OSMOPROTECTANTS AND OSMOREGULATION

Ashwani Kumar, Charu Lata, Pooja* and Anita Mann

ICAR-Central Soil Salinity Research Institute, Karnal – 132001, India

*ICAR- Sugarcane Breeding Institute, Regional Center, Karnal – 132001, India

Among abiotic stresses, drought and salinity stress are the major causes of historic

and modern agricultural productivity losses throughout the world. Both drought and salinity

result in osmotic stress that may lead to inhibition of growth. Consequently, most

angiosperms, including all major crop species, have a diminished capacity for Na+ transport

and tolerance to high salinity. New insights into the molecular mechanisms of Na+/K+

discrimination, Na+ extrusion and compartmentation, water transport, osmolyte biosynthesis

and function have led to genetically engineered plants with improved salt, drought, and cold

tolerance. Osmoprotectants are small molecules that act as osmolytes and help organisms

survive under extreme osmotic stress; examples include betaines, amino acids, and the sugar

trehalose. These molecules accumulate in cells and balance the osmotic difference between

the cell's surroundings and the cytosol. In extreme stress conditions, the cytosol and

osmoprotectants become a glass-like solid that helps stabilize proteins and cell membranes

from the damaging effects of desiccation. Compatible solutes have also been shown to play a

protective role by maintaining enzyme activity through freeze-thaw cycles and at higher

temperatures. Their specific action is unknown but is thought that they are preferentially

excluded from the proteins interface due to their propensity to form water structures. The

synthesis of glycine betaine, one of the most powerful osmoprotectants found in nature, also

plays an important role in the adaptation process of plants to a high-osmolarity environment.

Abiotic factors like drought, temperature, salt stress, etc. results in depletion of large number

food productions in today’s world and these considerations arouse strong interest in plant

abiotic stress responses.

What is Osmoregulation?

The active regulation of the osmotic pressure of an organism's fluids to maintain the

homeostasis of the organism's water content is known as Osmoregulation; this means it keeps

the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a

measure of the tendency of water to move into one solution from another by osmosis.

Osmosis is the movement of solvent molecules through a selectively-permeable membrane

into a region of higher solute concentration, aiming to equalize the solute concentrations on

the two sides. Higher the osmotic pressure of a solution, more water moves into the solution.

Pressure must be exerted on the hypertonic side of a selectively-permeable membrane to

prevent diffusion of water by osmosis from the side containing pure water. Various

metabolism involved in osmoregulation include polyoles, sugars, amino amino acids, proteins

and various other organic solutes. Act these metabolites play are important role in

maintaining osmotic potential of cytoplasm to maintain cell turgor. The concentration of

these compatible solutes/osmoprotectants within cell is maintained by either synthesis of

these compounds or combination of synthesis and degradation. Thus these osmolytes protect

one structure and maintain osmotic balance within the cell via continuous water influx.

Mechanism of Osmoregulation in Plants

While there are no specific osmo-regulatory organ in higher plants, the stomata are

important in regulating water loss through evapo-transpiration and on the cellular level,

vacuole plays an important role in regulatory the concentration of solvents in the cytoplasm.

The chemiosmotic regulatory systems of plant and fungal cells differ fundamentally from

those found in animal cells. In plants lack of plasma membrane Na+/K+-ATPases reported in

mechanism of osmoregulation. Thus, plants utilize H+-ATPases for primary extrusion or

sequestration of protons to generate H+ electrochemical gradients, which drive secondary ion

and nutrient transport processes via H+-symport/antiport systems. These H+-ATPase pumps

also modulate both intracellular and extracellular pH.

Except in the case of extreme halophytic archaebacteria, viable cellular processes in

animals, fungi, and plants depend upon the maintenance of low cytoplasmic Na+ and Cl−

concentrations and a high K+/Na+ ratio, because K+ counteracts the inhibitory effects of Na+

(and Li+). Like animal cells, most plant cells maintain cytosolic K+ concentrations in the

range of 100–200 mM and Na+ values in the low mM range (1–10 mM) up to a maximum of

100 mM (Maathuis and Amtmann, 1999). In contrast to K+, an essential cation for

maintaining biochemical interactions of the cytoplasm, Na+ is not essential for, but does

facilitate, volume regulation and growth in most plants. However, at high concentrations Na+

limits growth (Blumwald, 2000). Ironically, the productivity of irrigated agricultural regions

is generally many times greater than non-irrigated areas, yet irrigated crops are most

susceptible to detrimental salinity effects. Therefore, genetic engineering of crop plants to

improve their capacity for Na+ transport and sequestration is an important goal for meeting

the future food and fiber demands of a rapidly growing human population.

Many plants, such as extreme halophytes, display Na+ dependence for optimal growth

and development and have developed specialized structures such as salt glands and bladders

to accommodate high salt concentrations in tissues (Glenn et al., 1999). Others have

developed whole plant strategies for avoiding stress such as accelerated completion of

ontogeny. However, these specialized adaptations are lacking in most major crop species.

Furthermore, the precise impact of osmotic and ionic effects on cell growth, division,

phytohormone balance, and death in the context of the whole plant are complex and require

further investigation (Munns, 2001). Therefore, emphasis is placed on the molecular genetic

mechanisms controlling osmotic regulation at the cellular level, mainly because the action

and regulation of most osmoregulatory components has not been fully explored in the context

of the whole plant. Some channels associated for osmoregulation process are cation and anion

selective channels. Plant genomes encode multiple classes of ion channels that mediate K+

and Na+ transport (see Amtmann and Sanders, 1999; Schachtman and Liu, 1999;

Zimmermann and Sentenac, 1999; Czempinski et al., 1999;). Ion channels transport ions

from the soil solution, secrete ions into xylem sap, and participate in signaling (Zimmermann

and Sentenac, 1999). In contrast to carriers, ion channels conduct rapid, “downhill”

dissipation of transmembrane electrochemical gradients, often under the control of membrane

potentials that dictate the gating properties of the channel. Monovalent cation-selective

plasma membrane channels are divided into three classes depending on their

electrophysiological behavior and ion selectivity.

In plants generally betA and betB genes synthesized against drought stress and

produce BetB and BetA proteins. The deduced BetB and BetA proteins showed significant

similarity to soluble glycine betaine aldehyde dehydrogenases and membrane-bound choline

dehydrogenases, respectively, from a variety of organisms. Evidence is presented that BetA is

able to oxidize both choline and glycine betaine aldehyde and therefore can mediate both

steps in the synthesis of glycine betaine. Finally, glycine betaine synthesis in higher plants

involves a BADH in combination with a choline monooxygenase (Munns, R. 2001.)The

challenge for the coming decade will be to integrate information gathered at the molecular

genetic and cellular levels with the complexity of whole plant physiology. But to understand

genetic and molecular mechanism of osmoregulation is very difficult due to different types

of stress, any type of stress is converted ultimately in to drought stress and the genes

produced for any particular stress are different and encode different proteins to encounter

stress.

Refferences

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9. Wikipedia.org.osmoprotectants

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