The genome of A. thaliana was found to encode a total of fourteen sulfate transporter
genes belonging to five groups. Two genes (SULTR5;1 and SULTR5;2) have MOT
domain instead of STAS domain, which is involved in Molybdenum transport, are
not affected by levels of sulfate, and thus are not considered sulfate transporters.
These have been re-annotated as MOT1 and MOT2 and now are considered as
molybdenum transporters (Gasber et al. 2011; Baxter et al. 2008). There are thus
now only four groups of sulfate transporters. Group 1 have two homologs in
A. thaliana (SULTR1;1, SULTR1;2) both of them are high affinity transporters
and are involved in sulfate uptake from soil. Group 2 transporters (SULTR2;1,
SULTR2;2) are involved in long distance transport and in sulfate flux through the
plant (Takahashi et al. 2000). Members of group 3 include chloroplast sulfate
transporters (SULTR3;2, SULTR3;3, SULTR3;4; but not SULTR3;5) and low
affinity long distance transporter (SULTR3;1). Group 4 transporters are involved
in efflux of sulfate from the vacuole in pericycle and xylem parenchyma cells. Few
group 1 transporters are also involved in long distance transport within plant
(Yoshimoto et al. 2003; Howarth et al. 2003; Takahashi et al. 2011). Members of
group 1, 2, and 4 are regulated during sulfate deprived conditions (Yoshimoto et al.
2003; Takahashi et al. 2000. Other than Arabidopsis, sulfate transporters have been
identified and characterized from a variety of plants such as G. max (Ding et al.
2016), Brassica (Buchner et al. 2004b), M. truncatula (Casieri et al. 2012), O. sativa
(Kumar et al. 2019; Godwin et al. 2003), T. aestivum (Buchner et al. 2010), Z. mays
(Huang et al. 2018), Populus (Dürr et al. 2010), to name a few.
The first step in assimilation and metabolism of sulfate is adenylation to adenosine 5’ phosphosulfate (APS) by ATP sulfurylase (ATPS), which functions as a
homotetramer and is localized in cytosol as well as chloroplast. Four copies of gene
encoding ATP sulfurylase are present in A. thaliana. APS can act via two pathways:
(1) convert SO 4
2À into sulfite by the action of APS reductase (APR) and (2) generate
3’-phosphoadenosine 50-phosphosulfate (PAPS) with the action of APS kinase
(APK). The enzyme APR is encoded by three paralogous genes in A. thaliana and
acts a dimer; APK is encoded by four genes in A. thaliana. Cysteine is the first stable
product in sulfate assimilation. It is formed by the action of O-acetylserine (thiol)
lyase (OAS-TL) for which O-acetylserine (OAS) and sulfide are substrates (Wirtz
and Hell 2006). Serine acetyltransferase (SAT) is involved in OAS synthesis through
serine and acetyl coenzyme A. The first pathway results in production of cysteine
that acts as precursor for synthesis of methionine. Methionine in turn is a precursor
for several amino acids and glutathione, essential for several plant metabolites.
There are several other key regulators of sulfate uptake and assimilation. SULFUR LIMITATION 1 (SLIM1), a member of the ethylene insensitive 3-like (EIL)
transcription factor family, is considered a key transcriptional regulator of sulfate
uptake and metabolism, and acts via miR395 (Maruyama-Nakashita et al. 2006;
Kawashima et al. 2011; Mathewman et al. 2012). SLIM1 has been shown to also act
under Cadmium stress and promote sulfate uptake (Yamaguchi et al. 2020). SLIM1
also negatively regulates members of the MYB pathway such as MYB34 and may
influence glucosinolate biosynthesis, an assimilation product of sulfate in
Brassicaceae (Maruyama-Nakashita et al. 2006; Takahashi et al. 2011). Sulfate
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E. Bhardwaj et al.
genes belonging to five groups. Two genes (SULTR5;1 and SULTR5;2) have MOT
domain instead of STAS domain, which is involved in Molybdenum transport, are
not affected by levels of sulfate, and thus are not considered sulfate transporters.
These have been re-annotated as MOT1 and MOT2 and now are considered as
molybdenum transporters (Gasber et al. 2011; Baxter et al. 2008). There are thus
now only four groups of sulfate transporters. Group 1 have two homologs in
A. thaliana (SULTR1;1, SULTR1;2) both of them are high affinity transporters
and are involved in sulfate uptake from soil. Group 2 transporters (SULTR2;1,
SULTR2;2) are involved in long distance transport and in sulfate flux through the
plant (Takahashi et al. 2000). Members of group 3 include chloroplast sulfate
transporters (SULTR3;2, SULTR3;3, SULTR3;4; but not SULTR3;5) and low
affinity long distance transporter (SULTR3;1). Group 4 transporters are involved
in efflux of sulfate from the vacuole in pericycle and xylem parenchyma cells. Few
group 1 transporters are also involved in long distance transport within plant
(Yoshimoto et al. 2003; Howarth et al. 2003; Takahashi et al. 2011). Members of
group 1, 2, and 4 are regulated during sulfate deprived conditions (Yoshimoto et al.
2003; Takahashi et al. 2000. Other than Arabidopsis, sulfate transporters have been
identified and characterized from a variety of plants such as G. max (Ding et al.
2016), Brassica (Buchner et al. 2004b), M. truncatula (Casieri et al. 2012), O. sativa
(Kumar et al. 2019; Godwin et al. 2003), T. aestivum (Buchner et al. 2010), Z. mays
(Huang et al. 2018), Populus (Dürr et al. 2010), to name a few.
The first step in assimilation and metabolism of sulfate is adenylation to adenosine 5’ phosphosulfate (APS) by ATP sulfurylase (ATPS), which functions as a
homotetramer and is localized in cytosol as well as chloroplast. Four copies of gene
encoding ATP sulfurylase are present in A. thaliana. APS can act via two pathways:
(1) convert SO 4
2À into sulfite by the action of APS reductase (APR) and (2) generate
3’-phosphoadenosine 50-phosphosulfate (PAPS) with the action of APS kinase
(APK). The enzyme APR is encoded by three paralogous genes in A. thaliana and
acts a dimer; APK is encoded by four genes in A. thaliana. Cysteine is the first stable
product in sulfate assimilation. It is formed by the action of O-acetylserine (thiol)
lyase (OAS-TL) for which O-acetylserine (OAS) and sulfide are substrates (Wirtz
and Hell 2006). Serine acetyltransferase (SAT) is involved in OAS synthesis through
serine and acetyl coenzyme A. The first pathway results in production of cysteine
that acts as precursor for synthesis of methionine. Methionine in turn is a precursor
for several amino acids and glutathione, essential for several plant metabolites.
There are several other key regulators of sulfate uptake and assimilation. SULFUR LIMITATION 1 (SLIM1), a member of the ethylene insensitive 3-like (EIL)
transcription factor family, is considered a key transcriptional regulator of sulfate
uptake and metabolism, and acts via miR395 (Maruyama-Nakashita et al. 2006;
Kawashima et al. 2011; Mathewman et al. 2012). SLIM1 has been shown to also act
under Cadmium stress and promote sulfate uptake (Yamaguchi et al. 2020). SLIM1
also negatively regulates members of the MYB pathway such as MYB34 and may
influence glucosinolate biosynthesis, an assimilation product of sulfate in
Brassicaceae (Maruyama-Nakashita et al. 2006; Takahashi et al. 2011). Sulfate
150
E. Bhardwaj et al.