ION Transport In Higher Plants: Regulation Of Sodium, Chloride And Potassium Transport

Regulation of Potassium Transport in Plants

Discuss about the ION Transport in Higher Plants.

The natural environment has many causes of salinity. Higher plants endure salinity, at the seashores and estuaries where the salt water and fresh water meet. Furthermore, water is moved from the soil via translation and evaporation therefore leading to concentration of salts in the soil. The salinity affects cells of higher plants (Zeigar & Lincoln, 2009). The literature review examines the regulation of sodium, chloride and potassium transport in higher plants. Furthermore, it illustrates use of Goldman’s equation to calculate multiple ion concentrations or E values. In addition to that, it explains the role of chemiosmosis theory, ABC transporters, potassium channels, H+ ATP ases and the electrogenic ion pumps in ion transport.

Saline soils have a higher concentration of sodium, chloride, potassium ions. High concentration of saline soils injures plants and degrades the structure of soils therefore decreasing the permeability of water and soil porosity (William, 2014).  Incursion of salts into the soil causes deficits of water in leaves and prevents metabolism and growth of plants. Furthermore, high concentrations of sodium and chloride ions denature proteins and destabilize membranes (Dietrich, Kusker & Beker , 2010)

Potassium is an essential micronutrient in higher plants. It plays an important role in plant physiological processes such as plant growth and development. The concentration of potassium ions in plants is generally low and it is ever fluctuating. Plants perceive the external changes in K⁺ ions and generate physical and chemical signals in the cell. The signals can be moved across the plasma membrane into the cytosol. Consequently, this regulates downstream targets especially the K⁺ transporters and channels. Therefore, homeostasis due to potassium ions in the plant cells is modulated hence facilitating the adaptation of plants to K⁺ deficient conditions (Eduardo et al., 2014).

Every plant takes up sodium ions from the environment when exposed to salinity conditions; sodium is distributed within the cells, tissues, and organs once it is in the symplast. It can lower the water potential of the cells once it is in the symplast. However, it can also pose some toxic effects to the cells (Frans & Maathus, 2014). Therefore, the regulation of sodium ions is  important to the plant (Swarrendo & Usha, 2016). Several regulatory mechanisms have been developed depending on the signaling of calcium ions, cyclic nucleotides, hormones, reactive oxygen species, or the transcriptional and post translational changes in the gene and expression of proteins (Bob, Wilhelm & Russel, 2015).

Regulation of Sodium and Chloride Ions in Plants

The mechanisms of sodium ion transport in higher plants are similar. Na⁺ gain entry into the cell through several channels within the plasma membrane. As the sodium ions within the cytoplasm get to toxic levels above the threshold level, it is forced through the Na⁺/H+ antiports in the plasma membrane, which are energized by proton gradients. Furthermore, Na⁺ in the cytoplasm can be compartmentalized by antiports in vacuolar Na⁺/ H+. The transporters use energy provided by the proton gradient, it is created by H⁺ -PPiase and the vacuolar H⁺ ATP ase. This is the regulation of sodium ions by the higher plants (Rusells, Helen & Howard, 2012).

The Goldman’s equation is used in plant physiology to determine the potential of a membrane when there is no flow of ions from one side of the membrane to the next. The equation is named after Alan Lloyd, Bernard Kartz, and David Goldman of Columbia University (Verma, 2009).

The following is the Goldman’s equation for monovalent ions.

The Goldman’s equation determines the resting potential of the membrane. V_m is the membrane potential. The main contributors to the membrane potential are K⁺, Na⁺, and Cl^- ions. The Vm is measured in Volts. When the ion channels are closed, the relative permeability is zero. For instance, if K⁺ channels are closed, P_K= 0 (Verma, 2009).

The universal gas constant is represented by letter R. The constant is normally (8.314 J.K^-1.mol^-1).The temperature is represented by letter T. The SI unit for temperature is Kelvin.

The Faraday’s constant is represented by letter F, the constant is (96485 C.mol^-1). The potassium ions’ permeability value for the membrane is denoted as p_K.  However, permeability values lack a unit. Moreover, the permeability value for potassium ions has a reference value of 1, because in most living cells the permeability value for potassium is higher that of sodium and chloride ions (Verma, 2009).

The relative membrane permeability value for sodium ions is denoted by p_Na. The relative membrane permeability value for chloride ions is denoted by p_Cl. The concentration of potassium ions in extracellular fluids is denoted as [K⁺]_o. The concentration of potassium ions in intracellular fluid is denoted by [K⁺]_i .The concentration of sodium ions in extracellular fluids is denoted by [Na⁺]_o. The concentration of sodium ions in intracellular fluids is denoted by [Na⁺]_i .The concentration of chloride ions in extracellular fluid is denoted by [Cl^-]_o. Finally, the concentration of chloride ions in intracellular fluid is denoted by [Cl^-]_I (Verma, 2009).

Use of Goldman's Equation to Calculate Multiple Ion Concentrations or E Values

The Gold man’s equation calculates the membrane potential obtained through relative permeability values and concentration gradient values of the ions. Furthermore, open channels need to be available. The magnitude of permeability for a particular ion can be physiologically regulated and this determines the contribution of that ion to the membrane potential. Translocation of an ion down the electrochemical gradient results into the movement of the membrane potential to its equilibrium potential. Therefore, the larger the permeability of a particular ion results into a large contribution of the ion of the membrane potential (Conde, Manuela,  & Geros, 2011).

Chemiosmosis is the movement of ions across a semi permeable membrane via diffusion Ions transported include, Cl- or Na +. Movement of ions in chemiosmosis occurs down an electrochemical potential gradient (potential energy). Chemiosmosis is a type of diffusion, therefore, movement of ions across a membrane occurs from a region of high ion concentration to a region of low ion concentration (Hanson, 2007).

Chemiosmosis is important in the production of adenosine triphosphate (ATP). ATP is the main molecule used in the production of energy in the cell.ATP is produced via cellular respiration in eukaryotic mitochondria. NADH molecules and FADH2 are first derived from the citric acid cycle; electrons are then moved to an electron transport channel that releases energy. The energy released allows for the movement of protons (H+) down a protein gradient through chemiosmosis (Walter, 2010).

Consequently, the enzyme ATP synthase produces ATP. Proton flow down the concentration gradient turns the ATP synthase rotor and state therefore making it possible for a phosphate group to link up with adenosine diphosphate (ADP).Therefore, ADP is formed. ATP production during respiration is known as oxidative phosphorylation.ATP is finally created through phosphorylation of ADP via glucose (Walter, 2010).

Chemiosmosis plays a vital role in ATP production; the process enables living organisms to produce their energy. Dr. Peter D. Michelin first formulated the idea of ATP synthesis via chemiosmosis in 1961. Michelin was awarded a Nobel Peace Prize due to the contributions to Chemistry in 1976 (Heldt & Bridgit, 2010).

It is involved in transport of H⁺ ions across the cell membrane during ATP production. The most common type of chemiosmosis is production of ATP in cellular respiration. Eukaryotic organisms have a mitochondria. However, bacteria and archaea lack mitochondria, they also produce ATP during chemiosmosis via phosphorylation. The process has an electron transport chain. However, it takes place across the inner membrane of the microorganisms, since they lack mitochondria. Plants are involved in chemiosmosis via the production of ATP during photosynthesis (Caroline, Martin & Alyson 2008).

Chemiosmosis Theory

They consist of several subunits divided into transmembrane proteins and ATP ases. ATPase subunits use ATP energy obtained from hydrolysis. Furthermore, they provide energy used in the uptake or retake of substrates across membranes (Cornelia & Alexis, 2017)

ABC transporters belong to the ABC super family depending on organization and sequence of their ATP binding cassette domains. It can be noted that integral membrane proteins may have evolved several times on their own; therefore, they have different protein families.ABC uptake porters transport a wide variety of biosynthetic precursors, nutrients, vitamins and trace metals while exporters transport sterols, lipids, drugs and a wide variety of metabolites (Cornelia & Alexis, 2017)

Functions of ABC transporters

ABC transporters utilize ATP energy from hydrolysis. They are divided into three functional categories; importers allow for the transport of nutrients such as amino acids, ions, peptide sugars, and other hydrophilic molecules into the cells. Eukaryotes lack the importers .Exporters act as pumps that remove toxins from the cell. Exporters transport lipids and other polysaccharides from cytoplasm to periplasm in Gram-negative bacteria. The other group of ABC transporters is involved in the repair process of DNA and translation (Birgt, 2011).

ABC transporters are categorized into 13 subfamilies depending on their sizes, the overall amino acid sequence, and orientation. The largest subfamily is the P- glycoproteins that has 22 members. ABC transporters have the B subfamily that is found on the plasma membrane. In addition to that, they are heterologously expressed in E coli, Schizosaccharomyeces pombe and Saccharomyces cerevisae to determine the specificity of a substrate. ABC B transporters move phytohormone indole -3 –acetic acid (IAA) or auxin. This hormone is an important regulator of plant growth and development (Birgt, 2011).

They are found in all living organisms. In addition to that, they form selective pores that are potassium selective in the cell membrane. Moreover, they are found in many types of cells and control a variety of functions in plants (Carraretto et al., 2015).

They conduct potassium ions down the electrochemical gradient. This transport of ion occurs either rapidly to the rate of diffusion of potassium ions in bulk water or selectively by excluding sodium. Potassium channels set and reset the resting potential in cells. They regulate cell processes such as hormone secretion (Dreyer & Uozumi, 2011).

Potassium channels are divided into four classes. First, are the potassium channels that are calcium activated. They open or close depending on the presence of calcium ions. Second, are potassium channels that are inwardly rectifying, these pass current easily in an inward direction into the cell. Third, are known as tandem pore ions. They open constitutively and they have a high basal activation. Fourth, are the voltage gated potassium channels that open or close depending on the response of transmembrane voltage (Conde, Manuela  & Geros, 2011).

Functions of Chemiosmosis

Two processes regulate the flux of ions via potassium channel pores, the processes include inactivation and gating. The opening or closing of a channel in response to external stimuli is known as gating. When a current from an open channel is stopped and the channel inhibits conducting, this is known as inactivation. The two processes regulate conductance of potassium channels(Vanessa, C., Teardo, E., Elide, F. & Elizabeth, 2013).

These blockers stop the flow of potassium ions through the channels. They compete with the binding of potassium within the selectivity filter or they bind outside the filter to stop conduction of ions. An example is the ammonium ions, which bind at the central cavity of the channel or the extracellular face. Potassium ion channels can also be blocked by barium ions that bind with a high affinity in the selectively filter (Carraretto et al., 2015).

Plasma membrane H⁺ATP ases (P- Type).

This is a protein that belongs to the hydrolase family, especially hydrolases that act on acid anhydrides. The enzyme catalyzes movement of substances across the transmembrane. The protein is a member of the P – type ATPase family. The enzyme belongs to the ATP phosphohydrolases class also referred to as H⁺ exporting. The protein has several other names such as proton ATPases or a proton pump (Yang, 2016).

The proton pump creates an electrochemical gradient in plasma membranes of fungi, plants, protists or prokaryotes. Proton gradients that are created drive secondary transport processes. Therefore, this is very important for the uptake of metabolites and plant response to the external environment (Vadim & Frans, 2010).

The H⁺ATPases are specifically for protists, fungi and plants while Na⁺ , K⁺ ATP ases are concisely meant for animal cells. The two groups of P- type ATP ases perform complementary functions in plants and animal cells. Nevertheless, they do not belong to the same subfamily, their function includes creation of an electrochemical gradient that is used as source of energy for secondary transport (Vadim & Frans, 2010).

Structural studies of proton pump H+ ATPases are scarce. The only structural information available for the fungal H + ATPase is from Neurosporacrassa, it includes low-resolution2D H⁺ ATP ase crystals. Structure of Arabidopsis thaliana is available for the plants. The crystal structure of the plasma membrane H+ ATP ases is obtained from 3D crystals. The crystals have a resolution of 3.6A. The structure identifies three cytosolic domains that correspond to the nucleotide binding (N), phosphorylation (P), and actuator (A) domains (Vadim & Frans, 2010).

Moreover, the structure shows the ATP ase to exist in a quasi-occluded E1 state. The catalytic unit fold reveals structural similarity of a high degree to the SR Ca²⁺ ATP ase and Na⁺ K⁺ ATP ase. The domain arrangement is similar to SR Ca 2+ ATP ase (Vadim & Frans, 2010).

Regulation of the activity of PM H+ ATPase is very important to the plant. Over expression of the protein is compensated by the down regulation of activity. The removal of an isoform demands compensation by redundancy and augmented activity of other isoforms through increasing post-translational modifications. Auto inhibition of the PM H+ ATP negatively regulates activity of the pump .Therefore, it deactivates the enzyme into a low activity state, in this state, ATP hydrolytic activity is partially uncoupled from ATP hydrolysis. The auto inhibition is achieved in terminals of the protein. Communication between the two termini allows for the control of the pump activity. The PM H⁺ ATP as is the first ATP ase in which both C and N termini takes part in regulating protein activity (Carraretto et al. 2015).

The plasma membrane ATP ases are found all over the plant. Nevertheless, some plant cells have higher concentrations of PM H⁺ ATP ases. These types of cells are used in active transport. Furthermore, they also accumulate solutes from the external environment. Studies on the roles of these proteins were formulated from genetic research studies on the Arabidopsis thaliana , a multigene family with 12 different ATP ase genes (Birgt, 2011).

H+ ATP ases are involved in some important physiological processes such as loading of the phloem: The phloem tissue transports organic compounds over a long distance. Furthermore, it transports sugar from the leaves to other regions of the plant. H⁺ ATP ases power the transport of sugar via the sucrose / H⁺ transporters. In addition to that, it is very important in loading sucrose to the phloem (Birgt, 2011).

The second function is the uptake of solids by the roots. H⁺ ATP ase provides energy for the uptake of solid nutrients from the soil into the roots. Furthermore, it is involved in loading of solutes into the xylem that are used in transport of micronutrients and water ( Birgt, 2011).

The third function is in tip-growing systems. Root hairs and pollen tubes are good examples of tip growing systems in plants. In these systems, the cell only grows in one direction, a direction that is controlled by a proton gradient. The protons go through the extreme tip and they are pumped out just below the tip ( Laurence, Xavier & Adain, 2003).

The ATPase affects the size of aperture in stomata. The pore of a stomata controls Carbon dioxide diffusion into the leaves during photosynthesis. Two guard cells from the pore   control size of the stomatal pore. They swell as they respond to the activity of H+ ATP ase. Opening or closing of the pore is controlled partially by regulating H ⁺ ATP ase (Hamilton, Schlegel, & Haswell, 2016).

Movement of organs in a plant is controlled by motor cells that change the cell turgor. (Wang,et al., 2015). The movements of organs include solar tracking to increase orientation of leaves to photosynthesis, and reactions of some plant parts in plant species such as the carnivorous plants. The process of swelling and shrinking takes place via large fluxes of water and ions (Francis & Xavier, 2017).  The activation of H⁺ ATPase leads to the hyperpolarization of plasma membrane and opening of potassium channels that are voltage sensitive. The influx of K+ leads to the uptake of water and increase in turgor (Birgt, 2011).

The next function is osmotolerance and salt. The salt conditions are stressful to the cell; the first stress is loss of turgor because of hypertonicity of the extracellular medium. Next, is the effect of toxic ions on metabolism. Plants have therefore developed defense mechanisms against the effects of salinity. When a plant is facing salt stress, Na/ H+ antiporter is engaged, it is powered by the H+ ATP ase action (Birgt, 2011).

These are primary active transporters, which hydrolyze ATP. Furthermore, they use the energy derived from hydrolysis of ATP to facilitate ion transport across biological membranes. Consequently, net charges are transported across membranes. An example of such a pump is the Na⁺ /K⁺ ATP ase also known as the sodium pump.It transports three sodium ions out of the cell and two potassium ions into cells during each transport cycle. Therefore, it leads to movement of a net positive charge out of the cell therefore making the process electrogenic (Frans & Maathus,  2014).

Higher plant cells have vacuoles that play a vital role in homeostasis. High energy metabolites pump protons into the vacuole thus establishing a proton electrochemical gradient that allows for the transport of ions. Regulation of potassium, sodium and chloride ions in plant cells is important for growth and development. Furthermore, chemiosmosis is a form of diffusion that enables the transport of ions from a region of higher concentration to a region of lower concentration. Ions in plant cells need to be regulated in order to attain homeostasis and allow for better growth.

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