ION HOMEOSTASISIN PLANT CELLS

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’

• 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

Significance of Ion homeostasis

Electrochemical potential-

-Concentration gradient

-Electric-potential gradient

-Hydrolytic pressure

Membrane permeability-The 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 ofspecific substances.

Active transport-Movement of solutes against a chemical potential and requires energy input.

Passive transport-Transport of solutes down a chemical gradient (e.g., by diffusion)

Membrane transport proteins

•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.

Channels

Types of 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

K⁺ 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.

Ca² ⁺ Channels

•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

Carriers

•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 transportrefers to ion transport involving the net movement of charge across the membrane.•Electroneutral transportas the name implies, involves no net movement of charge.

Primary Active Transport

•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 P-type ATPases, which are phosphorylated as part of the catalyticcycle that hydrolyzes ATP•H⁺ -ATPase molecules can be reversibly activated or deactivated by specific signals, such as light, hormones, pathogen attack etc.

The Plasma Membrane 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 thatthe pH of the vacuolar sap is typically about 5.5, while the cytoplasmic pH is 7.0 to 7.5.

The Vacuolar H ⁺ -ATPase

•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 abackup 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 byATP-binding cassette (ABC) transporters.•Examples- H ⁺ /K ⁺ ATPase, Ca² ⁺ ATPase

The H ⁺ -Pyrophosphatase

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”

•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 transportSymporter•the two substances are moving in the same directionAntiporter•to coupled transport in which the downhill movement of protons drives the active (uphill) transport of a solute in the opposite direction

Secondary Active Transport

Sucrose-H ⁺ Cotransporter

Sodium-Potassium Cotransporter

Sodium- Calcium Antiporter

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

•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

Techniques to Study Ion Homeostasis

•Excess salt in soil or in solutions interferes with several physiological and biochemical processes•Problems-ion imbalance, mineral deficiency, osmotic stress, ion toxicity and oxidative stress•The major ions involved in salt stress signaling-Na ⁺, K ⁺, H ⁺ and Ca² ⁺•It is the interplay of these ions, which brings homeostasis in the cell.

High salinity Stress

http://www.knowledgebank.irri.org/ricebreedingcourse/Breeding_for_salt_tolerance.htm

• 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

Salt stress on plant cells arise from the following

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

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.

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.

•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.

Role of Ca2+ in relation to salt stress

•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.

•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.

Strategies for developing salinity stress resistance plants

Fig:Algorithm for discovering stress tolerance determinants

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•BiochemistryAuther-strayerResearch 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 salinityStress 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

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