MOLECULAR BIOLOGY OF ABIOTIC STRESS TOLERANCE

This contains Abiotic stress tolerance related information. Defination scope and types and method of tolerance and avoidance statragies.

1. Molecular Biology of Abiotic
Stress Response
Cold, high temperature, submergence,
salinity and drought

2. Introduction
•In the face of a global scarcity of water resources and the
increased salinization of soil and water, abiotic stress is a
major limiting factor in plant growth and will soon become
even more severe as desertification covers more and more
of the world’s terrestrial area.
•Drought and salinity are widespread in many regions, and
are expected to cause serious salinization of more than
50% of all arable lands by the year 2050 (Ashraf, 1994) .
•In a world where population growth exceeds food supply,
agricultural and plant biotechnologies aimed at overcoming
severe environmental stresses need to be fully
implemented.
•Plant adaptation to environmental stresses is controlled by
cascades of molecular networks.

3. The complexity of the plant response to abiotic stress

4. •In contrast to plant resistance to biotic
stresses, which is mostly dependent on
monogenic traits, the genetically complex
responses to abiotic stresses are multigenic,
and thus more difficult to control and
engineer.
•For this reason, biotechnology should be fully
integrated with classical physiology and
breeding

5. Stress-associated genes and proteins:
expression and proteomics
Acquired plant stress tolerance can be enhanced by manipulating
stress-associated genes and proteins and by over expression of stress
associated metabolites.

6. A. Signaling cascades and
transcriptional control

7. A. Signaling cascades and
transcriptional control
•Genes involved in signaling cascades and in transcriptional
control are:
Mitogen-Activated Protein (MAP)
Salt Overly Sensitive (SOS) kinases
Phospholipases
Transcription Factors:
(e.g. heat shock factor [HSF] and the C-repeat-binding
factor /dehydration- responsive element binding protein
[CBF/DREB] and ABA-responsive element binding
factor/ABA-responsive element [ABF/ABRE] families)

8. A Generic Pathway for the Transduction of Cold,
Drought, and Salt Stress Signals in Plants.

9. Major Types of Signaling for Plants during Cold,
Drought, and Salt Stress

10. Major Types of Signaling for Plants during Cold,
Drought, Heat and Salt Stress

12. B. Hsps, chaperones & LEA proteins

13. B. Hsps, chaperones & LEA proteins
•Drought, salt, and high temperature can cause them is folding
and dysfunction of many RNAs and proteins. Some stress
induced genes encode proteins to protect the conformation
of other proteins, RNAs, or cell structures.
•HSPs and CSPs are required for quick adaptation to
temperature changes as well as for rapid recovery after heat
or cold release.
•LEA proteins accumulate in embryos during seed desiccation
and are also induced in vegetative tissues by dehydration,
cold, salt, and ABA treatment.
•LEAs are extremely hydrophilic and are involved in adaptive
responses to hyperosmotic conditions through the
maintenance of protein or membrane structure,
sequestration of ions, binding of water, and their operation as
molecular chaperones (Bray, 1997).

14. Pathways for the Activation of the LEA-Like Class of
Stress-Responsive Genes with DRE/CRT and ABRE cis Elements.

15. C. Ion & Water transport

16. C. Ion & Water transport
•Osmotic stress, ion toxicity and high salt content in the soil and the
irrigation water, especially Na
+
and Cl
-
), significantly impair plant growth.
•Ion transporters selectively transport ions and maintain them at
physiologically relevant concentrations while Na
+
/H
+
antiporters also
play a crucial role in maintaining cellular ion homeostasis, thus
permitting plant survival and growth under saline conditions.
•The Na
+
/H
+
antiporters catalyze the exchange of Na
+
for H
+
across
membranes and have a variety of functions, such as regulating
cytoplasmic pH, sodium levels and cell turgor (Serrano et al. 1999).
•Regulation of cellular ion homeostasis during salinity stress is critical for
plant salt tolerance.
•Salt-Overly-Sensitive (SOS) pathway is involved in the plant's response
to ionic stress.
•Molecular analysis of sos mutants of Arabidopsis led to the identification
of components (SOS1, SOS2 and SOS3) of a pathway that transduce a
salt stress-induced Ca
2+
signal to reinstate cellular ion homeostasis (Zhu,
2002).

17. •In plants, protons are used as coupling ions for ion transport
systems, and the proton gradient, generated by proton pumps
found in the cell membrane, is the driving force for nutrient uptake
(Serrano et al. 1999). Three distinct proton pumps are responsible
for the generation of the proton electrochemical gradients (Sze et
al. 1999):
•the plasma membrane H-ATPase pump (PM H-ATPase) which
extrudes H
+
from the cell and thus generates a proton motive
force;
•the vacuolar-type H-ATPase pump (V-ATPase);
•the vacuolar H-pumping pyrophosphatase pump (H-PPase).
•The latter two acidify the vacuolar lumen and other
endomembrane compartments.
•Arabidopsis plants were transformed with a vacuolar H
+
-PPase pump
that is encoded by a single gene, AVP1 (Gaxiola et al. 2001), which
can generate an H
+
gradient across the vacuolar membrane, similar
in magnitude to that of the multisubunit vacuolar H
+
-ATPase pump.
These transgenic plants expressed higher levels of AVP1 and were
more resistant to salt and drought than wild-type plants.

18. Regulation of ion homeostasis by the SOS
pathway during salt stress.

19. Heat stress in plants

20. D. ROS Scavenging & Detoxification

21. D. ROS Scavenging & Detoxification
•Stress-induced production of reactive oxygen species (ROS) is
another aspect of environmental stress in plants.
•Alleviation of oxidative damage by the use of different antioxidants
and ROS scavengers can enhance plant resistance to salt and
drought.
•Transgenic tobacco plants overexpressing Chlamydomonas
glutathione peroxidase in the cytosol and in the chloroplast
displayed increased tolerance to oxidative stress, which was
imposed using methylviologen, chilling and salt stress (Yoshimura,
2004).
•Overexpression of the aldehyde dehydrogenase AtALDH3 gene in
Arabidopsis conferred tolerance to drought and salt stress (Sunkar,
2003). Aldehyde dehydrogenase catalyzes the oxidation of toxic
aldehydes, which accumulate as a result of side reactions of ROS
with lipids and proteins.

22. Generation and scavenging of Reactive Oxygen
Species

23. Stre
ss-associated changes in
metabolite
s and metabolomics

24. Stre
ss-associated changes in
metabolite
s and metabolomics
•A wide range of metabolites that can prevent
these detrimental changes have been identified,
including amino acids (e.g. proline), quaternary
and other amines (e.g. glycine-betaine and
polyamines) and a variety of sugars and sugar
alcohols (e.g. mannitol and trehalose).
•Two general strategies for the metabolic
engineering of abiotic stress tolerance have been
proposed: increased production of specific
desired compounds or reduction in the levels of
unwanted (toxic) compounds

25. •Severe osmotic stress causes detrimental changes in
cellular components.
•In stress-tolerant transgenic plants, many genes
involved in the synthesis of osmoprotectants that
accumulate during osmotic adjustment
i.organic compounds such as amino acids (e.g. proline),
ii.quaternary and other amines (e.g. glycinebetaine and
polyamines) and
iii.a variety of sugars and sugar alcohols (e.g. mannitol,
trehalose and galactinol)

26. •Amino acids:
Proline accumulation was correlated with improved plant performance under salt stress.
Proline-level increments can be achieved in planta by overexpressing ∆
1
-pyrroline-5-
carboxylate synthetase (P5CS), as found, for example, in tobacco (Konstantinova et al.,
2002).
•Amines:
Glycine-betaine is a widely studied osmoprotectant, its accumulation lead to modifications of
several metabolic steps. Betaine aldehyde decarboxylase from the halophyte Suaeda
liaotungensis was introduced into tobacco plants and the in vitro plantlets were significantly
resistant to salt conditions (Li et al., 2003).
Similar results were obtained by transforming rice with the choline dehydrogenase gene
(codA) from Arthrobacter globiformis; the gene product of codA catalyzes the oxidation of
choline to glycine betaine via betaine aldehyde as intermediate. The transgenic rice plants
recovered from salt stress and set seeds, in contrast to wild-type plants (Mohanty et al.,
2002).
•Sugars and Sugar alcohols:
Overall carbon metabolism and the levels of specific sugars are severely affected by abiotic
stress.
In Setaria sphacelata, a naturally adapted C4 grass, photosynthetic carbohydrate content was
studied under conditions of both rapid and slow water deficit (Silva & Abracca, 2004). In
short-term stress experiments, a decrease in sucrose and starch content was observed. In
long-term experiments, a higher amount of soluble sugars and a lower amount of starch
were found under stress. The shift of metabolism towards sucrose might occur because
starch synthesis and degradation are more affected than sucrose synthesis (Silva & Abracca,
2004).
Mannitol is sugar alcohol that accumulates upon salt and water stress and can thus alleviate
abiotic stress. Transgenic wheat expressing the mannitol-1-phosphatase dehydrogenase gene
(mtlD) of E. coli was significantly more tolerant to water and salt stress (Abebe et al., 2003).

27. Strategies for the genetic engineering of
drought tolerance.