ISOLATION, CHARACTERIZATION AND EXPRESSION ANALYSIS OF PUTATIVE DROUGHT RESPONSIVE EXPRESSED SEQUENCED TAGS FROM GOSSYPIUM ARBOREUM ROOTS

A. Jamal1,2*, M. N. Shahid3, B. Aftab4, A. K. Johargy1, M. S. Alshmemri1, B. Rashid2, and T. Husnain2

1College of Nursing, Umm Al-Qura University, Makkah-715, Kingdom of Saudi Arabia.
2Plant Genomics Lab, Centre of Excellence in Molecular Biology, University of the Punjab, 87-West Canal Bank Road, Thokar Niaz Baig, Lahore-53700, Pakistan.
3Department of Botany, Division of Science and Technology, University of Education, Lahore, Pakistan.
4Department of Biological Sciences, Faculty of Fisheries and Wildlife, University of Veterinary and Animal Sciences-Ravi Campus, Pattoki-55300, Pakistan.   
*Corresponding author Address  Email: adiljamalcemb@gmail.com, aajamal@uqu.edu.sa

ABSTRACT

Cotton is an important economic fibre crop. Seasonal water shortages and long term water deficit can effect cotton yield. Current study aimed to explore the cotton root transcriptome under drought stress. mRNA extracted from cotton roots subjected to osmotic stress treatment (5% gravimetric humidity) was used to construct cDNA library. Expressed sequence tags submitted to gene bank EST database (JK757087-JK757798) further annotated to predict the homology and function. Total 104 transcripts with an E-value less than 1e-33 revealed 82 (78.84%) known homologs and 22 (21.15%) with uncharacterized proteins. Gene ontology and KEGG analysis of these drought responsive ESTs elucidated their key role in biological regulation, molecular functions and cellular organelles. Expression pattern of 10 unigenes were validated by RT-qPCR in roots and leaves. These unigenes included WD repeat, FRIGIDA, peroxidase, E3 ubiquitin ligase, U-box domain, RNA binding, calcyclin binding, Glutathione S transferase, endochitinase and metallothionine like protein. cDNA library was successfully constructed from cotton roots and revealed several key drought responsive transcripts. Novel sequences identified in this study can be valuable resource for further exploration studies to exploit their role in genomics of drought responsive mechanism.

https://doi.org/10.36899/JAPS.2020.3.0088                                                                               Published online March 25, 2020.

INTRODUCTION

                Cotton is an important economic fibre crop, grown in tropical zones especially in U.S, Uzbekistan, China, India, Brazil, Pakistan and Turkey (Riaz et al., 2013). As stated by measurable surveys, India, USA, China, Brazil and Pakistan are top five principal cotton producing countries in the world (Statista 2019; OECD-FAO 2019). Cotton production declined in India, China, USA and Pakistan mainly due to water shortage and pest problems (OECD-FAO 2019). Unfavourable environmental abiotic stresses are leading factors in decreasing agricultural productivity (Grayson, 2013). Indeed, cotton being as glycophyte shows higher degree tolerance to abiotic stresses. However, extreme natural factors like drought affect growth, productivity and as well fibre quality of cotton (Parida et al., 2007). The molecular biology approaches play key role in genome alteration of higher plants against environmental factors for better growth and yield (Edgerton, 2009; Lawlor 2013). Incapacitating abiotic factors that reduce the crops yield has been main area of discussion. Drought, key abiotic factor that significantly affects plant biomass and yield (Chaves and Oliveira, 2004). The cotton genome is large as compared to other plant species, making it challenging to study. Thus, key understanding of drought stress tolerance can disclose the variable expression and regulation of key genes that may augment cotton drought tolerance.    

                Drought includes a variety of plant responses, including stomatal regulation, gene expression alteration, build-up of abscisic acid, generation of osmotic compounds and production of defensin proteins that destroy free radicals, ROS or act as nucleic acid binding factors (Wang et al., 2003). These events are controlled by complex networks at molecular level that trigger stress responsive mechanism to restore homeostasis, defend and revive impaired cellular components (Ramachandra et al., 2004). Responses to abiotic factors are genetically intricate and also complicated to understand. Previously, gene expression encoding dehydrins, antioxidants involved in the generation of structural and functional metabolites were used for modification of stress tolerance in plants (Park et al., 2005). Currently, approaches to utilize genes with their role in signalling and monitoring networks and pathways have revealed remarkable potential (Umezawa et al., 2006).

                Previous studies have focused on the cotton drought resistance on aboveground plant tissues while less knowledge have been documented on underground plant tissues. For instance, 3,517 unigenes were differentially expressed in Gossypium herbaceum leaves and roots of cotton with involvement of the 28 biological pathways significantly to drought stress (Ranjan and Sawant, 2015); cDNA library analysis of the 92 positive drought stress responsive clones with differential expression (Zhang et al., 2009); 6,047 high-quality expressed sequence tags (ESTs) from G. barbadense revealed enrichment of transcription factors and stress-related genes (Zhou et al., 2016).

                Roots are the vital organs of plants with key role in absorption and translocation of nutrients and water. Being as major connection between the plant and soil stresses, root generate specific chemical messenger from root to shoot that initiate stomatal closure and ultimately reduce evaporation losses (Davies and Zhang, 1991; Jia and Zhang, 2008). However, limited information prevails at molecular level regarding cotton root responses under water deficit stress (Graya and Brady, 2016). A key understanding of principal genes involved in osmotic stress is necessary for the plant development that sustain more yields under osmotic stress.

                Sequenced and re-sequenced cotton genomes are simply the foundation; the main challenge is to discover the features of the genome to elucidate the biology. The next stage of cotton genomics will entirely expose these biologically genome active states, as has been made for other model crop plants where high density genetic and fine maps, SNP array platforms, transcript abundance epigenetic regulations and modifications (Ashraf et al., 2018). Recently, most promising molecular approach is transcriptome profiling for demonstrating how information obtained from sequence data can be transformed into an extensive knowledge of gene function. Genome sequence and latest approaches like NGS technology practiced and reported several reports in cotton using RNA-Seq analysis. For instance, cotton root transcriptome analysis under water deficit stress (Bowman et al., 2013; Zhang et al., 2016) has been reported. However, RNA-Seq technique being among latest molecular approaches to study gene annotation faces some challenges such as library construction hence screening cDNA libraries screening as high throughput approach is an efficient way to identify functional and stress-tolerance genes in cotton (Li et al., 2019) and other plant species (Wang et al., 2019; Dossa et al., 2019).

                Despite of being the massive cotton genome sequence information using latest genomic approaches like whole genome sequencing and re-sequencing, still there large information gaps as compared to other model plants like tobacco and Arabidopsis. Hence, next era of cotton genomics require re-sequencing broad diversity panels, draft genome refinement including the development of high throughput functional genomics tools and integrating multidisciplinary approaches including transcriptomics, epigenomics, proteomics and bioinformatics to further explicate its genome and functional characterization.     
                Our lab has previously reported the abiotic stress responsive genes in Gossypium arboreum using multiple molecular approaches and tools (Maqbool et al., 2008; Barozai and Husnain 2012; Shahid et al.,2012). Previous findings revealed the elucidative role under multiple abiotic stresses. Taking into account the Gossypium arboreum as potential gene pool of abiotic stress responsive genes, present study was planned to explore the key putative drought responsive transcripts in cotton root by cDNA library. In this study, we report the identification and functional characterization of young root drought responsive based EST’s of cotton.

MATERIALS AND METHODS

Plant growth, drought stress induction, RNA isolation and mRNA purification: Gossypium arboreum cv FDH-786 was selected for evaluation. Delinted seeds were grown in a mixture of peat, sand, soil (1:1:1) under controlled environmental conditions in green house at 25+2oC; relative humidity 45-50% and 1500 µmolm2s-1 light intensity provided by metal halide lamps (400 W). The water stress treatment was inducted following previous studies (Maqbool et al., 2007; Jamal et al., 2014). The amount of water held by the soil was measured as gravimetric humidity (GH). Forty days old cotton seedlings following two moisture stress treatments 10% and 5% GH along 15% GH as control treatment were taken into study. Water stress treatment was maintained periodically for 15 days and monitored gravimetrically by weighing the pots daily. The fresh roots and leaves were harvested, immediately frozen and grinded in liquid nitrogen for RNA isolation. To construct the drought responsive cDNA library, RNA extraction was done form plants maintained at 5% GH.

                The relative water content (RWC) of leaves were measured for the second fully expanded leaves. The RWC was measured as described earlier (Barrs and Weatherly, 1962).

RWC (%) = [(fresh wt – dry wt) / (turgid wt – dry wt)] × 100

                Total RNA isolation from the roots and leaves was performed as described earlier with minor modifications (Jakola et al., 2001). Isolated RNA was further treated with DNase I, RNase-free (Thermo Fisher Scientific, USA) to avoid genomic DNA impurity before the synthesis of mRNA. To check the integrity of RNA samples, RNA samples were electrophoresed on 0.9% agarose. RNA concentration was measured using spectrophotometer (ND-1000 NanoDrop Technologies, Inc.). RNA samples having A260/280 ratio of 1.8-2.0 were used further. mRNA extraction and purification was performed using oligotex mRNA mini kit (Qiagen, Valencia USA) following manufacturer directions.

cDNA library construction, clones amplification, Sequencing and bioinformatics: cDNA library was constructed using CloneMinerTM cDNA library construction kit (USA, Invitrogen) following manufacturer instructions. Blunt end ds cDNA was size fractioned using low melt agarose gel (0.8%) with size ranged between > 100 bp - <1 kb. The ds cDNA was eluted using DNA gel extraction kit (Thermo Fisher Scientific, USA) following manufacturer guidelines. The eluted cDNA was proceeded for BP recombination reaction. Electroporation was performed for fractioned cDNA aliquot by adding to thawed ElectroMAX DH10B TM T1 phage competent resistant cells. Electroporated cells were incubated at 37oC for 1 h at 200 rpm for the expression of kanamycin. Incubated cells were further pooled with equal volume of sterile freezing media. These aliquots were prepared from pooled samples and stored at -80oC. Clones were screened from white colonies on agar plates with kanamycin selection. Positive transformants were confirmed by colony PCR. Colony PCR was performed using M13 sense and antisense primers following amplification program of initial denaturation at 94oC for 5 min; denaturation at 94oC, annealing at 52oC and extension at 72oC each of 35 cycles at 45 s, 45 s, 60 s respectively and final extension at 72oC for 10 min.

                For sequencing, isolated plasmid DNAs from randomly selected clones were used for Sanger sequencing. Isolated plasmids were extracted using alkaline lysis method (Sambrook et al., 1998).Clones were sequenced using cycle sequencing kit (ABI PRISM Foster, USA) on Applied Biosystems Sequencer model 3100/3700. The vector and adaptor sequences present at both 5´ and 3´ of sequences were removed using Vec Screen online available tool (https://www.ncbi.nlm.nih.gov/tools/vecscreen/). NCBI database was used to study the non-redundant nucleotide sequence (BASTN) and non-redundant protein sequence (BLASTX) similarities between ESTs and other databases sequences. Unisequences with an E-value < 1e-33 were compared to swissprot and blastx (Altschul et al., 1990). Analyzed blastx predicted sequence homology against Gossypium species were further used in Cotton Functional Genomics Database (CottonFGD) (https://cottonfgd.org/) to find the Gene ID of the respective sequences. The gene ids of the respective sequences saved were used for further annotation and functional assignments. The gene annotation (functions) of cotton was performed using PANTHER (ver 13) (http://pantherdb.org/) using corresponding Arabidopsis orthologs. KEGG Mapper (Kyoto Encyclopedia of Genes and Genomes) (Ogata et al., 1999) (http://www.genome.jp/kegg/mapper.html) was used to study the KEGG orthology and associated pathways online available.

Quantitative real time PCR analysis (RT-qPCR): Total RNA was treated with DNase I to avoid any genomic DNA residues. First strand cDNA was synthesized using cDNA synthesis kit (Fermentas, Germany). Primers were designed against using online available tool (http://bioinfo.ut.ee/primer3-0.4.0/) (Table 1) with selection of no primer dimer synthesis. Cotton GAPDH primer was used as reference gene for normalization (Zahur et al., 2012). Quantitative real time PCR was performed using iQ5 (Bio-Rad, USA) with IQTM SYBR Green supermix (Fermentas, USA). Amplification program consisted of 95 oC for 3 min, then 40 cycles of amplification at 95oC for 30 s, 60oC for 30 s, 72oC for 30 s. Melting curves were obtained from 70oC to 95oC at 0.1oC/s by continuous monitoring of fluorescent signals to check the specificity of amplicons and primers dimers. Each reaction was set in triplicate for both technical and biological to minimize any variation. Reactions were set in volume of 20 µL containing 200 ng cDNA, 15 µL 2X SYBR Green supermix and 1.0 µL (10 µM) of each primer. CT (cycle threshold) values were analyzed later using iQ5 software (Bio-Rad Ver 1.0). The cycle threshold (CT) values were exported to MS Excel for further analysis. To study the relative gene expression level, comparative ct method was adapted. The CT values were normalized with internal standard and the fold differences were calculated using delta approach (Livak and Schmittgen, 2001).

RESULTS

Physiological performance of cotton under drought: RWC from cotton leaves were found to be 70.23%, 55.12% and 44.30% with plants grown at 15%, 10% and 5% GH (gravimetric humidity) levels respectively. A pronounced reduction in RWC of leaves was measured at 5% GH level in comparison to other treatment.
Construction of cDNA library and functional characterization of cDNA sequences: Total 800 clones were randomly selected for sequencing. After sequencing, 711 clones showed an insert size of 100-800 bp. Total 711 unigenes (Accession# JK757087-JK757798) were submitted to NCBI Genebank. NCBI BLASTN revealed that 76% (541 sequences) showed sequence similarity to known sequences. Majority of blast match hits belonged to Gossypium spp, Populus trichochorpa spp, Oryza sativa, Zea Mays, Glycine max, Medicago, Nicotiana spp, A. thaliana and Ricinus spp, Atriplex and other plant species. Similar ESTs were simultaneously annotated for their protein functions to categorize into their classes. The BLASTX results demonstrated 104 unigenes showing significant similarity to known genes, 82 uniESTs displaying significant similarity to genes of predicted proteins, and 22 uniESTs remain uncharacterized in NCBI database.
                The gene ontology terms were further used to classify the gene products with an E value 1e-33 in functional GO categories and simplified into plant-specific annotations (GO classification) to obtain additional insights into the putative functions of unigenes. Of the 104 G. arboreum ESTs, 82 (78.84%) were assigned GO terms in any category (biological, cellular and molecular), and the other 22 (21.15%) ESTs were uncharacterized proteins without GO terms annotations. EST's which had no Arabidopis homology in NCBI genebank, failed to obtain a GO term, fell into distinct categories like uncharacterized, predicted and hypothetical proteins. We identified 82 unigenes from the 711 total ESTs, representing non-redundant unigenes, that share similarities with defense realted genes and stress response according to GO classifications. These 82 unigenes with Arabidopis based known homologs were further annotated to molecular function 30 (83%), 62 (100%) to biological process and 44 (100%) to cellular components. Many of the EST’s in molecular function category (53%) were associated with catalytic activity followed by binding activity (30%), whereas the remaining ESTs were involved in structural molecule, translation and transport activity (Fig 1A). Within the category of biological process, 23 EST’s (37%) were relegated to metabolic process, 19 (30%) to cellular process while others assigned to response to stimulus, biogenesis, localization and regulation (Figure 1B). Based on the cellular components, 20 (45%) EST’s contributed maximum to cell part and 10 (22.7%) to macromolecule complex followed by organelle, membrane and cell junction (Figure 1C). EST’s were also grouped based on the protein categories such as nucleic acid binding & hydrolase (19.6% each), enzyme modulator (10.7%) followed by chaperone, transferase, isomerase, lyase, cytoskeletal, oxidoreductase, signalling, storage, membrane trafficking, carrier, transporter, cell adhesion and calcium binding protein (Figure 1D).
                BLASTX results showed significant drought responsive genes (Table 2). Some of these genes revealed sequence homology with transcription factor JUNGBRUNNEN 1, WD repeat, heat shock, FRIGIDA, peroxidase P7, glutathione S-transferase, potassium channel KAT1, ubiquitin carboxyl-terminal hydrolase, U-box domain, serine/threonine kinase, polyubiquitin, Zinc finger, lysine histidine transporter, junction-mediating and regulatory, 26S protease regulatory subunit, endochitinase, metallothionein, translational activator, CBL-interacting protein, ubiquitin, calcyclin-binding, RNA-binding like proteins isoforms. The detailed descriptions of 104 unigenes was obtained by BLASTX (Table 2).
Predicted KEGG pathways and validation of differentially expressed selected unigenes: The predicted KEGG pathways included HSP20 family protein, phosphatidylinositol glycan, large subunit ribosomal protein L23e, chromodomain-helicase-DNA-binding protein, DNA polymerase delta subunit, peroxidase, DNA-3-methyladenine glycosylase II, Peptidyl-prolyl cis-trans isomerase A, phosphoglucomutase, enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase, translation initiation factor 1A, Ubiquitin, 1,3-beta-glucan synthase, crossover junction endonuclease EME1, 26S proteasome regulatory subunit T1, NADH dehydrogenase, carbonic anhydrase, proline iminopeptidase, calcyclin binding protein, auxin influx carrier, protein O-GlcNAc transferase, histone H2A, small subunit ribosomal protein S8e, gibberellin 2- oxidase, U3 small nucleolar RNA-associated protein, cell division protease, calmodulin and glutathione S-transferase (Table 3). This ultimately proves and evidence that the potential drought responsive unigenes in our study showing the BLASTX results strongly support their direct and indirect involvement in different pathways with similar role (Table 2, Table 3).
                We selected ten ESTs with their known function in response to stresses: WD repeat (WD, JK757101), FRIGIDA (FRIGIDA, JK757130), Peroxidase P7-like isoform (POX, JK757160), U-box domain (U-box, JK757206), E3 ubiquitin-protein ligase (E3LIG, JK757286), RNA-binding (RBP, JK757358), calcyclin-binding (CBP, JK757361), glutathione S-transferase (GST, JK757362), endochitinase (ECHT, JK757585), metallothionein (MTT, JK757720) like proteins to further validate the expression of these genes in response to drought stress in cotton roots. The real time PCR showed significantly elevation of all 10 selected unigenes in root tissues as compared to leaf tissues. Among the 10 unigenes, WD (JK757101), was most up-regulated (223.8 fold) followed by FRIGIDA (188.40 fold), POX (100.22 fold), U-Box (72 fold), E3LIG (63.13 fold), RBP (53.46 fold), CBP (41.86 fold), GST (31.65 fold), ECHT (7.89 fold) and MTT (6.64 fold) at 5 % GH. Variable expression was measured with similar sequential pattern under 10% GH (Fig. 2). In leaves, subjected to drought stress at 5% GH, the expression pattern was significantly higher than 10% GH but it was significantly less than that of expression pattern observed in root tissues. WD (JK757101) showed the maximum up-regulation (51.73 fold), followed by U-Box (31.90), POX and FRIGIDA with similar expression level, CBP and E3LIG with similar expression pattern, RBP (18.40 fold), GST (15.81 fold), MTT (6.79) and ECHT (5.09) (Fig 2).

Table 1. Sequences of qRT-PCR primers used in this study.

Table 2. Homology analysis of the transcripts with E-value < 1e-33.

Table 3. Gene ID’s along with KEGG orthologs and their associated pathways

DISCUSSION

                Many agro-physiological parameters related to drought tolerance have been established, RNA content, Relative water content (RWC) with decrease in water supply (Deblonde et al., 1999). The RWC is an important index in plants to measure plant water status, imitating its metabolic activity in tissues for dehydration tolerance (Anjum et al., 2011). Pronounced decline in RWC in plants leaves was observed with increasing water deficit in our study. Earlier studies also report decline in relative water contents with increase in water deprivation (Kumar et al., 2011; Meher et al., 2018). Relatively higher RWC observed in progressive mild stress than severe stress indicates that plants have the ability to sustain their water content under mild stress, whereas this ability lost under severe stress treatment in case of our findings. Alterations in RWC may be ascribed as ability of the variation to absorb more water and/or the ability to control water loss through stomata under osmotic stress (Bayoumi et al., 2008).   
                It is crucial to identify the differentially regulated genes and thorough understanding of stress tolerance at molecular and cellular levels (Ghorbel and Murphy, 2011). The crops whose complete genome sequence is not yet available, researchers have to opt the way by studying model genomes to explore EST sequences (Ewing et al. 1999). cDNA library constructed in our study reconnoitered drought responsive ESTs that can help in better understanding the molecular basis of drought tolerance in cotton. Overwhelming evidences also highlighted the role of stress genes and functionally efficient proteins involvement in biological processes, molecular functions and cellular structures in leaves and root tissues (Jiaa et al., 2015; Zhang et al., 2017).
                Earlier cDNA libraries have been constructed for cotton related to drought responsive genes in leaves and roots (Zhang et al., 2009; Ranjan and Sawant, 2015). Still there is lack of molecular information regarding cotton root responses to drought stress. In this study, cDNA library containing 711 clones was constructed. Of these 711, 82 ESTs with an E value 1e-33 had significant homology / similarity to reported genes in database. While remaining (22 ESTs) with uncharacterized function to any genes in Genbank databases suggesting that these uncharacterized unigenes probably embroil in abiotic stress tolerance mechanism. These unknown genes are of particular interest and can be explored further for their role, sequence and structure at protein level. In our study, though a large number of sequences were potential drought responsive transcripts. These genes were involved in the catalytic, structural molecule, cellular, metabolic, nucleic acid binding, transporter and hydrolase activities assuming their role in drought induction. This implies that we successfully constructed cDNA library and have identified cotton root drought responsive genes.
                The potential drought stress homologs engaged in biological processes reported in our study reveal their direct and / or indirect involvement in multiple stresses. Endochitinase (JK757105, JK757585) also known as EP3 chitinase has chitin binding activity and performs active biological functions in defense response, plant-type hypersensitive response, response to bacterium, wounding, somatic embryogenesis. Endochitinase being as type of pathogenesis related proteins are prompted under drought stress (Wang et al., 2016). JK757160 is peroxidase homolog (POX) P7 like protein involved in oxidative stresses. Different isoforms of POX are triggered by environmental stresses besides its involvement in plant growth and development. Various isoforms of POX are expressed under abiotic and biotic stresses (Li et al., 2009; Chiang et al., 2015). The homolog to the potassium channel (KAT) identified in this study was JK757186. KAT1 is the member of shaker family potassium ion channel located as integral membrane component and its active involvement in potassium ion transmembrane transport, membrane potential regulation and stomatal movement. Charged K+ ion channels are involved in the regulation of guard cell volume. The cytosolic phosphoglucomutase (PGM) (JK757202) identified in roots cDNA library expresses during plant growth and developmental stages. PGM being as proteins of carbon/nitrogen metabolism was reported to be more abundant in plant roots under drought stress (Mohammadi et al., 2012a; Mohammadi et al., 2012b). JK757242 in our study shows close similarity with callose synthase central role in plant development and multiple abiotic stresses. Previous findings also identified the accumulation of callose in plasma membrane in plant tissues to various biotic (wound) stresses (Chen and Kim, 2009).
                In our study, few unigenes have key biological role of proteasome-mediated ubiquitin-dependent protein catabolic process. 26S protease regulatory subunit (JK757321, JK757367 and JK757435) is proteasome complex with ATPase, hydrolase and peptidyl-prolyl cis-trans isomerase activity. 26S proteasome required for ubiquitin-dependent degradation, plant development and stress responses (Kurepa et al., 2009). Previous report illustrate that reduced 26S proteasome biogenesis results into increased hypersensitivity of heat shock while increases 26S proteasome biogenesis leads to boost cell capacity to destroy oxidized proteins which ultimately leads enhance stress tolerance during oxidation (Kurepa et al., 2009).
                Metallothionein (MT) (JK757467, JK757711 and JK757720) have been reported in root elongation inhibition (Zhigang et al., 2006), biotic stress resistance (Wong et al., 2004) and abiotic / metal stress tolerance (Kholodova et al., 2010). In current study, JK757596 identified close homolog of universal stress protein (USP). USP improves the cell survival rate in response to stress when exposed to longer period of times, and may accomplish plants with broader spectrum of stress tolerance (Raphael et al., 2011). Previous findings suggest dignified involvement of USP to abiotic stresses like oxidative, salt, heat and drought (Zahur et al., 2009).
                Our study enclosed few unigenes JK757125 JK757219 and JK757358 imparting their role as translation initiation factors (eIF). Previous studies reported TaeIF an overexpression in response to mild osmotic stress (Singh et al., 2007), elevated expression in roots (Yang et al., 2017), increased oxidases activities by enhancing protein synthesis and augmentation of ROS scavenging (Wang et al., 2012). EST JK757130 encodes FRIGIDA (FRI), a transcription factor functions in dehydration avoidance strategy (Lovell et al., 2013; Schmalenbach et al., 2014). Few unigenes JK757127, JK757312, JK757424 and JK757487 reported in current study are homologs to 60S and 40S ribosomal protein, involved in nucleic acid binding, translation and expressed during different growth and developmental plant stages. Environmental stresses regulate the ribosomal proteins and their overexpression in plants but still their abiotic stress mechanism is not very well understood (Xu et al.,2013; Liu et al., 2014). Ubiquitin carboxyl terminal based homologs JK757195 and JK757402 are engaged in hydrolase activity. The ubiquitin/26S proteasome proteolytic pathway plays an important role in development, stress responses and environmental adaptation by degrading short-lived and abnormal proteins (Hershko and Ciechanover, 1998; Callis and Vierstra, 2000). Among the unigenes involved in molecular processes, JK757233 serine threonine kinsase (SnRK) based homolog is involved in plant response to abiotic stresses, abscisic acid (ABA)-dependent plant development (Afzal et al., 2008) and metabolic signalling (Halford et al., 2003).
                WD repeat (JK757101) identified in this study is expressed in different plant tissues during different plant developmental stages. WD repeats proteins are key players in abiotic stresses (Lee et al., 2010). Calcineurin binding protein- interacting protein kinases (CBL- CIPK) (JK757507, JK757510) perform key role in Ca2+ signals perception besides plant development (Eckert et al., 2014). Overexpression of CBL-CIPK confers drought tolerance in plants through the regulation of stomatal movement (Wang et al., 2016). Coatomer (JK757302) is clathrin adaptor complexes medium subunit family protein located in golgi, cytosol and regulate bodies development in endosperm under drought stress (Chen et al., 2017).
                cDNA library showed that 21.15% ESTs had no homology to any protein in the NCBI database. These uncharacterized ESTs may provide novel and putative candidate genes for investigation to elucidate their role in drought stress. Transcriptome studies in past unveiled the uncharacterized transcription factors (Kumar et al.,2015), hypothetical proteins (Ding et al., 2014) and uncharacterized  potential sequences (Govind et al., 2009) that modulate the drought tolerance. Furthermore, the identification of uncharacterized genes as stress responsive provides a function to these genes that could not be identified under non-stressed conditions.
                Cellular, biological and molecular responses of plants to these stresses have been studied intensively (Hasegawa et al., 2000 ; Xiong et al., 2002). In our studies, we found different categories of genes had variable and differential expression in both roots and leaves tissues. Few unigenes affianced in biological processes showed an over expression under osmotic stress. We confirm the presence of cotton peroxidases (POX; JK757160) supposedly involved in abiotic stress responses of roots and leaves with a real time quantitative PCR (RT-qPCR) in root and investigate their role in under osmotic stress as reported earlier (Csiszár et al., 2012) suggesting that this transcripts may be components of the antioxidative defense mechanism activated especially in the drought tolerant cultivar. Another key unigene involved in biological function reported as E3 ubiquitin protein (E3LIG; JK757286) ligase found to upregulated in both roots and leaves tissues under stress that is supposed to have its functions in the drought stress response via the ABA-signaling pathway. E3 ubiquitin ligases been reported to be involved in ubiquitination-mediated degradation via the 26S proteasome by regulating ABA receptors degradation (Li et al., 2016). Previous reports suggested an overexpression of E3LIG based regulatory components in response to drought stress in plants (Kim et al., 2014).
                Gluathione S transferase (GST) gene expression patterns to abiotic stresses demonstrated in plant systems explicate their role in enhancing stresses tolerance (Edwards and Dixon, 2005). A homolog (GST; JK757362) reported in this study displayed elevated expression pattern both in roots and leaves emphasizing its role in plant acclimation towards drought tolerance. Previous findings revealed the upregulated expression of GST to multiple abiotic stresses in above ground and underground plant tissues (Diao et al.,2010; Ding et al., 2017). Our results are in accordance with previous findings assuming responsiveness of GST to abiotic stresses.Current study entails homolog Endochitinase (ECHT; JK757585) explicating its inducible expression to osmotic stress emphasizing role in plants defense mechanism (Chen et al. 1994). Elevated ECHT expression have been reported in underground and above ground plant parts to multiple abiotic stresses (Behringer et al., 2015) and induced due to soil borne fungi and confers to biotic resistance and stress (Wu et al., 2012). Our findings indicate that enhanced expression of the ECHT could be responsible for the increased drought tolerance as supported by earlier reports Furthermore; metallothionein (MTT) based homolog JK757720 showed variable expression under osmotic stress as in plants. MTT gene family play distinct and overlapping biological processes by the regulation of gene expression or signalling networks. Higher expression was of MTT was reported in current study and this overexpression of MTT gene family results in higher tolerance against abiotic stresses in plants due to scavenging of ROS production (Xue et al., 2009).
                In the second category, few transcripts involved in molecular functions assessed by real time PCR exhibited an elevated expression in cotton roots and leaves tissues. Many RBPs have been shown their involvements in abiotic stresses (Ambrosone et al., 2012; Jung et al., 2013). Increased mRNA expression of RBP (JK757358) in different tissues is due to active participation of RBP ABA-dependent mechanisms of response to salt and drought stress (Ambrosone et al., 2015). Another molecular function based homolog FRIGIDA (JK757130) was up-regulated expounding its role under water deficit stress. Previous studies also reported overexpression of FRIGIDA suggesting that it enhances drought tolerance accumulating proline during water stress (Chen et al., 2018).
                U-box domain containing protein (U-BOX JK757206), a class of E3 ubiquitin ligases exhibited an elevated expression in our experiment as its attributed towards U box role during ubiquitination, a cellular process plays an important role in the perception and signal transduction of hormone and various stress responses in higher plants (Hellmann and Estelle, 2002; Xu et al., 2015). Studies conducted earlier also reported strong up-regulation of UBOX genes in the roots under drought and salt stress (Cho et al., 2008). Our findings are best supported by previous studies signifying UBOX overexpression and enhanced drought tolerance (Liu et al., 2011). Expression of Calcyclin binding protein (CBP; JK757361), an Arabidopsis homologue of SIP (SIAH-interacting protein) was elevated in roots and leaves tissues in our studies. Increased CBP level may be due to the rapid generation of ROS primarily superoxide (O2−) and hydrogen peroxide (H2O2) and production of oxidative brust as well (Grant and Loake, 2000). Reports discussed earlier mentioned the highest expression of SIP in roots, root hairs and root tips and relatively low level in leaves, which is consistent with our studies (Kim et al., 2006).
                In third category of expressed genes, WD (JK757101) was up-regulated in both roots and leaves compared to the control plants, which indicate the role of this gene during water deprivation. Increased expression of WD in plant roots with involvement in nodule formation, cell wall formation (Guerriero et al., 2015), response to hormones and abiotic stresses is well understood which supports our findings (Chuang et al., 2015).  
Conclusion: We successfully constructed cDNA library from cotton roots and several potential transcripts encoding drought related proteins homology were identified. These unigenes were involved in multiple biological processes and different molecular processes under osmotic stress. Differential regulation of few drought responsive genes was validated through real time quantitative PCR. Several novel transcripts with no known functions may reveal their involvement in drought tolerance and these needs to be explore further. These genes with unknown functions need further exploration of novel mechanism that may be dynamic in cotton.
Acknowledgements: we cordially thank to the Center of Excellence in Molecular Biology (CEMB), University of the Punjab, Lahore for providing us all the necessary facilities, timely support and valuable guidance during all stages of work. Our special thanks to CEMB sequencing core facility for their valuable support and cooperation. The work was partially funded by the Higher Education Commission (HEC), Islamabad, Pakistan.
Conflict of interest: The authors declare no conflict of interest.

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