1. Resistance Mechanism and
Molecular Responses to the Salinity
Presented by - Mr. Suraj Singh
Enrollment Number - 210504014
Programme - M.Sc. In Life Sciences
Semester - II
Course - Plant Physiology
Name of the Guide - Dr. Seema Rawat & Dr. Anju Pappachan

2. Introduction
The term "salinity" refers to the concentrations of salts in water or
soil.
Salinity can take three forms, classified by their causes: primary
salinity, secondary salinity , and tertiary salinity.
1. Primary salinity (aka natural salinity)
Primary salinity is caused by natural processes such the
accumulation of salt from rainfall over many thousands of
years or from the weathering of rocks
2. Secondary salinity (aka dry land salinity)
Secondary salinity is caused where groundwater levels rise,
bringing salt accumulated through 'primary' salinity processes
to the surface
3. Tertiary salinity (aka irrigated salinity)
Tertiary salinity occurs when water is reapplied to crops over
many cycles, either directly or by allowing it to filter into the
groundwater before pumping it out for re-application.
Each time the water is applied, some of it will evaporate and the
salts in the water remaining will become more concentrated;
very high salt concentrations can result from multiple cycles of
reuse. 2
Class Electric
conductivity
- ECe (dS/M)
Description
Salt free ECe <4 Crop growth remains
uniform.
Slightly saline 4<ECe>8 Crop growth is
uneven & patchy.
Moderately
saline
8<ECe>15 Plant growth is very
patchy & restricted.
Strongly
saline
ECe>15 Salt tolerant crops
are possible to grow.
Table : It shows the classes of saline soil & their
correlation with plant growth

3. TYPES OF PLANTS
(On the basis of their response to the salinity)
1. Halophytes
Halophytes are native to saline soils
where they can grow satisfactorily,
compete with other species in the
same habitat and complete their life
cycle.
Example : Thellungiella halophila
2. Non-halophytes or
glycophytes.
Non-halophytes or glycophytes are
plants which cannot resist salts to
the same degree as halophytes.
Example : Arabidopsis thaliana
Fig.1. Thellungiella halophila, a halophytic relative of Arabidopsis,
tolerates high levels of salt stress. (a) Plants before NaCl treatment. (b)
Plants after watering with 500 mM NaCl for 10 days. For comparison,
Arabidopsis plants (left) were grown in the same pot as T. halophila
(right).
3

4. Thellungiella halophila Generates New Knowledge on Essential and
Critical Components of Abiotic Stress Tolerance in Plants
4
• In September 2004, a group of researchers from around the globe gathered
in Paris to discuss the prospects of a small cruciferous plant to become a
new model for molecular research into plant stress tolerance (Bressan et al.,
2001; Zhu, 2001; Amtmann et al., 2005).
• The plant under question was Thellungiella, a member of the Brassica
family and a close relative of the queen of plant models, Arabidopsis
thaliana (Bressan et al., 2001; Zhu, 2001; Warwick et al., 2006).
• Thellungiella, or ‘salt cress’ as it is also referred to, resembles its famous
cousin in many assets that make this plant so popular with geneticists and
molecular biologists, including short lifecycle, small genome
(approximately twice the size of the Arabidopsis genome), and copious
seed production.
• In addition, Thellungiella is a real ‘tough cookie’, able to grow and
reproduce under conditions of extreme cold, drought, and salinity.
• Based on the experience gathered with Thellungiella in the individual
laboratories, the Paris workshop participants decided to recommend
Thellungiella as an extremophile plant
• model system and to promote its usage by sharing protocols, seed stocks,
and BAC and cDNA libraries (www.thellungiella.org).

5. Fig.2. The three aspects of salt tolerance
in plants (homeostasis, detoxification and
growth control) and the pathways that
interconnect them; homeostasis is broken
down into ionic and osmotic
homeostasis. The SOS pathway mediates
ionic homeostasis and Na+ tolerance. A
mitogen activated-protein kinase
(MAPK) cascade similar to the yeast
HOG1 pathways is proposed to mediate
osmotic homeostasis. The two primary
stresses, ionic and osmotic stresses, cause
damage or secondary stresses such as
oxidation. Leaf type stress proteins such
as RD29A are proposed to function in the
detoxification or alleviation of damages.
CBF/DREB transcription factors mediate
some of the stress protein gene
expression in response to secondary
stresses caused by high salt
concentrations, cold, drought or abscisic
acid (ABA). The ionic homeostasis,
osmotic homeostasis and detoxification
pathways are proposed to feed actively
into cell division and expansion
regulation to control plant growth.
5

6. 6
Figure 1. Comparative flowchart model of the mechanism of response under salinity stress in glycophyte (Arabidopsis thaliana)
and halophyte (Thellungiella salsuginea and Cakile maritima) plants to regulate K+/Na+ ratio. HAK5, High Affinity K+
transporter 5. NHX, Sodium-Proton exchanger. SOS1, Salt Overly Sensitive.

7. Plant Responses towards Salinity
Plants grown under saline conditions may encounter three types of stress.
1. One of the most striking effects of high salt content is reduction in plant water
uptake. Water is absorbed by the roots only along an osmotic gradient between
soil and plant. Increasing salt content in soil solution lowers the osmotic
potential (Munns & Tester, 2008). Thus, high salt levels in soils lead to
water‐deficit stress for plants.
2. Salts may cause severe ion toxicity, because Na+ is not readily sequestered in
vacuoles (Nawaz, 2007).
3. Salinity increases the production of reactive oxygen species (ROS) such as
H2O2, O2− and OH-, in plant tissues, which may lead to cell membrane damage
and hence cell death (Hernandez et al., 2001).
7

8. Fig. 3. Mircroscopic image of root tips of durum wheat grown in absence (A, B) or presence of 100 mM NaCl (C, D)
8

9. Mechanisms of salinity tolerance — An
overview
Salt tolerance mechanisms can be classified into three main categories:
1. Osmotic tolerance, which is regulated by long distance signals that reduce shoot
growth and is triggered before shoot Na+ accumulation
2. Ion exclusion, where Na+ and Cl- transport processes in roots reduce the
accumulation of toxic concentrations of Na+ and Cl- within leaves; and
3. Tissue tolerance, where high salt concentrations are found in leaves but are
compartmentalized at the cellular and intracellular level (especially in the vacuole).
But a plant may have ion exclusion as its primary tolerance mechanism at moderate
salinity level, but then has tissue tolerance as its main tolerance mechanism when
the exclusion processes are ‘swamped’ at high salinity level.
9

10. Fig.4. The three main mechanisms of salinity tolerance in a crop plant.
10

11. 1. Ion exclusion
• The high affinity potassium transporter (HKT) gene family and the salt overly
sensitive (SOS) pathway have both been implicated in having an important role
in regulating Na+ transport within a plant.
• Manipulation of the expression of these genes has been frequently reported to alter
accumulation of Na+ in the shoot.
• Of the two families of HKTs, the HKT1 have the greatest potential for improving
the salinity tolerance of crops.
• An HKT2 has been reported to increase salinity tolerance, by unkonwn
mechanism but not through Na+ exclusion.
11

12. Role of Vacuole
• There are two mechanisms used by the plant to exclude salt reaching the leaf from the
cytoplasm. Salt ions can accumulate in the apoplast or move to the vacuole.
• Build‐up of salt ions in the apoplast leads to an increase in the osmotic gradient between the
inside and outside of the cell.
• To adjust a thermodynamic equilibrium, water inside the cell diffuses to the intercellular
spaces, leading to progressive cellular dehydration and, eventually, cell death. Therefore,
salt-tolerant traits are more associated with the amount of salt ions that accumulate in the
cell vacuole (Volkmar et al., 1998).
• Salt ions pass across the cell membrane and the cytoplasm to enter the vacuole. The
quantity of salt ions that pass across the cell membrane must not be more than the amount
deposited into the vacuole to minimize the risk of salt hazard (Volkmar et al., 1998).
• The amount of salt flow is controlled by the storage capacity of the root and the salt
concentration in the soil solution. Therefore, salt-tolerant plants require an active vacuolar
compartmentation capacity to store the high amount of salt ions delivered from the xylem
to the leaf (Lauchli & Epstien, 1990). 12

13. Fig.7. Salt stress signaling pathways of rice.
The CBL-CIPK calcium signaling network plays
a critical role in sensing salt-induced Ca2+ signals
and in the regulation of Na+/K+ ion homeostasis.
Salt-induced elevation in cytosolic Ca2+ activates
the CBL/CIPK protein kinase complexes which
modulates the activity of Na+/H+ antiporter
OsSOS1 and K+ transporter OsAKT1. OsCCD1
can bind cellular Ca2+ and enhance transcription
levels of transcription factors under high salinity.
The OsGORK governed by ATP is a K+ efflux
channel under salinity stress. OsRBOHA/F are
involved in the production of ROS at plasma
membrane, and accumulated ROS is scavenged
by OsAPXs which is regulated by OsCPK12.
MAP kinase cascades, consisting of
OsMAPKKK63, OsMKK1, and OsMPK4 are
involved in the ROS signal transduction pathways
triggered by salinity stress. Activated OsMPK4
transduce signals to downstream transcription
factors. OsSAPKs can be activated by ABA
signal and directly phosphorylate downstream
transcription factors. In the vacuole, OsNHXs
energized by either V-ATPase or V-PPase H+
pumps, slow-vacuolar (SV) ion channels and fast-
vacuolar (FV) ion channels are involved in the
regulation of ion homeostasis under high salinity.
The dashed lines indicate uncertain pathways that
remain to be identified. Solid arrows indicate
established direct regulation. 13

14. 2. Shoot tissue tolerance
• Upto date, three main mechanisms contributing to shoot tissue tolerance have been targeted:
1. Accumulation of Na+ in the vacuole,
2. Synthesis of compatible solutes and
3. Production of enzymes catalyzing detoxification of reactive oxygen species.
Increasing the abundance of vacuolar Na+ /H+ antiporters (NHX), vacuolar H+
pyrophosphatases (e.g. AVP1), proteins involved in the synthesis of compatible solutes such
as proline (P) and glycinebetaine (GB) and enzymes responsible for the detoxification of
ROS.
Although there is still uncertainty about the primary ions being transported by NHX proteins
in plants, and a new role has recently been proposed for AVP1, salinity tolerant plants appear
to have been developed by the overexpression of NHX and vacuolar pyrophosphatase genes.
14

15. Shoot tissue tolerance (contin.)
• It requires compartmentalization of Na+ and Cl− at the cellular and intracellular level to avoid
toxic concentrations within the cytoplasm, especially in mesophyll cells (R Munns & Tester,
2008) and synthesis and accumulation of compatible solutes within the cytoplasm.
• Compatible solutes play a role in plant osmotolerance by protecting enzymes from
denaturation, stabilising membrane or macromolecules or playing adaptive roles in
mediating osmotic adjustment (Ashraf & Foolad, 2007).
• Compatible solutes are able to replace water at the surface of proteins or membranes
(Hasegawa, Bressan, Zhu, & Bohnert, 2000) and also protect cellular structures through
scavenging ROS (Hasegawa et al., 2000; Zhu, 2001).
• Among the best known compatible solutes, P and GB have been reported to increase greatly
under salt and drought stresses (R. Munns, 2002; Sakamoto & Murata, 2002) and constitute the
major metabolites found in under salt stress in Graminaceous plants (Ashraf & Foolad, 2007).
15

16. Fig.6. Chemical structures of some important compatible compounds. 16

17. Shoot tissue tolerance (contin.)
• Proline normally accumulates in large quantities in response to drought or salinity stress (Szabados &
Savouré, 2010).
• Proline act as
• an osmolyte for osmotic adjustment,
• contributes to stabilizing sub-cellular structures (e.g. membranes and proteins),
• scavenging ROS,
• buffering cellular redox potential under stress conditions,
• also rapid breakdown of proline upon relief of stress may provide sufficient reducing agents that
support mitochondrial oxidative phosphorylation and generation of ATP for recovery from stress
and repairing of stress induced damages (Carillo, Mastrolonardo, Nacca, Parisi, Verlotta, & Fuggi,
2008).
• GB act as
• an osmoregulator, help in interacting both hydrophilic and hydrophobic domains of
macromolecules, such as protein complexes (Chen and Murata 2008) and enzymes, stabilising their
structures and activities, and maintaining the integrity of membranes against the damaging effects of
excessive salt, cold, heat and freezing (Sakamoto and Murata 2002).
• also plays a role as a scavenger of ROS generated during these various stresses ( Ashraf and Foolad
2007).
17

18. 18

19. 19

20. 3. Osmotic tolerance
• Upto date we are not aware of any specific candidate genes for osmotic tolerance,
although some genes highlighted above may be involved.
• Differences in osmotic tolerance are likely to involve long-distance signaling,
control of cell cycle and processes involving perception of signals from the roots
in the shoots.
• The introgression of TmHKT1;5-A from Triticum monococcum (Einkorn wheat)
into the (Triticum durum) durum wheat, Tamaroi, resulted in a significant
improvement in grain yield in salt stressed, field grown durum by increasing its
ion exclusion, but only in plants with highly saline soils.
• However, the yield of Tamaroi with TmHKT1;5-A was similar to that observed in
the Tamaroi cultivar without the introgressed gene, under low and moderate saline
conditions suggesting that osmotic stress was having a greater effect on the end
yield of these plants growing in low to moderate salinity, than ionic stress. 20

21. 4. Stomatal regulation
• High salinity causes low soil–water potential and reduces the ability of plants to
take up water, resulting in a marked decrease in stomatal conductance to
minimize water loss.
• Stomatal aperture in response to salt stress is mainly mediated by ABA and
H2O2, which function as signalling molecules.
• ABA-independent DST-mediated pathway is reported in rice.
• Although abscisic acid, stress and ripening-induced protein 5 (OsASR5) was
characterized to modulate stomatal closure via controlling the H2O2
accumulation through the ABA-dependent pathway, suppression of DST and its
downstream gene Prx 24 was found in OsASR5 overexpressing plants.
• Loss-of-function mutation of DST protein in rice plants enhances drought and
salt tolerance by reducing stomatal opening and stomatal density, consequently
resulting in enhanced drought and salt tolerance in rice.
• Thus, it appears that reducing stomata density comes with the same beneficial
role for salinity tolerance not only in halophytes and salt-tolerant glycophytes
such as barley but also in salt-sensitive glycophytic rice. 21

22. 22
Fig. 8. The gene networks regulating stomata
closure and ion transportation under salinity
stress of rice.
In leaves, DCA1 and DST complex binds to
genes regulating H2O2 homeostasis such as
Prx24, OsSR01c, and LP2, thereby influencing
stomatal closure under salinity stress. Na+ uptake
in the root epidermis is regulated by nonselective
cation channels including cyclic nucleotide-gated
channels. OsHKT2;1 also plays an important role
in Na+ uptake. The outward-rectifying K+ efflux
GORK channels are central to salinity stress-
induced K+ loss. The cellular Na+/K+ homeostasis
is regulated by active Na+ efflux channels via
SOS1 Na+/H+ exchangers and K+ influx channels
controlled by OsHAKs, OsKAT1, and OsAKT1.
Na+ loading into the xylem is mediated by
OsSOS1 while Na+ withdrawal from the xylem
is achieved by high-affinity K+ transporter
OsHKT1;5. OsHKT1 genes control both Na+
unloading from the xylem and Na+ loading into
the phloem in leaves for Na+ recirculation.
Vacuolar Na+ sequestration is conferred by
OsNHXs fueled by either V-ATPase or V-PPase
H+ pumps. Slow-vacuolar (SV) ion channels and
fast-vacuolar (FV) ion channels allow Na+ and
K+ to leak back to the cytosol from vacuole.

23. Fig.9. Salt Stress Signaling Pathways

24. Fig.9. Salt Stress Signaling Pathways (Explanation of previous diagram)
The SOS signaling pathway, consisting of SOS3/SOS3-like calcium-binding protein 8 (SCaBP8), SOS2, and SOS1, is important for
sensing salt-induced Ca2+ signals and in the regulation of ion homeostasis by extruding excessive Na+ out of cells.
Ca2+-mediated binding of PKS5 with 14-3-3 (DEAE chromatography (14th fraction) and purification involving gel electrophoresis
(fraction 3.3) ) releases the inhibition on SOS2.
GIPCs act as salt stress sensors that directly bind to Na+ and trigger Ca2+ influx via an unknown Ca2+ channel & Ca2+ are required for the
activiation of SOS pathway.
RbohD/F are involved in the production of ROS at the plasma membrane, and ROS can activate the ANN1-mediated Ca2+ signaling
pathway.
AKT1, which is regulated by SCaBP8, mediates the influx of K+ to the cytosol under salt stress.
MAP kinase cascades, including MAPKKK20, MKK2,MKK4, MPK3, MPK4, and MPK6, are involved in the relay of salt stress signals.
Salt stress-induced accumulation of ABA activates subclass III SNF1-related protein kinase2s (SnRK2s) via the PYR/PYLs-PP2Cs-
mediated regulatory module.
Subclass I SnRK2s are activated via an ABA-independent pathway under osmotic stress.
Activated MPKs and SnRK2s transduce signals to downstream transcription factors, including ABFs, zips, MYBs, NACs, WRKYs, and
AP2/ERFs, in the nucleus to induce the expression of stress-responsive genes.
In the apoplast, cell wall-localized leucine-rich repeat extensins LRX3, LRX4, and LRX5, together with secreted peptides RALF22/23 and
receptor-like kinase FER, function as a module to sense salt stress-induced cell wall changes.
FER, RALFs, and LLG1 form a complex at the plasma membrane to trigger Ca2+ signaling and consequently activate the cell wall repair
pathway.
FER also inhibits the activity of AHA2 to regulate apoplastic pH.
In the vacuole, NHXs, CAX1, TPK1, and H+-ATPase are involved in the regulation of ion homeostasis under high salinity.
24

25. Fig.10. The Biological Functions of Phytohormones in the Regulation of Salt Stress Response in Plants
25

26. References :
1. https://www.water.wa.gov.au/water-topics/water-quality/managing-water-quality/understanding-salinity
2. https://www.researchgate.net/publication/330857277_ELECTRICAL_CONDUCTIVITY_BASED_CLASSIFIC
ATION_AND_MAPPING_OF_SALT_AFFECTED_SOILS_IN_KAMPE-OMI_IRRIGATION_SCHEME
3. https://www.sciencedirect.com/science/article/abs/pii/S1360138500018380
4. https://www.researchgate.net/publication/284837596_Physiological_mechanisms_of_salt_stress_tolerance_in_pl
ants_An_overview
5. https://www.researchgate.net/publication/215517568_Salinity_Stress_and_Salt_Tolerance
6. https://www.sciencedirect.com/science/article/pii/S0958166913007192
7. https://www.researchgate.net/publication/284837596_Physiological_mechanisms_of_salt_stress_tolerance_in_pl
ants_An_overview#:~:text=It%20seems%20that%20the%20main,solutes%20%E2%80%A2%20change%20in%
20photosynthetic
8. https://www.sciencedirect.com/science/article/pii/S2214514121000714#:~:text=Further%2C%20salinity%20tole
rance%20is%20governed,stage%20(pollination%20and%20fertilization).
9. https://www.sciencedirect.com/science/article/pii/S2666675820300175
26

27. Thank You
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