Last updated: July 8, 2019
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ReviewCOTTON: – Cotton belongs to the family Malvaceae,  is an important commercial crop cultivated intropical and sub-tropical regions.

It’s a perennial shrubby dicot plant with avariable range in height (3-20 ft.). The leading cotton growing countries areChina, India, USA, Australia, Mexico & so on, contributes more than 80% oftotal cotton production in the world.Thecotton fiber mainly made up of cellulose 1which is a pivotal material in textile industries.

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Cotton is also used in oilindustries as seeds are rich in oil. Linters, contains cellulose, for makingplastics, explosives, high-quality paper and so on. Effectof non-biological factors to hinder the natural growth of an organism is termedas abiotic stress viz.drought, salinity, temperature, & so on. Among them,the drought or water deficient condition is the major problem regarding thecrop production 2. It severely createsa negative impact on growth & life cycle of a plant with the reduction inbiomass accumulation. Drought reduces the leaf size, stem elongation & rootproliferation by decreasing the cell division rate by diminishing the cropproductivity& water use efficiency. Dryness results of drought stressdecrease 30-50% yield 3.

Drought stressmainly affects the flowering stage 4the primary cause of the decrease in yield. Compareto rice, maize the cotton plant show a relatively higher rate of drought stresstolerance 5 but the growth of theplant, fiber yield and quality will reduce significantly 6. During stress condition geneticallysimilar cotton plant show 50% reduction in yield. Thus drought tolerance ofcotton genotypes and the mechanism behind this is a new avenue of research 7. Several numbers of physiological traits such as increase in number and weight of tap root, gear upthe development of root system 8,longer root length 9, reduction intranspiration rate 10 11 are then taken up bycotton plant to overcome this unfavorable condition.Towithstand drought stress plants adopt some morphological, physiological response.  Plants generatesome defense mechanism 12 to maintainnormal conditions during drought.

In case of the cotton plant, drought has someantagonistic effect on plant height, the number of nodes, the rate oftranspiration, stomatal conductance, net photosynthetic rate and waterpotential of leaves 13. Accumulation ofdry matter gets decreased up to 50% during drought condition in Gossypium barbadense 14. The drought tolerancemechanism can be categorized into four parts such as drought escape, droughtavoidance, drought tolerance, drought recovery 15,16.

Drought escape: -It can be gained through shortening of the life cycle or mainly shortening ofthe reproductive season as the flowering time is most crucial trait related todrought 17. As early maturity helps toescape the drought period thus the development of short duration varieties arevery efficient 18. But as the yield isdirectly proportional to the duration of crop, therefore average productivitywill be very less 19. Drought tolerance: -These are the mechanism where plants adopt some changes to withstand droughtthrough physiological activities 20;this changes related to xerophytic plants. By increasing the density, length andproliferation of root 21, by bearing alow number of leaves with less surface area 22plants can withstand drought. Again leaf pubescence and hairy leaf have asignificant effect on drought tolerant as they help the leaf from hightemperature and lowers the transpiration rate 23. Drought avoidance:-It is the mechanism by which plant can reduce water loss by maintainingstomatal regulation, developing more profiling root system 19, 21.

To sustain high tissue waterpotential, the layer of waxy bloom over leaf area consider as an essentialregulator 24, 25. These glaucous leavesare cooler and had a lower degree of senescence than unglaucous leaves 24. Droughtrecovery: – The strategies by which plants cancontinue their natural growth after facing a drought condition is called asdrought recovery. Under severe drought condition, the dehydration ratemaintained by stabilization of cell membrane and accumulating water-solublecarbohydrates 26-28. In grasslands, theplants accumulate fructans, as reserved carbohydrates and a shallow level ofstarch 29-32 and this higher level offructan level after drought improve plant survival 26, 33 and it may act as antioxidants 34.Effect of droughtstress on morphological and physiological; activity in plants: -Drought has somedrastic negative impact on both morphological as well as physiologicalactivities viz. photosynthesis 35,metabolism and mineral nutrition 36. Italso accelerates the growth of pests and weeds in the field 37, 38.

Root growth:-Thecotton plant consists of a tap root system, from where the secondary andtertiary branches of root development started. The roots consist of one layerof epidermis, one layer of endodermis and layers of cortex in between them.Endodermis surrounds stele and pericycle 39.The xylem here is endarch type.Droughtreduces growth, density, elongation of root in the cotton plant 40 but some genotypes get an elongated rootsystem which is more resilient to drought stress 41. In the initial stage of drought, root growth enhances but ifit persists for a long time, it generates an antagonistic effect on rootdevelopment 42  Again the numbers of tap roots increased atfirst encounter without expanding the root biomass, and even the dry weight ofsecondary root become less after drought 9, 43,are suspected to be a common symptom during the drought in cotton. Droughttolerance in transgenic with profuse root system is better than wild-type 44.

   Rate of Photosynthesis:-Photosynthesis ispivotal process controls all the dominant traits in plants health, productivityand so on. Drought creates a detrimental effect on the plant by decreasing therate of photosynthesis, as in water scare condition the stomata became closefollowed by reducing the level of CO2 influx thus it affects photosynthesis 45, 46. In cotton, as water stress reducesphotosynthesis rate 47 followed by thedecreased rate of yield because yield is directly correlated withphotosynthesis 48.

Reducephotosynthesis level resulted in lowering the C accumulation followed bydecrease level of biomass 49. Severedrought stress resulted in the decreased level of chlorophyll a, chlorophyll b andtotal chlorophyll 50. It alsosuppresses the activity of PS-II by lowering the electron transport chainactivity and by releasing the Ca+2, and Mg+2 ions bind with it 51-53. In a study of cotton field study, a decreasedyield resulted as that was lower photosynthesis rate followed by lessproductivity but the same genotype yield well when cultivated in waterabundance condition 54. Thus the rateof photosynthesis is inversely proportional to drought.Effect on Respiration:- Respiration is a cellular process by whichenergy is generated, and this energy is then used up by the otherenergy-dependent process in the cell or may be stored. The efficiency ofmetabolism in plants can be described by the fraction of carbohydrates lostthrough respiration 55.

Root is theprincipal place for using these photosynthates (i.e., carbohydrates) and usedit for the growth and dry matter production in plants as more than 50% ofaccumulated photosynthates were transported to root, and the maximum amount ofthem was respired 56. But therespiration rate decline during drought as soil moisture deficits can reduceroot respiration 57-60. But limitedroot growth and respiration under drought resulting from uncoupling between carbonproduction in leaves and use in the root 61,helps to improved growth on the plant in drought 62.In response to droughtstress in spite of regular electron transport system, plants take up analternate pathway where the electron directly passed to oxygen by an enzymealternative oxidase. Here the production of ATP is meager 63, 64. During drought stress, plants produceReactive Oxygen Species, which leads to the destruction of plant cell membrane.

But their alternate oxidase activity reduces the production of ROS duringdrought 65. In conclusion, droughtstress leads to an imbalance in carbon sources utilization, reduction inbiomass accumulation, reduce ATP level and geared up the generation of ROS.Mechanisms involved indrought tolerance: -The molecular mechanismin drought stress tolerance in cotton: To cope up with water deficientcondition plants change some of its balanced mechanism so that it could adapt.Several genes in several modified pathways are involved in this process inresponse to drought stress.Abscisic Acid: ABAis sesquiterpenoid with 15 carbon ring 66.When imposed to salt, drought, cold and any other abiotic stress, plantsresponse 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 ifstress persists.AsABA is induced during various stresses, considered as the stress hormone in theplant 67, 69. It plays some vital rolein the plant like seed dormancy, storage protein synthesis, leaf senescencedelayed germination and combat pathogenic infection 67.

Generally, ABA synthesis occurred in root and transported viavascular tissues 70 resulting the levelof ABA increased under drought stress resulting several gene expression andstimulating the closure of stomata 71-73.Among ABA-induced genes, 54%, i.e., 133 genes are induced during drought stress71 and ABA expression initiated underthe presence of the ABA-responsive element (ABRE), a cis-acting DNA bindingelement 74, 75. When the extracellularstress signal perceived by membrane receptors, receptors like kinase (RLK),histidine kinase (HK) and then activates several pathways leads to theformation of ABA, ROS, and Ca+ 69.

ThisCa+ ion is a secondary messenger and mediates crosstalk between severalsignaling pathways 76, 77. ABAbiosynthesis precursor ?-carotene gets activated by several drought-inducedgenes like zeaxanthin oxidase (ZEP for conversion of zeaxanthin toviolaxanthin), 9-cis epoxycarotenoid dioxygenase (NCED for Neoxanthin toXanthoxin), and ABA aldehyde Oxidase (AAO for the ABA synthesis from ABAaldehyde). Many of these genes are regulated by calcium-dependentphosphorylation 76-78.Overexpression of PYR(Pyrabactin Resistant) an ABA receptor genes induced during drought conferstolerance to this stress 79. In ABAsignaling pathway the main three components are PYL, PP2C (Protein Phosphatase2C) 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 2Cprotein phosphatase), e.

g., ABI 1 and ABI2 (ABA insensitive 1, 2), a negativeregulator of ABA. Inbound form, PP2C is not able to bind and dephosphorylateSnRK2 (sucrose nonfermenting kinase-1 Related protein kinase). This SnRK2 are aprotein of serine-threonine kinase having a role in drought condition 83. Thus the SnRK2 in the phosphorylatedstate (unbound and activated form), phosphorylate ABFs (ABA-responsive elementbinding factors), which again binds to ABRE and induce the ABA-responsivesignals 84-87. In upland cottonvarieties, several genes have been identified which have drought tolerantresponse by ABA-dependent manners such as GhATAF1 6, GhMKK3 88, GhNAC2 89 and GhCBF3 90.MAPK Signalling: Underfocus of water deficit condition for long time plant may evolve or adopt somesignaling pathways to combat severe dehydration conditions.

Mitogen-ActivatedProtein Kinase (MAPK) signaling is one of the major factors among them, theyare conserved in all eukaryotes from plant to animals, from insects to fungi 91, 92. MAP Kinase pathway is a cascadesignaling which composed of three different kinases viz. MAPKKK, MAPKK, MAPK 93, 94. During stress condition, anotherprotein kinase, MAPKKKK (i.e., MAP4K) may be activated 95. After getting external stress stimuli, first, the MAPKKKactivates by phosphorylation and then it phosphorylates the twoSerine/Threonine (or both) residue at S/TX3-5S/T motif situated in the MAPKKactivation loop.

This activated MAPKK then continues the cascade byphosphorylating conserved T-X-T motif of MAPKs, which Phosphorylates severalother transcription factors by their activation. Those TFs then mediatesseveral 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 geneGhMKK3 isolated which shows efficient tolerance to drought stress byup-regulating root hair elongation gene and reduce the rate of water loss bystomatal closure in Nicotiana benthamiana.Silencing this gene shows a high level of water loss and thus wilting duringdrought 88. In G.

raimondii genome, bioinformatics analysis helps to identify 28MAPK genes 99.Aquaporins:Aquaporinis a type of transmembrane protein present in all form of life due to itspivotal role in water balance in the cell, belongs to MIP (Major IntrinsicProtein) superfamily. It acts by a phosphorylation-dephosphorylation mechanism 100.

Aquaporin in plants consists of 5subfamilies PIP (Plasma membrane Intrinsic Protein), SIP (Small IntrinsicProtein), TIP (Tonoplast Intrinsic Protein), XIP (X or Unrecognised IntrinsicProtein), NIP (NOD-26 like Intrinsic Protein) 101.Most of them are expressed continuously throughout the life, but some of themexpressed 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 amongthem twenty-eight are PIP, twenty-three TIP, twelve NIP, seven SIP and rest one is XIP. All them shows common levelstructural similarity that is six alpha-helical structure with fiveinter-helical loops and a common AEP (Alanine-Glutamine-Phenylalanine) motif inN terminus and two NPA (Asparagine-Proline-Alanine) motif 106. Among five loops A, C and E areextracellular whereas B and D are intracellular. These two play an essentialrole in forming of water channel 107.

Aquaporin not only acts as water transporter channel but also a channel oftransport for glycerol and urea in the cell. In recent studied, related thatNt-AQP1, an aquaporin of tobacco plasma membrane, transport water, glycerol,and urea 108-110, NtTIPa, a tonoplastaquaporin mainly transport urea 110.Theplasma membrane aquaporin (PIP) is the leading component concerned with waterbalance in the plant cell. Opening and closing of plant plasma membraneaquaporins are depended on phosphorylation (open) and dephosphorylation (close)of particular amino acid(s).

Dephosphorylation of Ser115 in cytosolic loop Band Ser214 in C-terminus and of SOPIP2;1 of spinach leads to closure of thisaquaporin 103 again Leu197 along withHis99, Val104, Leu108 of cytosolic loop D makes a hydrophobic motif blockingthe water to enter. The diameter becomes 1.4Å and further becomes narrower to0.8Å near these residues, where the minimum diameter should be 2.1Å to passwater.

In case of open structure for the SOPIP2;1 aquaporin the N terminushelix 5 extends itself into the cytoplasm, followed by displacing the aminoacids blockage.  As a result, thecytoplasmic loop D moves 16Å from the blockage site due to phosphorylation ofSer115 and Ser274 also creating a free space for water entrance 111.Thuswhen water potential is high, the amino acids become phosphorylated followed bythe opening of the gate and in case of lower water potential dephosphorylationoccur resulted in the closing of the gate so that, no efflux of water from thecell takes place 112.

In case of droughtstress in cotton, GhPIP2;7 gene expressed in leaves and cotyledons and shows ahigh 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 droughttolerance 105.ROS Scavenging: ROSor Reactive Oxygen Species are continuously produced in various metabolicpathways as a byproduct in cellular organelles such as the chloroplast,mitochondria, peroxisome and so on. They can damage protein, DNA, and membranelipid 114. There is a subtle balancebetween production and scavenging of ROS so that it can’t cause any oxidativedamage. But several types of abiotic (as well as biotic) stresses like drought,temperature, Salinity, cold-induced the level of ROS followed by breaking thebalancing 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.

Plantsadopt two different kinds of mechanisms to scavenge the ROS- enzyme-dependent,and enzyme-independent 116. Here onlyenzymatic ROS scavenging activity will be discussed.Enzymatic ROSscavenging: Several enzymes like SOD (Superoxide dismutase), APX (AscorbatePeroxidase), 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 reactionresulting in the formation of O2 and H2O2 bydismutating the superoxide radical followed by deprotonation 118.                       O2- + O2-  +  2H+                        O2  +   2H2O2 Catalase (CAT): Catalases is aperoxisomal heme-containing tetrameric oxidoreductase type of enzyme which canreduce H2O2 by catalyzation reaction 119.

It has a higher affinity for H2O2 and can break it into the water and molecularoxygen 120.                                       2H2O2                               O2   +   2H2OAlthough it mainlyfound in the peroxisome, its presence also reported in cytosol, chloroplast,and mitochondria 121. Three genes havebeen identified so far, responsible for catalase enzyme production are cat1,cat2 and cat3 122 in Nicotiana plumbiginifolia. In earlydrought condition, the CAT2 gene is upregulated. But this expression became nilor very low during later stage of drought 123.The CAT1 gene expression is dependent on ABA level. This gene contains ahomologue of ABRE2 element in promoter sequence, which is the binding site ofCBF1 (Cat1 binding factor 1) and upregulate the expression of CAT1.

Duringosmotic stress, the ABA level increased followed by increasing of CAT1 enzymelevel 124. The CAT3 gene expression isregulated by a CDPK named CPK8. Both the CAT3 and CPK8 interaction lead toincrease ABA level and H2O2 homeostasis during drought. Yeast two hybrid assayproved this interaction. The CPK8phosphorylate the CAT3 in several sites specifically at T-408.  This phosphorylation leads to upregulation ofCAT3 gene 125.

 Glutathione reductase(GR):A central cysteinecontaining flavoprotein oxidoreductase enzyme (60-190 kDa. The oxidized form ofthis enzyme is more stable than its reducing form. But in reducing form, itcatalyzes GSSG with the help of NADPH as reducing agent and generate GSH 126, 127. The central cysteine residue is thecritical mediator of this reaction. GR1 and GR2, two types of GR have been identifiedso far with NADPH and FAD-binding domains 128.NADPH binding domain of GR has a highly conserved GXGXXA and an arginineresidue present therein having an essential role for NADPH binding 129.

GR provide tolerance against severalabiotic stresses in plants.                             GSSG + NADPH                  2GSH  + NADP+ Future prospective: -Asan essential fiber crop, the demand for cotton will always remain at a higherlevel. But several abiotic (as well as biotic) stress mainly the drought andrising temperature are the main hindrances for the production of the cotton. Asper a review, by 2025 the consumption of cotton will increase up to 49 milliontons 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 issuenowadays. Due to low and uneven rainfall pattern, the production of cotton innew areas, with little rainfall or rain-fed condition, by the conventional wayis somewhat troublesome.

Thus the production of drought tolerance cottonvariety is a critical approach now 130.The possible ways toproduce the drought-tolerant cotton varieties are to find genes related to the toleranttrait, marker-assisted selection, finding out drought-related orthologous genesby 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. Forexample, functional research of the critical genes, transcriptional factors, phytohormones,ROS scavenging enzyme activity, specific interaction of protein-protein orprotein-RNA can be studied to accomplish the objective. Future studies can alsofocus on the metabolomic studies to find out the metabolites which are solelyresponsible for drought tolerance and marking out the pathways they areregulating. Finding out the candidate genes related to drought tolerance by bioinformaticapproaches and overexpressing them which can play a significant role of signaltransduction by regulating phytohormones, transcription factors, miRNAs, andproteins will be proved to be a powerful tool for the development ofdrought-tolerant varieties.