The beginning of the sulfate uptake mechanism by the root tissue from the surrounding environment and the translocation of sulfate between different cell compartments is facilitated by specific sulfate transporters (SULTR) (
Figure 1). The expression levels of these transporter genes in specific organs, cell types and subcellular compartments are regulated by the transcriptional and post transcriptional mechanism which maintains the homeostatic balance between the uptake of sulfate and internal tissue distribution on the basis of sulfate availability and on-demand organic sulfur metabolites biosynthesis
[11]. SULTR, a large gene family, is involved in encoding this transporter which consists of 14 members in Arabidopsis and rice (
Oryza sativa L.). According to phylogenetic studies this gene family can be categorized into 4 closely related groups (
SULTR1 to
4), each containing 12 membrane-spanning domains and a STAS (sulfate transporter and anti-sigma antagonist) domain at the carboxy-terminal end
[12], and a fifth member of this group is SULTR5, distinct from the other, lacking that STAS domain
[13]. Interestingly, Tomatsu et al.
[14] mentioned that the Arabidopsis
Sultr5;2 gene was responsible for encoding a high-affinity root molybdate transporter, which raises a valid question about the role of group 5 transporter genes in the sulfate transport mechanism. Furthermore, group 1 and 2 transporters of sulfate are localized in the plasma membrane and are considered as the best categorized groups, being subjected to several studies. Members of group 1 sulfate transporters represent a high-affinity transport system which facilitate the uptake of sulfate by roots (SULTR1;1 and SULTR1;2) or translocate the sulfate from source to sink tissues (SULTR1;3)
[15][16][17][18][19]. In addition, the group 2 members consist of low-affinity sulfate transporters whose gene products may be involved in vascular tissues transportation and facilitate the translocation of sulfate throughout the plant
[20][13][18]. The literature suggests that group 3 is composed of low-affinity transporters localized at the plasma membrane, showing differential expression patterns in plant tissues and not stimulated by sulphur deficiency
[13]. In addition, a role of the SULTR3;5 transporter is noted in the transport of sulfate from root-to-shoot in cooperation with the SULTR2;1 transporter of Arabidopsis
[21]. The last group, i.e., group 4 sulfate transporters, has been identified in the vacuolar membrane: a study with SULTR4;1-GFP fusion protein showed that this transporter mainly accumulated in the vacuoles of roots and hypocotyls of young seedlings
[21]. Under sulphur sufficient and deficient conditions, the
Sultr4;1 transporter gene was expressed in roots and helps to efflux sulfate (SO
42-) from the vacuolar lumen to cytoplasm, and it also improved the storage capacity of sulfate in vacuoles
[21]. In contrast, gene expression of
Sultr4;2 was highly inducible by sulfur during the time of sulfur deficient condition in the same tissue. The double knock-out mutants, i.e.,
sultr4;1/sultr4;2, contained higher amounts of sulfate as compared to wild-type plants. Comparison between the single and double knock-out mutants
sultr4;1/
sultr4;2 demonstrated that
Sultr4;1 devotes a vital role and
Sultr4;2 has a supplementary effect
[22]. Whereas the sulfate transport system has been studied extensively in roots, to date, there are very few reports available based on the functions of individual sulfate transporters within seeds. Moreover, vacuoles may play an important role for the purpose of storage and unloading of sulfate within the developing seed, where the members of the SULTR4 transporters would play a key role. In this context, the Arabidopsis
Sultr4;1 gene was expressed strongly within the developing seeds, and it was observed that its disruption significantly enhanced the content of seed sulfate, depicting that SULTR4;1 was involved in the efflux mechanism of sulfate from vacuoles to the developing seeds. In addition, a proteomic study of
Sultr4;1 mutant seeds revealed the metabolic adjustment for the adaptations in altered sulfate compartmentalization, which indicates a SULTR4;1-mediated sulfate transport system for the establishment of defense mechanisms against oxidative stress during the time of seed development.
For instance, in mature mutant seeds of
sultr4;1, on average, the sulfate content was 1.7 times more than that of wild-type plants, whereas the total sulfur content in seeds remained unchanged. In mature seeds, sulfate contributes a significant fraction of the total sulfur content, i.e., 7.7% in wild-type and 13.24% in
sultr4;1 mutant seeds, respectively. In respect to mutant
sultr4;1 plant phenotypic response, no significant difference was observed in terms of yield parameters, leaf area and/or onset of flowering but a slight reduction in seed weight was noted as compared to wild-type plants. The sulfur ion fluxes into the developing seeds may not be affected in the
sultr4;1 mutant, which increases the sulfate contents in seed, but there is no relation for such a drastic perturbation of vegetative growth of this mutant. In mature seeds of the
sultr4;1 mutant, a significant increment of sulfate pool was observed, which may be related to a reduced efflux of sulfate from the vacuoles during the development of seeds. In continuation with this, in the reproductive growth phase of Arabidopsis, the
Sultr4;1 transporter was preferentially expressed in developing seeds during the time of transition between embryogenesis and the seed filling phase and a relatively higher amount of transcript availability was observed in comparison to
Sultr4;2, which was expressed at equal levels throughout the development of seeds. Additionally, the transporter SULTR4;1 also maintains the redox homeostatic balance at the time of seed development if any kind of oxidative stress outbreak takes place due to any environmental abnormalities. The proteomic study revealed this kind of dehydration tolerance capability with the due course of their development
[23][24]. In a nutshell, it can be suggested that the sulfate transporters SULTR3 and SULTR4 play a vital role in sulfate translocation, which is associated with seed development, by suppling sulfate and different S metabolites. In this context, it can also be noted that SULTR3 and SULTR4 homologs control the allocation of sulfate in between the seed compartments and help to modulate S metabolites and seed protein composition in Arabidopsis
[23][24].
Another experiment depicted that, at bolting stage, previously stored sulfate in vacuole of the source leaves is remobilized into the developing seeds of oilseed rape (
B. napus L.) and an up-regulated gene expression is noted of
SULTR4;1 and
SULTR4;2 transporters. These kinds of upregulated gene expression of two SULTR4-type transporters were also observed during the vegetative growth phase in old and mature leaves
[27] and in roots
[28] of oil seed rape (
Brassica napus L.). In addition, Gironde et al.
[29] revealed that at the reproductive stage, sulfate is the main source of S remobilized from the stored vacuolar sulfate of leaves via tonoplastic SULTR4-type transporters in S deficient condition. Moreover, Awazuhara et al.
[20] revealed that sulfate transporter SULTR2.1 is also involved in the transfer of S into developing seeds of Arabidopsis. In addition, when the amount of abscisic acid (ABA) in freshly harvested seeds of
sultr3;1 mutants was tested, 25–50% more ABA was found in comparison to wild-type Arabidopsis plants; this finding established the fact that SULTR3;1 affects ABA biosynthesis not only during the time of early vegetative growth but also in the seed filling stage
[30].