Mechanisms of abiotic stress such as cold drought and salt stress which takes place in plants. Molecular control activities the plant undergoes during stress.

1. MECHANISMS OF ABIOTIC STRESS

2. INTRODUCTION:
 Environmental stresses such as drought, salinity, cold and heat cause adverse
affects.
 These stresses affects the productivity of the crops.
 Drought and salt stress together with low temperature, are the major problems
for agriculture.
 The factors influencing and oxidative stress are often interconnected with each
other.
 This may induce cellular damage.
 The complex mechanisms of these stresses provide the plant cell with different
information.
 Signaling pathways are induced in response to environmental stress, molecular
and genetic studies have revealed that these pathways involve many
comonents.

3. MOLECULAR CONTROL MECHANISMS:
 Molecular control mechanisms for abiotic stress tolerance are based upon the
activation and regulation of specific stress-related genes.
 These genes are involved in the stress responses such as signalling, transcription
control, protection of membranes, and proteins and scavenging of free radicals
and toxic compounds.
 The stress-inducible genes also function in stress response.

4. SALT STRESS SIGNALLING AND TRANSDUCTION PATHWAYS:
 Salinity is increasingly becoming a major threat to crop production.
 This is due to inappropriate irrigation regimes and the increasing use of brackish
water for irrigation.
 Salt stress in plants occurs when electrical conductivity of saturated soil paste
extract (ECe) reaches 4.0 deci-Siemens per meter (dS/m; approximately 40mM
of NaCl).
 When the plants accumulate salts, osmotic stress, nutrient imbalance, etc.,
occur.
 Salt effects disrupts intracellular ion homeostasis, membrane function and
metabolic activity. The secondary effects which occur are decrease in root
epidermal cell division and elongation rates, inhibition of growth, etc.,

5.  To cope up with saline soils, plants exhibit a wide range of mechanisms that
range from exclusion of sodium from the cells to tolerance within the cells.
 Amongst the receptor proteins identified as the first detectors of salt stress are
G-protein-coupled receptors, ion channel, receptor-like kinase or histidine-
kinase.
 These receptors transduce signals which consequently generates secondary
signals such as Ca²⁺, inositol phosphates, ROS, nitric oxide (NO) and ABA.
 The signalling pathway associated with increased concentration of cytosolic Ca²⁺
is the most reported.
 Cytosolic Ca²⁺ activates CDPKs, calcineurin B-like proteins (CBLs) and CIPKs to
transduce signals to down-stream protein activity and gene transcription.

6. Continued…
 Transcription factors such as calmodulin-binding transcription activators
(CAMTAs), GT element-binding like proteins (GTLs) and MYBs have been
reported to be activated by Ca²⁺/calmodulin directly.
 Other commonly expressed TFs in response to salt stress include the basic
leucine zipper (bZIP).
 Examples of bZIP are OspZIP71 in rice, WRKY, APETELA2/ETHYLENE RESPONSE
FACTOR (AP2/ERF), MYB, etc.,
 These TFs regulate the expression of genes related to water potential decrease
due to osmotic stress caused by salinity.
 There are several genes associated with salinity tolerance.

7. Continued…
 The most reported are the genes encoding for salt exclusion proteins, e.g., SOS1
 The salt overly sensitive (SOS) Ca²⁺ sensor regulatory mechanism are conserved in higher
plants including monocots and dicots.
 SOS consists of three functionally interlinked proteins, SOS3/SCaBP8, SOS2, SOS1.
 SOS3 mainly functions in the roots. CBL10/CaBP8, is an alternative regulator of SOS2 and it
functions primarily in shoots.
 At high Na⁺ concentrations, the increased influx of Ca²⁺ is identified by SOS3 which encodes
EF hand.
 Upon Ca²⁺ binding, a change occurs and SOS3 activates the downstream serine/threonine
protein kinase, SOS2, and recruits it to plasma membrane.

8.  Subsequently, the SOS3-SOS2 complex stimulates the plasma membrane localized Na⁺/H⁺
antiporters SOS1 and the leads to the extrusion of the excess Na⁺ out of the cells.
 The recent discovery is that the plant growth promoting rhizobacteria (PGPR) populations
reduce Na⁺ concentration in the shoots. The PGPRs increase the expression of stress
responsive TFs, induce greater proline synthesis, improve plant biomass under salinity stress,
etc.,. Therefore, treatment with rhizospheric organisms, and knowing the mechanisms
associated with PGPRs leading to salt tolerance is an attractive option for improvement of
crop yields.

9. SALT STRESS
SIGNALING

10. ADAPTIVE
MECHANISMS OF SALT
TOLERANCE

11. DROUGHT STRESS SIGNALING AND PATHWAYS:
Drought and low temperatures cause major limitations on crop productivity.
Plants respond to dehydration with a number of physiological and developmental changes.
Various kinds of proteins and smaller molecules, including sugars, proline, and glycine betaine are
known to accumulate under these stresses.
Many genes are induced during dehydration and cold, but some of only respond to water deficit and
some only with cold.
Drought and high salinity leads to high levels of ABA, exogenous application of ABA also induces a
number of genes which responds to dehydration and cold stress.
Analysis of the expression of dehydration-inducible genes in Arabdiopsis consists of atleast four signal
transductional pathways for the induction of genes in response to dehydration.
DROUGHT STRESS SENSORS:
Drought stress’s perception is important for sustenance of plant’s survivability.
Stress sensing is exercised by membrane-bound receptor proteins.
Receptor-like kinases and histidine kinases are two important protein families in stress perception.
Corticular microtubules (CMTs) have role in salinity, cold, and stress perception.

12. HISTIDINE KINASES:
Histidine kinase, is a two component (histidine-aspartate) phosphorylated
system which plays an important role in various stress perception.
The majority of these receptors were identified in Arabdiopsis, most of them are
hormone receptors.
The four types of HKs are AHK1,AHK2,AHK3 and AHK4, which are responsible for
osmotic stress signalling.
AHK1 is a plasma-membrane-bound osmoreceptor and positively regulates the
stress response whereas AHK2, AHK3 and AHK4 are found in ER and negatively
regulate its response toward ABA. The AHK2, AHK3 and AHK4 are cytokinin
receptors and in an unstressed conditions they take place in CK signal
transduction. This inturn forms a link between abiotic stress and plant hormone
signal transduction pathways.

13. DROUGHT SIGNAL TRANSDUCTIONS:
Signal transduction is a very complex phenomenon, as drought may occur in various
stages of development.
The signal transduction is comprised of two mechanisms, one mechanism consists of
regulatory proteins, genes, and TFs, the second one is the effector mechanism and
includes the genes that govern the accumulation of solutes, water transport channels,
enzymes for the detoxification of ROS and protectants of macromolecules.
ABA DEPENDENT PATHWAY:
ABA is a key player in dehydration stress response.
It regulates the expression of numerous drought-responsive TFs and genes.
It is often applied externally to plant tissues for mimicry of drought response in
laboratory conditions.
There are many elements involved in ABA-dependent pathway.
ABA synthesis is the first to be effected in the ABA-dependent drought stress
signalling process.

14. Pyrabactin resistance 1 (PYL) is a regulatory component of ABA receptor (RCAR).
PYL are the ABA binding cytosolic proteins involved in stress-induced signalling.
Protein phosphatase 2C (PP2C) is a group of phosphatases that regulates ABA
signalling.
In the absence of ABA, PP2C binds to SnRK2, and dephosphorylates their kinase
domain.
SnRKs are the main kinases involved in ABA-dependent stress signalling.
They are also responsible for stomatal pore closure, as they phosphorylate the
slow anion channel (SLAC1) in the presence of ABA.
There are many TFs involved in ABA-induced signalling.
ABA responsive element binding or ABA-responsive element binding factor
protein (AREB/ABF) belong to the bZIP family of TFs.
AREBs act as a trans-acting TFs, which bind to the major cis-acting element ABRE
(PyCGTGGC) present in the promoter regions of the stress-activated genes.

15. Myeloblastosis (MYB) and Myelocytomatasis (MYC) are involved in ABA induced
signalling under drought stress in plants.
MYC and MYB bind to MYBR and MYCR.
NAM (no apical meristem), a subgroup of NAC domain TFs (ATAF1-2), and CUC2
(cup-shaped cotyledon 2) which also comes under the NAC respond to ABA
dependent pathway.
These NAC TFs are found to be activated against both abiotic and biotic stresses.
The promoters of NAC TFs are regulated by other TFs such as AREB, MYB, MYC
and dehydrative responsive element binding (DREB).
ZFPs are found in the drought and salt signalling pathways.
Nuclear factor-Y has CCAAT binding site improves drought tolerance in plants.

16. ABA DEPENDENT FOR
DROUGHT

17. LEA PROTEINS (late embryogenesis abundant)
LEA are a large group of hydrophilic proteins.
They were characterized as accumulating in seeds during maturation and
dessication.
LEA proteins participate in protecting cellular components from dehydration.
They also prevent the aggregation of proteins during water stress.
Seven different groups of have been defined, out of which three main groups are
group 1, 2, and 3.
Group 3 play a major role in cellular dehydration.

18. DROUGHT AND COLD
STRESS SENSORS

19. COLD STRESSS SIGNALLING PATHWAYS:
Different plants vary enormously in their ability to withstand cold and freezing
temperature.
Most tropical plants have virtually no capacity to survive in freezing conditions
Depending upon the regions, plants in cold region can survive in a temperature
of about -5 to -30 C. Plants in colder regions can withstand temperature less than
this.
It is known that plants can withstand cold or freezing stress when they are
subjected to a period of cold acclimation, at a low but non-freezing temperature.
During, the period of acclimation, plants produce a number of cold induced
proteins that play a role in cold resistance.
Some of them have already been identified in LEA proteins. Other groups of
proteins are encoded by class of genes which is known as cold responsive genes
(COR).
These genes are involved in the freezing tolerance by lessening the damaging
effects of dehydration associated with freezing.

20. SIGNAL TRANSDUCTION:
Plant cell can sense cold stress through low-temperature induced changes in
membrane fluidity, protein and nucleic acid confirmation, and metabolite
confirmation.
Cold induced Ca²⁺ increase in cytosol.
The secondary messengers are ABA and ROS which induces Ca²⁺ signals
impacting cold signalling.
The role of ABA in cold responses is still unclear. Only a few years ago, ABA was
thought to have a major role in cold responses.
The transient increase in ABA accumulation was found in response to chilling
treatment.
However, other studies do not seem to find ABA accumulation under cold stress.
Cold stress induces a transient increase in cytosolic Ca²⁺ levels and activates the
expression of the C-repeat (CRT) binding TFs CBF/DREB1 (C-repeat-binding
factor/DRE-binding protein)

21. CBF/DREB1 in turn triggers the expression of a subset of cold-responsive (COR) genes.
CBF3 is transcriptionally regulated by the TFs ICE1 (inducer of CBF expression 1) and
MYB15.
The ICE1–CBF–COR cascade is one of the primary cold signaling pathways involved in
plant responses to cold stress.
Significant progress has been made in the identification of stress genes and cis and
trans-acting factors that control stress-responsive expression. For example,
RD29A/COR78 has been shown to be responsive to a variety of stress signals.
The TFs that bind to DRE/CRT (dehydration responsive element/C-repeat) and ABRE
have been identified and shown to function in stress- and ABA-responsive gene
activation.
CIPK3 function appears to be most important in the cold induction of gene expression.
Of all marker genes examined (RD29A and KIN1/KIN2), induction was delayed most
dramatically under cold conditions in the cipk3 mutant plants, although the maximum
level of gene induction was not altered.
This finding suggests that the cold-induced expression of RD29A and KIN1/KIN2 genes
may consist of two components: the early phase and the late phase.

22. TFs for RD29A activation, such as DREBs/CBFs, are activated at the transcriptional
level by cold stress.
Although drought and cold are known to activate RD29A gene expression by
activating the same cis-acting element DRE/CRT.
TFs (DREB1 and DREB2) may be involved in drought and cold responses,
implicating separate pathways linking drought and cold to RD29A expression.
WRKY protein is the largest family of TFs in plants.
It regulates abiotic stresses such as drought, salt, and ABA signalling.
HARDY (HRD) gene improves water use efficiency and photosynthetic
assimilation.

23. ABA INDEPENDENT PATHWAY
OF DROUGHT AND COLD
SIGNALLING

24. CONCLUSION
Complex traits of abiotic stress phenomena in plants make genetic modification
for efficient stress tolerance difficult to achieve. Understanding the molecular
mechanism of plant responses to abiotic stresses such as drought, salinity, and
cold is very important, as it helps in manipulating plants to improve stress
tolerance and productivity. Several abiotic stress signaling pathway components
have been identified. ABA-dependent and ABA-independent pathways act in
parallel and also interact, thereby providing added coordination between stress
signals and ABA in the regulation of stress-inducible genes for the modulation of
plant tolerance to agriculturally relevant stressors such as salinity, drought, and
cold.

25. REFERENCES
Sagarika Mishra, Sanjeev Kumar, et.al., ‘Crosstalk between Salt, Drought, cold stress in plants: Toward Genetic
Engineering for Stress Tolerance’, 2016, Wiley-VCH Verlag GmbH and Co. KGaA.
Advances in Plant Tolerance to Abiotic Stresses.
http://dx.doi.org/10.5772/64350
Mayra Rodriguez, Eduardo Canales, et.al., ‘Molecular Aspects of Abiotic Stress in Plants’, Biotecnologia Aplicada
2005; Vol.22, No.1
Adrian Slater, Nigel W. Scott, and Mark R. Fowler, ‘Plant Biotechnology- the genetic manipulation of plants’,
second edition, 2008, Oxford university Press.
https://www.biotecharticles.com/Agriculture-Article/Abiotic-Stress-Signal-Transduction-Pathways-in-Plants-
4452.html
https://www.slideshare.net/mobile/kirtimehta16/drought-n-heat-abiotic-stress-in-plant
http://m.authorstream.com/presentation/T.Swapna-1924306-abiotic-stress-msc-1v-sem
Yamaguchi-Shinozaki, K. and Shinozaki, K. (2006) Transcriptional regulatory networks in cellular responses and
tolerance to dehydration and cold.
 Chen, L., Song, Y.,A Li., S., Zang, L., Zou, C., and D. (2012) The Role of WRKY transcription factors in plant abiotic stresses.
Biochim. Biophys. Acta, 1819, 120-128
 Hickman, R., Hill, C., Penfold, C.A., Breeze, E., Bowden, L., Moore, J.D., and Buchanan-Wollaston, V. (2013) A local
regulatory network around three NAC transcription factors in stress responses and senescence in Arabidopsis leaves.
Plant J., 75 (1), 26–39.

26. PLANTS HAVE MORE PROBLEMS THAN HUMANS