Ion Compartmentation

The sensitivity of cytosolic enzymes to salt is similar in both glycophytes and halophytes, indicating that the maintenance of high cytosolic K+/ Na+ ratio is a key requirement for plant growth under salt conditions (Glenn and Brown 1999). While dealing with Na+, the cell must also acquire nutrient K+. The Na+ ion is the foremost inorganic ion and a cheap source of osmoticum in the halo-phytes to maintain the osmotic balance under abiotic stresses. Under typical physiological conditions, plant cells require high K+ (100200 mM) and lower Na+ (less than 1 mM) and accordingly the high cytosolic K+/Na+ ratio to maintain the osmotic balance (Tester and Davenport 2003+ for proper functioning of the cell. Na+ competes with K+ for intracellular influx since both these are transported by common channels present on the membranes and, thus, subsequently increase K+ efflux from intracel-lular stores as against the higher Na+ stress built up outside the cell. To maintain a high K+/Na+ ratio in the cytosol, plant cell employs primary active transport, mediated by channels and co-transporters for Na+ extrusion and/or the intracellular compartmentalization of Na+ into the vacuole (Blumwald 2000). When halophytes are exposed to saline condition, a large increase in extracellular Na+ level establishes the Na+ electrochemical potential gradient more than the actual negative electrical membrane potential difference at the plasma membrane (-140 mV) which favor the passive transport of sodium ions from the outer environment inside the cell. Recently, uniporter or ion channel type transporters have been identified for the entry of Na+ into the cell; these are high-affinity potassium transporter (HKT), low-affinity cation transporter (LCT1), nonselective cation channels (NSCC) like cyclic nucleotide-gated channels (CNGCs) and glutamate-activated channels (GLRs) (Apse and Blumwald 2007). HKTs have been shown to function as Na+/K+ symporter and as Na+ selective uniporters (Horie and Schroeder 2004). In the process of elevated levels of Na+ outside the cell, the electrochemical gradient makes the sodium uptake passive; however, the efflux of Na+ outside cell is an active process and requires energy in the form of ATP. In this process, the Na+/H+ antiporter (NHX) present on the plasma membrane facilitates the Na+ efflux. This elec-troneutral exchange of sodium for protons to facilitate efflux is the only mode of transport that has been measured for efflux under physiological conditions (Apse and Blumwald 2007) . Besides the efflux of Na+ , some halophytes have developed mechanism to sequester the Na+ into the vacuoles as an efficient mechanism to avoid the toxic effects of Na+ in the cytosol. The transport of Na+ into the vacuoles is mediated by cation/H+ antiporters that are driven by the electrochemical gradient of protons generated by the vacuolar H+ translocating enzymes such as H+-ATPase and H+-PPiase (Gaxiola et al. 2007). These transporters play an important role in the sequestration of Na+ ions into the vacuole or exclusion outside the cell of the halophytes.

Abiotic stresses (Salinity, drought, extreme temperatures, toxic metals, etc.,)

Imbalanced water and nutrient uptake, stomatal closure, altered gaseous exchange and improper functioning of photosynthetic system

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