Uptake of sulphur, in the form of sulphate, from the soil and its transportation in the plant system is mediated by specific transporters.

1. DR. VIBHA KHANNA
Asso. Prof. (Botany)
SPC GOVERNMENT COLLEGE
AJMER (Rajasthan)

2. PLANT BIOCHEMISTRY
• BLOCK 2. : Sulphur metabolism
–PRESENTATION 2:
• Sulphate Uptake and Transport in Plants

3. Sulphate: the available form of S for plants
• Sulphur is available to plants primarily in the
form of anionic sulphate (SO4
2−) present in
soil.
• It is actively transported into roots and then
distributed, mostly unmetabolized,
throughout the plant.
• SO4
2− is a major anionic component of
vacuolar sap; therefore, it does not necessarily
enter the assimilation stream.

4. SULPHATE UPTAKE AND TRANSLOCATION IS
MEDIATED BY SPECIALIZED MEMBRANE PROTEINS
• Multiple transport steps through different membranes
are involved:
 plasma membrane transporters in the root present in the
outermost cell layers for initial uptake;
 plasma membrane transporters of vascular tissues for
long-distance translocation and of leaf mesophyll cells for
assimilation coupled with photosynthesis;
 inside cells, transporters associated with organelle
transport, in particular, plastids and vacuoles
 The phosphate/triose phosphate translocator of the inner
chloroplast membrane or a proton/SO4
2−symporter may
mediate SO4
2− transport into chloroplasts.

5. Sulphate Transporters
• Plasma membrane sulphate transporters are classified as
proton/sulfate cotransporters that mediate active sulphate
uptake driven by the transmembrane proton gradient.
• The uptake mediated by this transporter is pH dependent,
and the proton gradient is generated by the plasma
membrane proton ATPase.
• The sulphate transporter possesses 12 membrane-spanning
domains and belongs to a large family of cation/solute
cotransporters .
• Besides proton/sulfate cotransporters, anion channels, ABC
proteins, and oxaloacetate/sulfate transporters may
mediate sulphate transport in plant cells

6. Sulphur Uptake and Transportation
• Sulphur is mostly absorbed as sulphate from the soil solution
through the root system.
• Once absorbed through the roots, the sulphate is transferred
to the site of reduction or of storage via the xylem.
• Sulphate is then transferred between tissues via the phloem.
• In all cases, transport across the plasma membranes of cells
is required.
• Intracellular transport processes are also involved: reduction
occurs in the plastid, requiring specific transport inward
across the plastid membrane and as the vacuole is the major
storage pool, both inward and outward transport across the
tonoplast is required.
• In higher plants there exists a large gene family encoding
related transporters.

7. Sulfate Transport Across The Plasma Membrane
• Sulfate transport across the plasma membrane, is via a
specific carrier.
• It is driven by an electrochemical gradient established by a
proton ATPase.
• The sulfate transporter uses the gradient to transport
sulfate into the cell along with protons at a ratio of 1 sulfate
to 3 protons (3H+/sul· co-transport mechanism).
• Proton/sulphate co-transport in the plasma membrane of
root cells is the first step for the uptake of sulphate from
the environment by plants.
• Further intracellular, cell-to-cell and long-distance transport
must fulfil the requirements for sulphate assimilation and
source/sink demands within the plant.

8. Plant Sulfate Transporters
• Plant sulfate transporters belong to a superfamily of cation/proton
cotransporters found in eukaryotic organisms.
• Plants are able to take up sulfate from soil through the use of high
affinity and low affinity transporters localized in root epidermal and
cortical cells.
• Thus there are two uptake systems;
– high-affinity and
– low-affinity.
• A class of sulfate transporters has been identified in plant roots
which are part of the starvation-induced, high-affinity uptake system.
• A second class of sulfate transporter genes appear to encode low-
affinity transporters that may load sulfate into the vascular tissue in
roots and unload it into leaf cells, or which may transport sulfate
between cellular or subcellular compartments

9. Sulphur Transport in Plant Body
• Transport throughout the plant body involves
export of sulfate from root cells during the
loading of xylem and then import into the
cells within sink organs.
• Within cells, sulfate is transported to the
chloroplast where most reduction takes place,
and also to the vacuole where it serves as a
major anionic constituent.

10. The various sulfate transporters of Arabidopsis:
(Sultr 1 to 4)
• Group 1 consists of high affinity transporters expressed
primarily in roots of plants after sulfate starvation,
suggesting they function under sulfur-limiting
conditions.
• Group 2 transporters have a lower affinity for sulfate
and are expressed primarily in vascular tissues. They
may be involved in transporting sulfate throughout the
plant.
• Group 3 transporters are expressed primarily in leaves,
but their function has not yet been characterized.
• Finally, group 4 consists of a transporter (Sultr4;1)
localized to chloroplasts.

11. Sulphate Uptake: The Primary Step
• The primary uptake of sulphate by the root is mediated by two
different Group 1 sulphate transporter isoforms.
• The spatial expression of these two high-affinity transporters in the
root tip and root epidermis, including root hairs and in the cortical
cells of the mature root suggests that all these tissues have the
capacity for high-affinity sulphate influx across the plasma
membrane into the symplast
Simplified Arabidopsis root cross-section: cell specificity of sulphate transporters.

12. Subcellular Sulphate Movement
• Of fundamental importance to plant sulphur assimilation is
the effective delivery of sulphate to the plastid, the major
site of the assimilatory reductive pathway. In addition, the
requirement for cytosolic ion homeostasis leads to a flux of
surplus sulphate into the vacuole, which serves as an
internal nutritional reservoir

13. Long-distance Sulphate Transport

14. Sulphate Transport At The Whole Plant Level
• The radial transport from the epidermis through the
cortex and endodermis may be via plasmodesmata
without traversing plasma membranes.
• From the stele, sulphate has to be loaded into the
xylem.
• The nature of the efflux system for sulphate from plant
cells is unknown. There is no evidence that Group 1
and 2 sulphate transporters are able to act in the
reverse direction.
• A voltage-dependent anion channel, is activated by
sulphate and deactivated by nucleotides. Such a
channel may contribute to homeostasis, but may also
be involved in the delivery to the vascular system,
especially to the xylem from vascular parenchyma cells.

15. Group 2 sulphate transporters are responsible
for low-affinity sulphate transport.
• Expression of Arabidopsis Group 2 sulphate transporters have been
localized in the vascular tissues.
• The two Arabidopsis Group 2 transporters, AtSultr2;1 (Km 0.41 mM)
and AtSultr2;2 (Km 1.2 mM), have specific functions in the process
of vascular movement of sulphate.
• AtSultr2;1 is expressed
– In the leaves, in xylem parenchyma and phloem cells,
– but in the root in xylem parenchyma and pericycle cells,
• by contrast, AtSultr2;2 is localized specifically in the phloem of roots
and in vascular bundle sheath cells of leaves.
• The expression of AtSultr2;1 in the root pericycle and xylem
parenchyma cells may indicate an efflux of sulphate from the
endodermal cells leading to a high concentration of sulphate in the
apoplast of the vascular tissue. AtSultr2;1 would reabsorb this
sulphate and will optimize the amount of sulphate transferred to
the shoots during sulphate deficiency.

16. Group 2 sulphate transporters are responsible
for low-affinity sulphate transport. (Contd.)
The leaf phloem expression suggests a role in phloem
loading for sulphate transport to other organs.
• The leaf xylem parenchyma localization of AtSultr2;1 might
indicate absorption of sulphate from the xylem vessels or
reabsorption for further xylem transport.
• The localization of AtSultr2;2 in the root indicates a role in
sulphate transport via the phloem.
• In leaves, however, the expression in the bundle sheath
cells surrounding the vascular veins suggests the uptake of
sulphate released from xylem vessels at millimolar
concentrations for transfer to the primary sites of
assimilation in leaf palisade and mesophyll cells.

17. High Affinity Sulphate Transport
• Sulphate transport in vascular tissues is not restricted to
low-affinity transport.
• Under sulphate stress, plants are able to induce an
additional high affinity sulphate transport to maintain
vascular movement of sulphate under low sulphate
concentrations. The role of the AtSultr1;3 high affinity
transporter seems to be to mediate the inter-organ
transport of sulphate by specific expression exclusively in
the phloem.
• The up-regulation of the Arabidopsis Group 4
transporter, AtSultr4;1, under S-deficiency in roots as well
as in leaves, indicated the importance of vacuolar efflux of
sulphate regulated by the sulphur demand.

18. Regulation Of Sulphur Uptake And Transportation
• Sulphate transport consists of both constitutive and S-nutrition-
dependent regulated transport.
• A decreased intracellular content of sulphate, cysteine, and
glutathione is associated with increasing transporter activity.
• Gene and protein expression studies have confirmed that regulation
occurs predominantly at the level of the mRNA.
• A breakdown of the expression patterns of the genes of the
sulphate transporter family reflects a complex pattern of regulation:
– (i) expression of the Group 3 transporters with tissue/organ
specificity but no regulation by S-nutrition;
– (ii) cell-specific expression of some Group 1, 2, and 4
transporters under adequate S-nutrition which is up-regulated
by inadequate S-nutrition and
– (iii) cell/tissue-specific S-deficiency-related de-repression of
some Group 1 and 2 transporters.

19. REFERENCES
• Plant sulphate transporters: co-ordination of
uptake, intracellular and long-distance
transport : Peter Buchner, Hideki
Takahashi, Malcolm J. Hawkesford
(Journal of Experimental Botany, Volume 55,
Issue 404, August 2004, Pages 1765–
1773, https://doi.org/10.1093/jxb/erh206)