plant physiolgy

1. MECHANISM OF DROUGHT
TOLERANCE IN PLANTS

2. Why DROUGHT tolerance in plants?
 =Water deficit stress
 occurs when the soil water
potential falls between −0.5
to −1.5 MPa.
 most serious threat to world
food security.
 causes negative effects on
plant growth and total yield
by -
• Decreased leaves absorption
of photosynthetic active
radiation.
• Reduced radiation-use
efficiency.
• Minimized harvest index.

3. Plant Adaptations For Drought Stress
• Drought escape
• Drought avoidance
• Drought tolerance

4. MECHANISMS OF DROUGHT TOLERANCE

5. 1. Morphological Mechanisms
• changes at whole plant, cell, tissue, physiological
and molecular and genetic levels.
• Escape
-attained by the shortening of life cycle
-plants to reproduce only in the favorable
environment conditions. (Araus et al., 2002).
• Avoidance
-reduce water loss by
. due to regulation of stomatal transpiration,
.Glaucousness or waxy bloom on leaves helps with
maintenance of high tissue water potential,
(Richards et al. 1986; Ludlow and Muchow 1990)..
. water uptake through the extensive and prolific root
system (Kavar et al. 2007).
• Phenotypic Flexibility

6. 2. Physiological Mechanisms

7. Stomatal regulation

8. Osmotic Adjustment
• allow the maintenance of water uptake and cell turgor.
• A major source for glucose signals is transitory starch
breakdown from chloroplasts during the night.
• Huber et al. (1984) concluded that water stress has a larger
effect on carbon assimilation than on translocation.
• Drought induces synthesis of both chaperons (which
enhance stability of other proteins), and proteases and
ubiquitin (which cause degradation of proteins that
become denatured during water loss).

9. The phytohormones
• There are many signals that induce stomatal closure, among these the best
known signal is ABA.
• ABA is the central regulator coordinates a complex gene regulatory network
enabling plants to cope with decreased water availability (Cutler et al.,
2010; Kim et al., 2010).
• the production of ABA in roots which is transported to the shoots causing
stomatal closure and eventually restricting cellular growth.
• The ABA synthesis is probably dependent on plasma membrane localized
pressure sensitive receptors or ion channels (Guerrero et al.,1990).
• ABA induces K+ and Ca2+ fluxes in guard cells causing stomatal closure (to
improve the water use efficiency (WUE)) and regulating water loss.
• Drought induced changes in the photosystem (Giardi et al., 1996) reduce
the stromatal pH, which lead to release of the chloroplast
compartmentalized ABA into the cytosol.
• There it could act upon an intracellular receptor initiating a cascade of
signal transduction, or could be directly imported into the nucleus
triggering gene activation.
• ABA biosynthesis gene (NCED3), a cytochrome P450 CYP707A family has
been identified as ABA 80-hydroxylases, which play a central role in
regulating ABA levels during dehydration stress conditions (Umezawa et al.,
2006).

10. • ABA signals are perceived by
different cellular receptors-
The nucleocytoplasmic
receptors PYR/PYL/RCARs
(pyrabactin
resistance/pyrabactin
resistance-like/regulatory
component of ABA receptors)
bind ABA and inhibit type 2C
protein phosphatases (PP2Cs).
• Inactivation of PP2Cs leads to
accumulation of active
sucrose non-fermenting-1
(SNF1)-related protein kinases
(SnRK2s),
• which interacts with ABA-
responsive TFs, ABA-
responsive promoter
elements (ABREs) and ABRE-
binding protein/ABRE-binding
factors (AREB/ABF)
• to regulate transcription of
downstream target genes and
related physiological
processes.

11. ABA-dependent gene regulatory pathway in rice.

12.  Aquaporins
• Aquaporins are water channel proteins, belong to a highly
conserved family of major intrinsic membrane proteins (MIP) family.
• reduction in root hydraulic conductivity in response to water stress
-a closure of aquaporins.(North et al., 2004).
 Osmoprotectants
• The biosynthesis and accumulation of compatible solutes-enable
protection of cell turgor and maintains cellular water-
potential,stabilize membranes and scavenge ROS.
• Glycine betaine (GB), a fully N-methyl-substituted derivative of
glycine-accumulates in the chloroplasts-involved in cell membrane
protection.
• Choline is converted to betaine aldehyde by the choline mono
oxygenase (CMO) under drought.
• Betaine aldehyde is then catalyzed into glycine betaine by betaine
aldehyde dehydrogenase.
 Reactive Oxygen Species(ROS)
Plants keep ROS under control by an efficient and versatile
scavengingSystem.
• SOD is the front-line enzyme in ROS attack, it rapidly scavenges
superoxide dismutase (SOD)
• producing H2O2 (thiol inhibitor) and oxygen
• H2 O2 is then locally converted to water by ascorbate peroxidase
(APX).
 Specified protein synthesized under water scarcity/Changes in
plant metabolics (Proteins):
• Proline represents an important osmolyte, which has protective
role.
• LEA proteins participate in protecting cellular components from
dehydration (Reyes et al., 2005),Prevent aggregation of proteins
due to water stress and Overproduction of a wheat dehydrin
(DHN5) in Arabidposis enhanced the tolerance to osmotic stress.

13. 3. Genetic and Molecular mechanism
• changes in gene expression (up- and down-regulation).
• Various genes are induced in response to drought at
the transcriptional level.
• 3.1 Conventional breeding approaches
-Breeding approach started at the International Maize
and Wheat Improvement Center (CIMMYT), Mexico in
the 1970s for developing drought tolerant maize
,ICARDA, ICRISAT, IITA and CRI focused on different crop
worldwide.
• 3.2 Marker-assisted breeding (MAB)
- Drought stress is controlled by many minor genes
(polygenes) that have additive effects in their
expression (Khan et al., 2011). Thus, the loci on
chromosomes housing referred as quantitative trait loci
(QTL).
- The QTL mapping has been considered as imperative to
the use of DNA markers for improving tolerance
(Ashraf et al., 2010). Ashraf et al., (2010) have listed a
variety of DNA markers such as RFLPs, RAPDs, CAPS,
PCR indels, AFLPs, microsatellites (SSRs), SNPs, and
DNA sequences being currently in use to examine the
inheritance of tolerance.
• 3.3 Engineering crops for enhanced drought
tolerance—Transgenic approach

15. Molecular mechanism
• Drought tolerance in plants is the ability to survive and
produce stable yields under water scarcity during various
stages of crop growth.
• Plants respond to drought, like many other abiotic
stresses, by inducing cellular damage and secondary
stresses, such as osmotic and oxidative stresses.
• These secondary stresses induce initial stress signals
(e.g., osmotic and ionic effects and membrane fluidity
changes) that are perceived by membrane receptors
(sensors).
• The perceived signals are transmitted downstream to
trigger transcription, which is regulated by
phytohormones, transcription factor binding proteins
(TFBPs), cis-acting elements and miRNAs.
• transcription factors TFs bind to specific cis-elements to
induce the expression of targeted stress-inducible genes,
allowing for products to be transcribed that help with
stress response and tolerance.]
 Some of these include dehydration-responsive element-
binding protein (DREB),
 ABA-responsive element-binding factor (AREB),
 no apical meristem (NAM),

16. OVERVIEW OF ALL MECHANISMS