Ion homeostasis is a fundamental process that allows plants to maintain optimal ion concentrations under varying environmental conditions. High salt concentrations in soil disrupt ion homeostasis and hamper plant growth. Plants have developed strategies like accumulating potassium ions to retain water during drought stress and efficiently closing stomata to reduce water loss. Reactive oxygen species are produced under osmotic stress but plants use antioxidant enzymes and molecules to scavenge these harmful compounds and prevent cellular damage.

1. P:5 U:2
Vedanti S. Gharat
T. Y. B.Sc. Biotechnology
Roll No. 20
Exam seat no. 005018

2. IONIC HOMEOSTASIS IN PLANT STRESS
AND DEVELOPMENT
 Ion homeostasis is a dynamic process and a fundamental requirement for all
organisms.
 All living organisms have developed efficient systems to acquire and store
elements and maintain their cytosolic and organellar concentrations within a
specific physiological range that allows for normal development.
 The presence of high ionic concentrations in soil, especially of sodium
chloride, is a great problem for the plant's physiology.
 Salt tolerance may be a macro evolutionary self-destructive trait, as it is
gained frequently, but also is lost easily by reversal or extinction. This is
probably due to the high amount of energy required to maintain the ion
homeostasis under stress conditions, which hampers normal plant
development.

3.  Ion homeostasis determines pivotal functions such as the
compensation of the negative charges of macromolecules,
maintenance of electroneutrality, and the establishment of
cell turgor and volume.
 Ions are also essential components of biomolecules, such
as chlorophyll or hemoglobin, and they play a key role
such as stomatal aperture which controls transpirational
water loss, plant desiccation and cell elongation.
 In addition to direct effects of ions on plant cell
physiological function and homeostasis, a proper
membrane potential (inside negative) is maintained
through the maintenance of specific cation and anion
gradients across the cell membrane.

4. ION HOMEOSTASIS UNDER DROUGHT
STRESS
 The problem is increasing due to the rising water demand to feed
a growing population and the effects of climate change, especially
in drought-prone arid and semi-arid areas.
 In response to drought, plants activate a defense mechanism
devoted to the accumulation of water and potassium.
 High activity of K+ and Cl− uptake systems and a large root
system are desirable traits.
 In the leaf, K+ and Cl− allow for an efficient osmotic adjustment
of leaf cells, which is a key process to retain water within cells.
Efficient stomatal closure prevents excessive water loss and is
achieved by K+ and Cl− release from guard cells.

6. OSMOTIC HOMEOSTASIS
 Osmotic stress apparently reduces growth and productivity of
crop plants.
 One important response to osmotic stress is the accumulation
of the phytohormone abscisic acid (ABA), which induces
several responses to osmotic stress.
 Osmotic stress signaling consists of an ABA-dependent and an
ABA-independent pathway.
 Osmotic stress also caused increased ROS generation, which in
turn elicits various cellular signaling networks resulting into
physiological damage to plant cell.

7. Osmotic adjustment of cells helps maintain plant water
balance
 As the soil dries, its matric potential becomes more negative.
Plants can continue to absorb water only as long as their water
potential is lower (more negative) than that of the soil water.
 Osmotic adjustment, or accumulation of solutes by cells, is a
process by which water potential can be decreased without an
accompanying decrease in turgor or decrease in cell volume.
Osmotic Stress Induces Crassulacean Acid Metabolism
in Some Plants
Crassulacean acid metabolism (CAM) is a plant adaptation in
which stomata open at night and close during the day.

8.  Osmotic Stress Changes Gene Expression
Several genes coding for enzymes associated with osmotic
adjustment are turned on (up-regulated) by osmotic stress
and/or salinity, and cold stress. Several other genes that
encode well-known enzymes are induced by osmotic
stress.

9. REACTIVE OXYGEN SPECIES
SCAVENGING
 Induced osmotic stress resulted in overproduction of reactive oxygen species (ROS)
which was considered as a hallmark of plant stress response. To scavenge the toxic
consequences of ROS, plant deploys antioxidative mechanisms.
 Reactive oxygen species (ROS) were initially recognized as toxic by-products of
aerobic metabolism.
 ROS plays an important signaling role in plants, controlling processes such as growth,
development and especially response to biotic and abiotic environmental stimuli.
 The major members of the ROS family include free radicals like O•−
2, OH• and non-
radicals like H2O2 and 1O2.
 The ROS production in plants is mainly localized in the chloroplast, mitochondria and
peroxisomes. There are secondary sites as well like the endoplasmic reticulum, cell
membrane, cell wall and the apoplast.

10.  The cellular damages are manifested in the form of degradation
of biomolecules like pigments, proteins, lipids, carbohydrates,
and DNA, which ultimately combine in plant cellular death.
 To ensure survival, plants have developed efficient antioxidant
machinery having two arms
(i) enzymatic components like superoxide dismutase (SOD),
catalase (CAT), ascorbate peroxidase (APX), guaiacol
peroxidase (GPX), glutathione reductase (GR),
monodehydroascorbate reductase (MDHAR), and
dehydroascorbate reductase (DHAR);
(ii) non-enzymatic antioxidants like ascorbic acid (AA),
reduced glutathione (GSH), α-tocopherol, carotenoids,
flavonoids, and the osmolyte proline.
 These two components work hand in hand to scavenge ROS.

11. REACTIVE OXYGEN SPECIES (ROS) ARE
SYNTHESIZED IN TWO MECHANISMS
 The synthesis of ROS during the oxidative burst involves at least two
separate mechanisms: NADPH oxidase and extracellular peroxidases.
 The plasma membrane- spanning NADPH oxidase transfers an electron
from NADPH within the cell across the plasma membrane and uses this
to convert extracellular oxygen to the superoxide anion (O2).The
superoxide anions are then rapidly converted to hydrogen peroxide
(H2O2).
 Two extracellular (apoplastic) peroxidase enzymes (PRX) are also
sources of ROS production leading to the arrest of pathogen growth.
The ROS generated by the extracellular peroxidases likely have roles as
antimicrobial agents and in crosslinking specific polymers to the cell
wall to improve rigidity.

12. REFERENCE
 Ion Homeostasis in Plant Stress and Development by
https://www.frontiersin.org/
 Reactive oxygen species scavenging mechanisms
associated with polyethylene glycol mediated osmotic
stress tolerance in Chinese potato by
https://www.nature.com/
 Plant physiology by Taiz and Ziegler book.
 Plant stress biology notes.
 Images were taken from google.