2. Homeostasis in Plants
• Plant cells work best if they
have the correct
– Temperature
– Water levels
– Ion concentration
• The maintenance of a constant environment in the
plant body is called Homeostasis
• Control of the ion concentrations across the plant cell is
called ‘Ion Homeostasis’

3. Significance of Ion homeostasis
• Uptake of nutrients is in the
form of ions
(NO₃⁻,NH₄⁺,PO₄³⁻,K⁺,Ca²⁺,SO₄²⁻,
Zn²⁺,Fe²⁺,Mn²⁺,Cu²⁺,H₃BO₃,
MoO₄²⁻,
• Ion concentration maintains
osmotic and pH homeostasis
• Control of the ion
concentrations in the cytosol is
important for the regulation of
metabolic enzymes
• Ion concentrations are
controlled by passive (dashed
arrows) and active (solid
arrows) transport processes

4. Electrochemical
potential-Concentration gradient
-Electric-potential gradient
-Hydrolytic pressure

5. Membrane permeabilityThe extent to which a
membrane permits or restricts
the movement of a substance.
The permeability depends
on-the chemical properties of the
particular solute
-the lipid composition of the
membrane
-the membrane proteins that
facilitate the transport of
specific substances.

6. Active transportMovement of solutes against a
chemical potential and requires
energy input.
Passive transportTransport of solutes down a
chemical gradient (e.g., by
diffusion)

7. Membrane transport proteins

8. Channels
•Transmembrane proteins formed of glycoproteins
•Formed by aggregation of subunits made of proteins into cylindrical
configuration forming a pore in the centre
•Function as selective pores
•Transport depends on electrical potential or concentration gradient
•Transport specificity-The type ion crossing through the channels
depends on the size of a pore, the electrochemical configuration of
the protein subunits lining on the pore
•Transport is always passive
•Transport Ions or water
•Rate of trensport-10⁸ ions per second
•Gates that open and close the pore in response to external signals
such as- voltage changes, hormone binding, or light.

9. Types of Channels

10. K⁺ Channels
•The most abundant inorganic cation
•Essential mineral nutrient
•Osmoticum-cellular hydrostatic
pressure
•Enzyme activation
•Stabilization of protein synthesis
•Formation of membrane potential
•Maintenance of cytosolic pH
homeostasis
•Subdivided into two channel classes:
•Non-voltage-gated Or inward K⁺
channels open only at more negative
potentials for inward diffusion of K⁺
•Voltage gated Or outward K⁺ channels
open only at more positive potentials

11. Ca² ⁺ Channels
•Calcium signal transduction is a
central mechanism by which
plants sense and respond to
endogenous and environmental
stimuli.
•Cytosolic Ca²⁺ elevation- Ca²⁺
influx through Ca²⁺ channels in
the plasma membrane
•Ca²⁺ release from intracellular
Ca²⁺ stores
•Function in various cellular
responses, including hormone
responses, plant–pathogen
interaction, symbiosis, salt stress,
light signaling and circadian
rhythm.

12. Carriers
•Highly selective
•Binding causes a conformational change in the Protein
•Transport is complete when the substance dissociates from the
carrier’s binding site.
•Typically, carriers may transport 100 to 1000 ions or molecules
per second (10⁶ times slower than transport through a channel)
•Passive transport by a carrier is sometimes called facilitated
diffusion

13. Primary Active Transport
•Directly Coupled to Metabolic or Light
Energy
•The membrane proteins that carry
out primary active transport are called
Ion pumps
•Pumps are energy dependant
channels
•Electrogenic transport
refers to ion transport involving the net
movement of charge across the
membrane.
•Electroneutral transport
as the name implies, involves no net
movement of charge.

14. The Plasma Membrane H⁺-ATPase
•Active transport of H⁺ across the
plasma membrane creates
gradients of pH and electric
potential that drive the transport
of many other substances (ions and
molecules)
•H⁺ -ATPases and Ca²⁺ -ATPases are
members of a class known as Ptype ATPases, which are
phosphorylated as part of the
catalytic
cycle that hydrolyzes ATP
•H⁺ -ATPase molecules can be
reversibly activated or deactivated
by specific signals, such as light,
hormones, pathogen attack etc.

15. The Vacuolar H ⁺ -ATPase
•Drives Solute accumulation into
Vacuoles
•More closely related to the F-ATPases
of mitochondria and chloroplasts
•They are large enzyme
complexes, about 750 kDa, composed
of at least ten different subunits
•Vacuolar ATPases are electrogenic
proton pumps that transport protons
from the cytoplasm to the vacuole and
generate a proton motive force across
the tonoplast.
•This gradient accounts for the fact that
the pH of the vacuolar sap is typically
about 5.5, while the cytoplasmic pH is
7.0 to 7.5.

16. The H ⁺ -Pyrophosphatase
•A single polypeptide that has a molecular
mass of 80 kDa.
•Harnesses its energy from the hydrolysis of
inorganic pyrophosphate (PPi).
•The synthesis of the vacuolar H ⁺ -PPase is
induced by low O2 levels (hypoxia) or by
chilling
•The vacuolar H ⁺ -PPase might function as a
backup system to maintain essential cell
metabolism under conditions in which ATP
supply is depleted because of the inhibition
of respiration by hypoxia or chilling.
•Large metabolites such as
flavonoids, anthocyanins and secondary
products of metabolism are sequestered in
the vacuole.
•These large molecules are transported into
vacuoles by
ATP-binding cassette (ABC) transporters.
•Examples- H ⁺ /K ⁺ ATPase, Ca² ⁺ ATPase

17. Ca² ⁺ ATPase
Fig. Topology of plant calcium pump
•Belong to the superfamily of P-type ATPases comprising also the
plasma membrane H ⁺ -ATPase of fungi and plants
•Ca² ⁺ signal is not restricted to the changes in the Ca² ⁺
concentration but is also presented by its spatial and temporal
distribution
•All these characteristics are known as
“calcium signature”

18. Secondary Active Transport
•Transport solute against gradient of
electrochemical potential by coupling
of the uphill transport to the downhill
transport
•A membrane potential and a pH
gradient are created at the expense of
ATP hydrolysis.
•The proton motive force generated by
electrogenic H ⁺ transport is used in
secondary active transport
Symporter
•the two substances are moving in the
same direction
Antiporter
•to coupled transport in which the
downhill movement of protons drives
the active (uphill) transport of a solute
in the opposite direction

19. Sucrose-H ⁺ Cotransporter

20. Sodium-Potassium Cotransporter

21. Sodium- Calcium Antiporter

22. Overview of the various transport processes on the plasma membrane and tonoplast
of plant cells.

23. Techniques to Study Ion Homeostasis
•Photochemical tools for studying metal ion signaling and
homeostasis
•Patch-clamp techniques to study cell ionic homeostasis under
saline conditions
•Channel cloning, mutagenesis, and expression techniques
•Antibodies as tools for the study of the structure and function of
channel protein
•Electron microscopy

24. High salinity Stress
•Excess salt in soil or in
solutions interferes with several
physiological and biochemical
processes
•Problemsion imbalance, mineral
deficiency, osmotic stress, ion
toxicity and oxidative stress
•The major ions involved in salt
stress signalingNa ⁺, K ⁺, H ⁺ and Ca² ⁺
•It is the interplay of these
ions, which brings homeostasis
in the cell.
http://www.knowledgebank.irri.org/ricebreedi
ngcourse/Breeding_for_salt_tolerance.htm

25. Salt stress on plant cells arise from the following
• Disruption of ionic equilibrium: Influx of Na ⁺ dissipates the
membrane potential and facilitates the uptake of Cl¯ down
the chemical gradient.
• Na ⁺ is toxic to cell metabolism and has deleterious effect
on the functioning of some of the enzymes.
• High concentrations of Na ⁺ causes osmotic
imbalance, membrane disorganization, reduction in
growth, inhibition of cell division and expansion.
• High Na ⁺ levels also lead to reduction in photosynthesis
and production of reactive oxygen species
Fig. Yellowing and "burning" on tips of
leaves of orange tree, sensitive to both
salinity and sodium.
http://www.salinitymanagement.org/Salinity%
20Management%20Guide/sp/sp_7b.html

26. Maintenance of ion homeostasis and the possible
roles of ion transporters
•Ion homeostasis in saline
environments is dependent on
transmembrane proteins that mediate
ion fluxes, including H⁺ translocating
ATPases and pyrophosphatases, Ca²⁺ATPases, secondary active
transporters, and channels.
•A role for ATP-binding cassette (ABC)
transporters in plant salt tolerance has
not been elucidated, but ABC
transporters regulate cation
homeostasis in yeast which is very
similar to plants.

27. Osmolytes/Osmoprotectants
Listed are common osmolytes involved in either osmotic adjustment or in the protection of
structure. In all cases, protection has been shown to be associated with accumulation of these
metabolites.

28. Role of Ca2+ in relation to salt stress
•Externally supplied Ca²⁺
reduces the toxic effects of
NaCl, presumably by
facilitating higher K⁺/Na ⁺
selectivity
•SOS (salt overly sensitive)
pathway results in the
exclusion of excess Na+
ions out of the cell via the
plasma membrane
Na ⁺ /H ⁺ antiporter and
helps in reinstating cellular
ion homeostasis.

29. •The enhanced activity of H ⁺ /ATPase proton pumping activity would furnish plasma
membrane Na ⁺ /H ⁺ antiporter with a driving force to expel Na ⁺ out of the cytoplasm
•The NHX-type antiporters i.e. Na ⁺ /H ⁺ located in tonoplast have been reported to
increase salt-tolerance in many plant species by driving Na+ accumulation in vacuole
Fig. Cellular homeostasis established after salt (NaCl) adaptation.

30. Strategies for developing salinity stress
resistance plants
•Conventional breeding
•In vitro selection techniques
-Somaclonal variation
-Mutagenesis
•Genetic engineering
New varieties for Salt
tolerance developed in
following crops:
Canola or rapeseed,
Chickpea, Cotton, Rice,
Sorghum, Soybean, Sugar
cane, maize etc.
Fig:Algorithm for discovering
stress tolerance determinants

31. REFERENCES
Books
•Plant physiology(fifth edition)
Authers-Lincoln Taiz and Eduardo Zeiger
•Essential cell biology(second edition)
Authers-alserts, hopkin, johnson, lewis, raff, robert and walter
•Biochemistry
Auther-strayer
Research papers
•Shilpi Mahajan, Narendra Tuteja ‘Cold, salinity and drought stresses: an
overview’, Archives of biochemistry and biophysics 444 (2005) 139–158
•R. K. Sairam, Aruna Tyagi ‘Physiology and molecular biology of salinity
Stress tolerance in plants’, Current Science, vol. 86, no. 3, 10 february 2004
•Paul Hasegawa, Jian-Kang Zhu ‘Plant cellular and molecular responses to high
salinity’, Annu. Rev. Plant physiol. Plant mol. Biol. 2000. 51:463–99
•Fabien Jammes, Heng-Cheng hu ‘Calcium-permeable channels in plant cells’, FEBS
journal 278 (2011) 4262–4276
•Ingo Dreyer, Nobuyuki Uozumi ‘Potassium channels in plant cells’, FEBS journal 278
(2011) 4293–4303
•Katarzyna Kabała, Grayna Klobus ‘Plant ca2 ⁺ -ATPases’, ACTA PHYSIOLOGIAE
PLANTARUM Vol. 27. No. 4a. 2005: 559-574
•Michael G Palmgren ‘Plant plasmamembrane h ⁺ -ATPases: Powerhouses for nutrient
uptake’, Annu. Rev. Plant physiol. Plant mol. Biol. 2001. 52:817–45