the expression level of calmodulin. Calmodulin acts on the auxin-related transcription factor IAA28, which may inhibit the expression of auxin-induced genes [89]. In addition to the positive regulation pathway, in which S deficiency enhances root growth by increasing auxin synthesis, there is an IAA28-mediated negative feedback regulation pathway that prevents the enhancement of plant root development under S-deficient conditions, and in turn initiates a negative feedback regulation mechanism for restricting root growth [87,88]. Although this evidence indicates that auxin is involved in regulating the response of root morphology to S deficiency, its mechanism remains to be clarified.

Furthermore, n class="Chemical">auxin has class="Chemical">no effect oclass="Chemical">n the upregulatioclass="Chemical">n of high-afficlass="Chemical">nity placlass="Chemical">nt S traclass="Chemical">nsporter expressioclass="Chemical">n uclass="Chemical">nder S-deficieclass="Chemical">nt coclass="Chemical">nditioclass="Chemical">ns [74]. SURE, the key regulaclass="Chemical">n class="Chemical">tory element of the S deficiency response, contains a binding sequence (GAGACA) for the auxin response factor. However, there is no evidence to indicate that SURE is related to auxin signaling [75]. Recent studies have shown that the overexpression of auxin-related genes in response to S deficiency changes many metabolic processes in plants without affecting S metabolism [90].

7.3. Other Hormones

The n class="Chemical">ethylene sigclass="Chemical">nal pathway may be related to the respoclass="Chemical">nse of class="Chemical">n class="Species">tobacco LSU (response to low sulfur) family of proteins under S-deficient conditions [91,92]. Transcriptome analysis has shown that jasmonic acid (JA) is related to the expression of genes involved in S metabolism, and JA synthesis-related genes are induced by S deficiency [93,94]. These results imply that JA plays a positive role in the regulation of S metabolism. Recently, gibberellin has been implicated in the regulation of APR transcription and enzyme activity in Arabidopsis. JA, salicylic acid, ethylene, and CKs only regulate APR enzyme activity, whereas ABA (abscisic acid) does not affect the upregulation of APR under salt stress [95].

8. Systemic Signals Involved in Regulation of Plant S Nutrition

Systemic signals (i.e., long-distance signals) are messengers that connect plant roots and shoots. They can regulate the overall relationship between nutrient requirements in plants and nutrient supply in soil. Several n class="Disease">S-deficiency-responsive geclass="Chemical">nes (such as class="Chemical">n class="Gene">APR3) have been shown to be regulated by systemic signals, and are believed to be related to the decrease in OAS content in Arabidopsis [96]. However, systemic signals that are involved in regulation of plant S nutrition are relatively elusive [97]. In the process of regulating the expression of S transporters and S absorption activity, no systemic signals from the shoots to the roots were found in rape plants under S-deficient conditions [98]. In Arabidopsis, SULTR1.1, and SULTR1.2 are only regulated by local S deficiency, but not affected by systemic signals. In addition, the expression of SULTR1.1 and SULTR1.2 does not depend on OAS or GSH [77]. Overall, there are conflicting conclusions about whether plants have a systemic response to S-deficiency stress. Specific systemic signal molecules have not been determined. Further study is needed to elucidate the regulation mechanism of systemic signals under S-deficient conditions.

9. Crosstalk between S and Other Nutrition in Plants

There is a tendency for the balance of nutrient n class="Chemical">metabolism to be maiclass="Chemical">ntaiclass="Chemical">ned iclass="Chemical">n the placlass="Chemical">nt body, so the iclass="Chemical">nteractioclass="Chemical">n betweeclass="Chemical">n class="Chemical">nutrieclass="Chemical">nt elemeclass="Chemical">nts will affect the placlass="Chemical">nt respoclass="Chemical">nse to the class="Chemical">n class="Disease">stress of mineral nutrient deficiency [99]. However, there are relatively few reports on the interaction between S and other nutrients [8]. Therefore, more attention needs to be paid to the mechanism of crosstalk between S and other forms of nutrition in plants [100].

9.1. S and N

There is an interaction between the regulation of S and n class="Chemical">N class="Chemical">nutritioclass="Chemical">n class="Chemical">n class="Chemical">metabolism in plants [101]. N deficiency inhibits the response of SULTR1.1 and SULTR1.2 to S deficiency in Arabidopsis [74]. N deficiency downregulates expression of the key enzyme APR during S assimilation, and the Cys precursor OAS may be a node that connects N and S metabolism [102,103]. Indeed, N supply can induce the upregulation of genes responsible for S absorption and assimilation, thereby increasing the plant’s S absorption efficiency [104].

A lack of S nutrition affects the utilization of n class="Chemical">N iclass="Chemical">n placlass="Chemical">nts. class="Chemical">n class="Disease">S deficiency inhibits the uptake of nitrate and reduces the activity of nitrate reductase, leading to the accumulation of nitrate and a reduction in N utilization in plants [105]. S-deficiency stress leads to a decrease in the content of S and S-containing amino acids in plants, which may be the cause of hindered protein synthesis and the accumulation of non-protein forms of N [106]. However, in a study of rapeseed plants under short-term S deficiency, the total N content and N absorption were not affected [33].

9.2. S and C

The absorption and n class="Chemical">metabolism of S are also affected by C class="Chemical">n class="Chemical">metabolism in plants [101]. Sugar can upregulate the expression of the S transporters SULTR1.1 and SULTR1.2, and increase the activity and transcription level of APR [107,108]. Under CO2-free conditions, the absorption and transport of S are inhibited, while the activity and transcription level of APR are decreased in plants [107].

9.3. S and P

There is a correlation between S and P nutrition n class="Chemical">metabolism iclass="Chemical">n placlass="Chemical">nts [109]. class="Chemical">n class="Chemical">Sulfolipids are quickly synthesized to replace phospholipids under P-deficient conditions, and phospholipids can also replace sulfolipids under S-deficient conditions [110]. S and P transporters have similar topological structures and molecular regulation mechanisms [111]. For example, the absorption and transport of S and P in plants are both negatively regulated by cytokinins [83,112]. miR395, which is involved in the regulation of the S deficiency response [81], is also upregulated under P-deficient conditions [113]. Transcriptome analysis has shown that P deficiency upregulates the expression of S transporters in Arabidopsis [114]. PHR1 (Phosphate Response 1), which is a key transcription factor in the P-deficiency signaling pathway, may play an important role in the P and S nutrient interaction in Arabidopsis [115]. SULTR1.3, which is responsible for the transport of S in shoots, is upregulated by P deficiency, and its promoter region contains a PHR1 binding sequence [115]. By using phr mutants, the regulation of SULTR1.3 by P deficiency was proven to depend on PHR1 [115]. PHR1 also inhibits the expression of other S transporters (SULTR2.1 and SULTR3.4) [115]. Furthermore, the PHR1 homologous gene PSR1 in Chlamydomonas reinhardtii also negatively regulates S transporters [116]. Nevertheless, P deficiency has not shown any effect on the expression of SULTR1.1 and SULTR1.2 in Arabidopsis [74].

Overall, the regulation of a plant’s response to n class="Disease">S deficiency is iclass="Chemical">nterrelated with the class="Chemical">n class="Chemical">metabolism of N, C, and P; however, the specific regulation mechanisms are still not very well known.

10. Conclusions

S nutrition plays an important role in the growth and development of plants. It is closely related to the response of plants to biotic and n class="Disease">abiotic stresses, as well as the yield aclass="Chemical">nd quality of crops. Soil class="Chemical">n class="Disease">S deficiency has gradually become one of the main factors limiting plant growth and crop yields. However, compared with studies on other macronutrient elements, there is not enough research on plant S nutrition, and a mechanistic understanding of the absorption, metabolism, and regulation of S nutrition remains elusive. Therefore, more effort is needed to understand the plant’s response to soil S deficiency stress and its regulatory mechanisms, and this will form the basis for improving S nutrient utilization efficiency in crops.