Review
COTTON: – Cotton belongs to the family Malvaceae,  is an important commercial crop cultivated in
tropical and sub-tropical regions. It’s a perennial shrubby dicot plant with a
variable range in height (3-20 ft.). The leading cotton growing countries are
China, India, USA, Australia, Mexico & so on, contributes more than 80% of
total cotton production in the world.

The
cotton fiber mainly made up of cellulose 1
which is a pivotal material in textile industries. Cotton is also used in oil
industries as seeds are rich in oil. Linters, contains cellulose, for making
plastics, explosives, high-quality paper and so on.

Effect
of non-biological factors to hinder the natural growth of an organism is termed
as abiotic stress viz.drought, salinity, temperature, & so on. Among them,
the drought or water deficient condition is the major problem regarding the
crop production 2. It severely creates
a negative impact on growth & life cycle of a plant with the reduction in
biomass accumulation. Drought reduces the leaf size, stem elongation & root
proliferation by decreasing the cell division rate by diminishing the crop
productivity& water use efficiency. Dryness results of drought stress
decrease 30-50% yield 3. Drought stress
mainly affects the flowering stage 4
the primary cause of the decrease in yield.

Compare
to rice, maize the cotton plant show a relatively higher rate of drought stress
tolerance 5 but the growth of the
plant, fiber yield and quality will reduce significantly 6. During stress condition genetically
similar cotton plant show 50% reduction in yield. Thus drought tolerance of
cotton genotypes and the mechanism behind this is a new avenue of research 7. Several numbers of physiological &
morphological traits such as increase in number and weight of tap root, gear up
the development of root system 8,
longer root length 9, reduction in
transpiration rate 10 &
photosynthesis 11 are then taken up by
cotton plant to overcome this unfavorable condition.

To
withstand drought stress plants adopt some morphological, physiological &
biochemical response.  Plants generate
some defense mechanism 12 to maintain
normal conditions during drought. In case of the cotton plant, drought has some
antagonistic effect on plant height, the number of nodes, the rate of
transpiration, stomatal conductance, net photosynthetic rate and water
potential of leaves 13. Accumulation of
dry matter gets decreased up to 50% during drought condition in Gossypium barbadense 14.

The drought tolerance
mechanism can be categorized into four parts such as drought escape, drought
avoidance, drought tolerance, drought recovery 15,
16.

Drought escape: –
It can be gained through shortening of the life cycle or mainly shortening of
the reproductive season as the flowering time is most crucial trait related to
drought 17. As early maturity helps to
escape the drought period thus the development of short duration varieties are
very efficient 18. But as the yield is
directly proportional to the duration of crop, therefore average productivity
will be very less 19.

 

Drought tolerance: –
These are the mechanism where plants adopt some changes to withstand drought
through physiological activities 20;
this changes related to xerophytic plants. By increasing the density, length and
proliferation of root 21, by bearing a
low number of leaves with less surface area 22
plants can withstand drought. Again leaf pubescence and hairy leaf have a
significant effect on drought tolerant as they help the leaf from high
temperature and lowers the transpiration rate 23.

 

Drought avoidance:-
It is the mechanism by which plant can reduce water loss by maintaining
stomatal regulation, developing more profiling root system 19, 21. To sustain high tissue water
potential, the layer of waxy bloom over leaf area consider as an essential
regulator 24, 25. These glaucous leaves
are cooler and had a lower degree of senescence than unglaucous leaves 24.

 

Drought
recovery: – The strategies by which plants can
continue their natural growth after facing a drought condition is called as
drought recovery. Under severe drought condition, the dehydration rate
maintained by stabilization of cell membrane and accumulating water-soluble
carbohydrates 26-28. In grasslands, the
plants accumulate fructans, as reserved carbohydrates and a shallow level of
starch 29-32 and this higher level of
fructan level after drought improve plant survival 26, 33 and it may act as antioxidants 34.

Effect of drought
stress on morphological and physiological; activity in plants: –

Drought has some
drastic negative impact on both morphological as well as physiological
activities viz. photosynthesis 35,
metabolism and mineral nutrition 36. It
also accelerates the growth of pests and weeds in the field 37, 38.

Root growth:-

The
cotton plant consists of a tap root system, from where the secondary and
tertiary branches of root development started. The roots consist of one layer
of epidermis, one layer of endodermis and layers of cortex in between them.
Endodermis surrounds stele and pericycle 39.
The xylem here is endarch type.

Drought
reduces growth, density, elongation of root in the cotton plant 40 but some genotypes get an elongated root
system which is more resilient to drought stress 41. In the initial stage of drought, root growth enhances but if
it persists for a long time, it generates an antagonistic effect on root
development 42  Again the numbers of tap roots increased at
first encounter without expanding the root biomass, and even the dry weight of
secondary root become less after drought 9, 43,
are suspected to be a common symptom during the drought in cotton. Drought
tolerance in transgenic with profuse root system is better than wild-type 44. 

 

Rate of Photosynthesis:

Photosynthesis is
pivotal process controls all the dominant traits in plants health, productivity
and so on. Drought creates a detrimental effect on the plant by decreasing the
rate of photosynthesis, as in water scare condition the stomata became close
followed by reducing the level of CO2 influx thus it affects photosynthesis 45, 46. In cotton, as water stress reduces
photosynthesis rate 47 followed by the
decreased rate of yield because yield is directly correlated with
photosynthesis 48. Reduce
photosynthesis level resulted in lowering the C accumulation followed by
decrease level of biomass 49. Severe
drought stress resulted in the decreased level of chlorophyll a, chlorophyll b and
total chlorophyll 50. It also
suppresses the activity of PS-II by lowering the electron transport chain
activity and by releasing the Ca+2, and Mg+2 ions bind with it 51-53. In a study of cotton field study, a decreased
yield resulted as that was lower photosynthesis rate followed by less
productivity but the same genotype yield well when cultivated in water
abundance condition 54. Thus the rate
of photosynthesis is inversely proportional to drought.

Effect on Respiration:

 Respiration is a cellular process by which
energy is generated, and this energy is then used up by the other
energy-dependent process in the cell or may be stored. The efficiency of
metabolism in plants can be described by the fraction of carbohydrates lost
through respiration 55. Root is the
principal place for using these photosynthates (i.e., carbohydrates) and used
it for the growth and dry matter production in plants as more than 50% of
accumulated photosynthates were transported to root, and the maximum amount of
them was respired 56. But the
respiration rate decline during drought as soil moisture deficits can reduce
root respiration 57-60. But limited
root growth and respiration under drought resulting from uncoupling between carbon
production in leaves and use in the root 61,
helps to improved growth on the plant in drought 62.

In response to drought
stress in spite of regular electron transport system, plants take up an
alternate pathway where the electron directly passed to oxygen by an enzyme
alternative oxidase. Here the production of ATP is meager 63, 64. During drought stress, plants produce
Reactive Oxygen Species, which leads to the destruction of plant cell membrane.
But their alternate oxidase activity reduces the production of ROS during
drought 65. In conclusion, drought
stress leads to an imbalance in carbon sources utilization, reduction in
biomass accumulation, reduce ATP level and geared up the generation of ROS.

Mechanisms involved in
drought tolerance: –

The molecular mechanism
in drought stress tolerance in cotton: To cope up with water deficient
condition plants change some of its balanced mechanism so that it could adapt.
Several genes in several modified pathways are involved in this process in
response to drought stress.

Abscisic Acid:

ABA
is sesquiterpenoid with 15 carbon ring 66.
When imposed to salt, drought, cold and any other abiotic stress, plants
response to them mainly by ABA and phytochrome 67.
Several ABA genes loss of function mutation have been reported 68 which are susceptible to wilt and dry if
stress persists.

As
ABA is induced during various stresses, considered as the stress hormone in the
plant 67, 69. It plays some vital role
in the plant like seed dormancy, storage protein synthesis, leaf senescence
delayed germination and combat pathogenic infection 67. Generally, ABA synthesis occurred in root and transported via
vascular tissues 70 resulting the level
of ABA increased under drought stress resulting several gene expression and
stimulating the closure of stomata 71-73.
Among ABA-induced genes, 54%, i.e., 133 genes are induced during drought stress
71 and ABA expression initiated under
the presence of the ABA-responsive element (ABRE), a cis-acting DNA binding
element 74, 75. When the extracellular
stress signal perceived by membrane receptors, receptors like kinase (RLK),
histidine kinase (HK) and then activates several pathways leads to the
formation of ABA, ROS, and Ca+ 69. This
Ca+ ion is a secondary messenger and mediates crosstalk between several
signaling pathways 76, 77. ABA
biosynthesis precursor ?-carotene gets activated by several drought-induced
genes like zeaxanthin oxidase (ZEP for conversion of zeaxanthin to
violaxanthin), 9-cis epoxycarotenoid dioxygenase (NCED for Neoxanthin to
Xanthoxin), and ABA aldehyde Oxidase (AAO for the ABA synthesis from ABA
aldehyde). Many of these genes are regulated by calcium-dependent
phosphorylation 76-78.

Overexpression of PYR
(Pyrabactin Resistant) an ABA receptor genes induced during drought confers
tolerance to this stress 79. In ABA
signaling pathway the main three components are PYL, PP2C (Protein Phosphatase
2C) and SnRK2 (Sucrose non fermenting related protein kinase 2) 80, 81. In Arabidopsis Genome, there are PYR,
PYL or RCAR as ABA receptors 80, 82.
Once PYR/ PYL/ RCAR bound with ABA, these receptors able to bind PP2C (type 2C
protein phosphatase), e.g., ABI 1 and ABI2 (ABA insensitive 1, 2), a negative
regulator of ABA. Inbound form, PP2C is not able to bind and dephosphorylate
SnRK2 (sucrose nonfermenting kinase-1 Related protein kinase). This SnRK2 are a
protein of serine-threonine kinase having a role in drought condition 83. Thus the SnRK2 in the phosphorylated
state (unbound and activated form), phosphorylate ABFs (ABA-responsive element
binding factors), which again binds to ABRE and induce the ABA-responsive
signals 84-87. In upland cotton
varieties, several genes have been identified which have drought tolerant
response by ABA-dependent manners such as GhATAF1 6, GhMKK3 88, GhNAC2 89 and GhCBF3 90.

MAPK Signalling:

Under
focus of water deficit condition for long time plant may evolve or adopt some
signaling pathways to combat severe dehydration conditions. Mitogen-Activated
Protein Kinase (MAPK) signaling is one of the major factors among them, they
are conserved in all eukaryotes from plant to animals, from insects to fungi 91, 92. MAP Kinase pathway is a cascade
signaling which composed of three different kinases viz. MAPKKK, MAPKK, MAPK 93, 94. During stress condition, another
protein kinase, MAPKKKK (i.e., MAP4K) may be activated 95. After getting external stress stimuli, first, the MAPKKK
activates by phosphorylation and then it phosphorylates the two
Serine/Threonine (or both) residue at S/TX3-5S/T motif situated in the MAPKK
activation loop. This activated MAPKK then continues the cascade by
phosphorylating conserved T-X-T motif of MAPKs, which Phosphorylates several
other transcription factors by their activation. Those TFs then mediates
several gene expressions in stress condition 84,
92, 96, 97. MAPK signaling components are activated by ABA, cold,
drought, pH  98. To shut the signaling pathway down MKPs (MAPK phosphatase)
are there which provide a cut off connection during favorable condition.

In cotton, a MAPKK gene
GhMKK3 isolated which shows efficient tolerance to drought stress by
up-regulating root hair elongation gene and reduce the rate of water loss by
stomatal closure in Nicotiana benthamiana.
Silencing this gene shows a high level of water loss and thus wilting during
drought 88. In G. raimondii genome, bioinformatics analysis helps to identify 28
MAPK genes 99.

Aquaporins:

Aquaporin
is a type of transmembrane protein present in all form of life due to its
pivotal role in water balance in the cell, belongs to MIP (Major Intrinsic
Protein) superfamily. It acts by a phosphorylation-dephosphorylation mechanism 100. Aquaporin in plants consists of 5
subfamilies PIP (Plasma membrane Intrinsic Protein), SIP (Small Intrinsic
Protein), TIP (Tonoplast Intrinsic Protein), XIP (X or Unrecognised Intrinsic
Protein), NIP (NOD-26 like Intrinsic Protein) 101.
Most of them are expressed continuously throughout the life, but some of them
expressed only during imbalance of environmental factors like drought,
salinity, temperature, cold, Ph, blue light and all that 102-104. There are total 71 aquaporin genes 105 found in cotton by in silico method among
them twenty-eight are PIP, twenty-three TIP, 
twelve NIP, seven SIP and rest one is XIP. All them shows common level
structural similarity that is six alpha-helical structure with five
inter-helical loops and a common AEP (Alanine-Glutamine-Phenylalanine) motif in
N terminus and two NPA (Asparagine-Proline-Alanine) motif 106. Among five loops A, C and E are
extracellular whereas B and D are intracellular. These two play an essential
role in forming of water channel 107.
Aquaporin not only acts as water transporter channel but also a channel of
transport for glycerol and urea in the cell. In recent studied, related that
Nt-AQP1, an aquaporin of tobacco plasma membrane, transport water, glycerol,
and urea 108-110, NtTIPa, a tonoplast
aquaporin mainly transport urea 110.

The
plasma membrane aquaporin (PIP) is the leading component concerned with water
balance in the plant cell. Opening and closing of plant plasma membrane
aquaporins are depended on phosphorylation (open) and dephosphorylation (close)
of particular amino acid(s). Dephosphorylation of Ser115 in cytosolic loop B
and Ser214 in C-terminus and of SOPIP2;1 of spinach leads to closure of this
aquaporin 103 again Leu197 along with
His99, Val104, Leu108 of cytosolic loop D makes a hydrophobic motif blocking
the water to enter. The diameter becomes 1.4Å and further becomes narrower to
0.8Å near these residues, where the minimum diameter should be 2.1Å to pass
water. In case of open structure for the SOPIP2;1 aquaporin the N terminus
helix 5 extends itself into the cytoplasm, followed by displacing the amino
acids blockage.  As a result, the
cytoplasmic loop D moves 16Å from the blockage site due to phosphorylation of
Ser115 and Ser274 also creating a free space for water entrance 111.

Thus
when water potential is high, the amino acids become phosphorylated followed by
the opening of the gate and in case of lower water potential dephosphorylation
occur resulted in the closing of the gate so that, no efflux of water from the
cell takes place 112.

In case of drought
stress in cotton, GhPIP2;7 gene expressed in leaves and cotyledons and shows a
high range of tolerance than plants than those doesn’t have that gene 113. In G.
hirsutum expression of GhPIP1;3 and GhPIP1;1 shows good result in drought
tolerance 105.

ROS Scavenging:

ROS
or Reactive Oxygen Species are continuously produced in various metabolic
pathways as a byproduct in cellular organelles such as the chloroplast,
mitochondria, peroxisome and so on. They can damage protein, DNA, and membrane
lipid 114. There is a subtle balance
between production and scavenging of ROS so that it can’t cause any oxidative
damage. But several types of abiotic (as well as biotic) stresses like drought,
temperature, Salinity, cold-induced the level of ROS followed by breaking the
balancing bridge. Main types of ROS are like superoxide radical (O2?-,
generated due to partial reduction of oxygen); singlet oxygen (O21,
formed by reaction of oxygen with diffused state chlorophyll), hydroxyl radical
(OH?, produced due to reaction between H2O2 and O2?-  catalyzed by Fe3+ or Fe2+)
and the  H2O2
(formed during protonation of O2?-) 115.

Plants
adopt two different kinds of mechanisms to scavenge the ROS- enzyme-dependent,
and enzyme-independent 116. Here only
enzymatic ROS scavenging activity will be discussed.

Enzymatic ROS
scavenging: Several enzymes like SOD (Superoxide dismutase), APX (Ascorbate
Peroxidase), CAT (Catalase), MDHAR (Monohydorxyascorbate peroxidase) and GR
(Glutathione reductase) to scavenge the ROS.

Superoxide dismutase
(SOD):

SOD is a mettalozyme,
containing Cu and Zn/Fe/Mn as a cofactor, present in all aerobic organisms.
SODs are of three types viz. Cu/Zn-SOD (CSD1, CSD2, and CSD3), Fe-SOD (FSD1,
FSD2, and FSD3), Mn-SOD (MSD1) 115, 117.
The mechanism of removal of ROS molecules by SOD is a catalyzing reaction
resulting in the formation of O2 and H2O2 by
dismutating the superoxide radical followed by deprotonation 118.

                       O2-
 + 
O2-  +  2H+                        O2  +   2H2O2

 

Catalase (CAT):

Catalases is a
peroxisomal heme-containing tetrameric oxidoreductase type of enzyme which can
reduce H2O2 by catalyzation reaction 119.
It has a higher affinity for H2O2 and can break it into the water and molecular
oxygen 120.

                                      2H2O2                               O2   +   
2H2O

Although it mainly
found in the peroxisome, its presence also reported in cytosol, chloroplast,
and mitochondria 121. Three genes have
been identified so far, responsible for catalase enzyme production are cat1,
cat2 and cat3 122 in Nicotiana plumbiginifolia. In early
drought condition, the CAT2 gene is upregulated. But this expression became nil
or very low during later stage of drought 123.
The CAT1 gene expression is dependent on ABA level. This gene contains a
homologue of ABRE2 element in promoter sequence, which is the binding site of
CBF1 (Cat1 binding factor 1) and upregulate the expression of CAT1. During
osmotic stress, the ABA level increased followed by increasing of CAT1 enzyme
level 124. The CAT3 gene expression is
regulated by a CDPK named CPK8. Both the CAT3 and CPK8 interaction lead to
increase ABA level and H2O2 homeostasis during drought. Yeast two hybrid assay
proved this interaction. The CPK8
phosphorylate the CAT3 in several sites specifically at T-408.  This phosphorylation leads to upregulation of
CAT3 gene 125.

 

Glutathione reductase
(GR):

A central cysteine
containing flavoprotein oxidoreductase enzyme (60-190 kDa. The oxidized form of
this enzyme is more stable than its reducing form. But in reducing form, it
catalyzes GSSG with the help of NADPH as reducing agent and generate GSH 126, 127. The central cysteine residue is the
critical mediator of this reaction. GR1 and GR2, two types of GR have been identified
so far with NADPH and FAD-binding domains 128.
NADPH binding domain of GR has a highly conserved GXGXXA and an arginine
residue present therein having an essential role for NADPH binding 129. GR provide tolerance against several
abiotic stresses in plants.

                            GSSG + NADPH                  2GSH  
+ NADP+

 

Future prospective: –

As
an essential fiber crop, the demand for cotton will always remain at a higher
level. But several abiotic (as well as biotic) stress mainly the drought and
rising temperature are the main hindrances for the production of the cotton. As
per a review, by 2025 the consumption of cotton will increase up to 49 million
tons and for fulfilling this requirement total 65.5 million ha will be needed.
But due to increasing population, the scarcity of land is an alarming issue
nowadays. Due to low and uneven rainfall pattern, the production of cotton in
new areas, with little rainfall or rain-fed condition, by the conventional way
is somewhat troublesome. Thus the production of drought tolerance cotton
variety is a critical approach now 130.

The possible ways to
produce the drought-tolerant cotton varieties are to find genes related to the tolerant
trait, marker-assisted selection, finding out drought-related orthologous genes
by bioinformatic approaches and by manipulating the genomic consequences.
Therefore, the clasping of the system biological and bioinformatical approach,
as well as wet-lab experiments, may enclose several gateways of this study. For
example, functional research of the critical genes, transcriptional factors, phytohormones,
ROS scavenging enzyme activity, specific interaction of protein-protein or
protein-RNA can be studied to accomplish the objective. Future studies can also
focus on the metabolomic studies to find out the metabolites which are solely
responsible for drought tolerance and marking out the pathways they are
regulating. Finding out the candidate genes related to drought tolerance by bioinformatic
approaches and overexpressing them which can play a significant role of signal
transduction by regulating phytohormones, transcription factors, miRNAs, and
proteins will be proved to be a powerful tool for the development of
drought-tolerant varieties.