GhGASA10-1 promotes the cell and ber elongation through the phytohormones IAA-induced

Background: Cotton is an important cash crop. The ber length has always been a hot spot, but multi-factor control of ber quality makes it complex to understand its genetic basis. Previous reports suggested that OsGASR9 promotes germination, width, and thickness by GAs in rice, while the overexpression of AtGASA10 lead to a reduction in silique length, which is likely to reduce cell wall expansion. Therefore, this study aimed to explore function of GhGASA10 in cotton bers development. Results: To explore the molecular mechanisms underlying ber elongation regulation concerning GhGASA10-1, we revealed an evolutionary basis, gene structure, and expression. Our results emphasized the conservative nature of GASA family with its origin in lower fern plants S. moellendori. GhGASA10-1 was localized in the cell membrane, which may synthesize and transport secreted protein to the cell wall. Besides, GhGASA10-1 promoted seedling germination and root extension in transgenic Arabidopsis, indicating that GhGASA10-1 promotes cell elongation. Interestingly, GhGASA10-1 was upregulated by IAA at ber elongation stages. Conclusion: We propose that GhGASA10-1 promotes ber elongation by regulating the synthesis of cellulose induced by IAA, to lay the foundation for future research on the regulation network of GASA10-1 in cotton ber development.

GhGASA10-1 promotes the cell and ber elongation through the phytohormones IAA-induced Baojun  Sciences the N-terminal signal peptide sequence, the hydrophilic, andC-terminal regions as GASA domain. GASA domains are generally comprised of 60 amino-acids, including 12 cysteine, key residues for the functional domain [14]. Exact subcellular localization of GASTs proteins is the sticking point to determine the protein function. Many in vivo studies demonstrated that GASTs proteins were discovered in the cell wall and/or apoplast. However, a few GASTs proteins were localized in plasma membrane, cytoplasm and nucleus. The above mentioned reports implies the divergent functional trends of GASTs, which are likely corresponding to functional association with cell elongation and division [15]. The presence conserved motifs is the main reason for GASTs to be conserved in vascular plants. Further, the phylogenetic studies of GASTs family genes from plant kingdom showed that GASTs might be initially evolved in S. moellendor i [11]. GAST1, which is one of GAST family genes, initially detected in tomato [16] with subsequent discoveries in different species, such maize (Zea mays L.) [10], Arabidopsis [11], Glycine max [17], Grapevine (Vitis vinifera L.) [12].
Previously published reports emphasized the regulatory role of GAST family genes in cell elongation and cell division, but also involved in responses towards abiotic and biotic stresses [18][19][20]. AtGASA genes family members have been reported with their regulatory role in harmone synthesis such as ABA, GA, BR, IAA, JA, SA in Arabidopsis [15], while the OsGASR9 regulated grain length, widthen and thickness in rice [21]. Interestingly, two vertical homologies from Arabidopsis (AtGASA10) and rice (OsGAST9) showed conserved (Functional and structural) positive regulatory role in germination, between dicot and monocot plants. Phytohormones may regulate both AtGASA10 and OsGASR9 through the signal element in their promoter reigons and correspond to the signal element in likely feedback/responce cycles in GA/ABAmediated regulation [11]. GAs are among the universal plant phytoharmones that play crucial role in the growth and development processes including germination, stem elongation, photosynthesis, owering, and seed development, while GASA-like genes possess a key role in GAs signaling pathway [12,22,23] in different plants viz. gerbera [24], and maize [10]. Some GASA genes are also regulated by other hormones, as RSI1 was regulated by IAA [25], and OsGSR1/2 was regulated by GA3 [26].
Reported statistics suggested that different GASA-like genes may exhibit differential expression patterns, mostly built on the spatiotemporal pattern of gene expression for regulating their speculated functions [27]. In A. thaliana, AtGASA5 is highly expressed in the shoot tip and the in orescence meristems of the reproductive stage [27]. Interestingly, AtGASA6 is the key node in GA, ABA, and glucose signal interaction network, which is involved in the regulation of GA, ABA, and glucose in seed germination mesocotyl elongation [28]. The expression analysis in Moso bamboo revealed that most PhGASTs might be related to M. bamboo ower development and shoot growth. Importantly, PhGASR1 was presumed to act a key role in rapid shoot growth involving in the ABA pathway [29]. However, the expression of AtGASA10 was not signi cantly in uenced by exogenous GA treatment of suspension cells in Arabidopsis [30].
However, the factors that the mechanism of exogenous plant phytohormones IAA and GA3 promoted in cotton ber cells remain unknown. Given the important roles of GASA family proteins by exogenous plant phytohormones IAA and GA3 promoted in plant development and cell elongations, the analysis of GASA family is greatly valuable. Our study systematically aimed to uncover transcrptomic landscape of GASA genes in ber development of G. arboretum, G. barbadense, G. hirsutum, speci cally AtGASA10 and its vertical homologous genes, by utilizing RNA-Seq and qRT-PCR expression pro les. Furthermore, we exploited speci c feature of GASA gene family including gene structures, conserved motifs, tissues speci c expression and subcellular localization, expression patterns, overexpression of Arabidopsis as well as ovule culture in vitro. Using IAA, GA3 and their transport inhibitors, we demonstrated that GhGASA10 plays a vital role promoted cell elongations in the overexpression of Arabidopsis and cotton bers.

Results
Identi cation, phylogenetic analysis of the GASA gene family in the plant kingdom According to the results of BLASTP and Query Sequence Searching of TBtools, the GASA protein sequences from different plants were collected and revised the correctness. To study the evolutionary relationships of GASA genes, the protein sequences, including Arabidopsis, G. darwinii, G. mustelinum, G. tomentosum, G. raimondii, G. arboretum, G. barbadense, G. hirsutum were exploited using phylogenetic tree (Fig. 1A). GASA proteins have a quite conservative in phylogenetic relationships between Arabidopsis and Gossypium, and further classi ed into three subfamilies viz. GASA1/2/3/9/11/14, GASA4/5/6/12/13, and GASA7/8/8L/10. The vertical homologous gene corresponding to each AtGASA gene could be found in different Gossypium species, and most GASA genes did not have doubling occurred in diploid cotton compared to Arabidopsis such as GASA1/2/3/4/5/6/9/14/12, and occurred doubled such as GASA7/8/8L/10/11/13. These results showed that the GASA7/8/8L/10/11/13 might act as a key role for cottons. Allotetraploid cotton should have twice the number of diploid cotton genes, but the number of GASA genes were less than twice. This result showed that during the evolution process some GASA genes were lost, which is inline with the previously published statistics demonstrating higher gene losses in allotetraploid cotton as compared to diploid cotton [31]. Subsequently, GASA genes were explored in 20 species, extending from lower plants to higher plants, to make certain the origin and evolutionary relationship of these genes (Fig. 1B). Based on the genes number analysis, GASA genes were present in lower fern plants Selaginella moellendor i, but not in the lower algae plants Micromonas pusilla, Ostreococcus tauri, and Volvox carteri and moss Physcomitrella patens, indicating that the GASA genes might have originated in fern. From the origin of ferns to angiosperms, the number of GASA genes has hardly changed in diploid plants. This result indicated that the number of GASA family gene are conserved in most plants.

Structural characterizations of GhGASA genes and motif and analyses
To exploit the evolutionary relationships GhGASA family genes along with their structure and function, an unrooted phylogentic tree was constructed utilizing GhGASA protein sequences. In general, GhGASA genes possessed one to three exons. Furthermore, genes structure and phylogenetic relationship displayed highly correlation. In total, 20 conserved motifs were identi ed in GhGASA protein sequence (Fig. S1). The number of conserved motifs in each GhGASA varied from 3 to 11. Most GASA proteins contain the conserved Motif 1-3, showing that the three Motif of GASA proteins may have important role for the functional conservative.
To study the evolutionary relationships and functional divergence of the prominent GASA gene-family members, we extracted and examined the upstream 2.0 kb promoter regions. Many cis-acting regulatory elements, including 13 elements related to plant growth (including, Photo-responsive, cell cycle, and seedspeci c reulation) and stress responses (including, hormone-response, wound-response, and defense response towards stresses), were analyzed (Fig. 2B).
We mainly focused on cis-acting regulatory elements to verify gene functions concerning cotton ber development. GhGASA genes promoter active elements and cotton transcriptomic data revealed that ber development might not be linked to the cell cycle regulation, and seed-speci c regulation, it might be linked to hormone-responsive elements such as IAA, GAs ( Fig. 2A). It might be suggested that IAA and GAs act important roles in ber development and cell elongation.
RNA-seq expression pro le of GASA genes in three major cotton species Expression pro le and tissue speci city were explored using transcriptomic data of G. arboretum, G. barbadense, G. hirsutum. Firstly, we constructed phylogenetic tree of GASA family genes of three major cotton species using the relative expression pro les with TBtools. The majority of GASA family-genes from the same subfamily had similar expression patterns in six varieties in three major cotton species (Fig. 3, Fig. S2, and Fig. S3).
In G. hirsutum (Fig. 3), only four genes are highly expressed at ber development stages. Gh.A09G018000 and Gh.D04G053600 depicted relatively high expression at the early stages of ber development. However, the Gh.A04G144000 and Gh.D04G1827 were highly expressed throughout whole ber development stages, especially the critical period of ber elongation for 5-15 DPA.
In G. arboretum (Fig. S2), the study found that the Ga14G0224.1 signi cantly higher expression level in all the tissues. While Ga07G1350.1 was only high expressed in leaves, and Ga04G0326.1 was only highly expressed in different ber development stages, especially during the critical period of ber elongation.
In G. barbadense (Fig. S3), only three genes were highly expressed at crucial ber development stages; especially two genes viz. Gbar.D04G017490 and Gbar.A04G012790 showed signi cant expression levels at the critical period for ber elongation (10-DPA).
Interestingly, the ve genes Ga04G0326.1, Gbar.D04G017490, Gbar.A04G012790, Gh.A04G144000, and Gh.D04G182700 were all vertical homologous of AtGASA10 in three cotton species. Regardless of the evolutionary relationship, gene structure, and expression changes were consistent among the six varieties of G. arboretum, G. barbadense, G. hirsutum. These results emphasized that higher expression and tissue speci city of these genes viz. Ga04G0326.1, Gbar.D04G017490, Gbar.A04G012790, Gh.A04G144000, and Gh.D04G182700 might play a direct critical role in ber development and ber cell elongation.
In this study, we identi ed ve GASAs genes sited at A04/D04, they were explicitly expressed at critical ber development stages in three major cotton species, which emphasized that the GASA10 might have a vital role in ber development, speci cally ber cell elongation. To understand the genetic basis, characteristics, and functions of GhGASA10-1 (Gh.D04G182700), we further performed functional veri cation. Subcellular localization of GhGASA10-1 According to the online tool analysis, TMHMM2.0 (http://www.cbs.dtu.dk/services/TMHMM/) predicted that GhGASA10-1 has the 26 N-term signal sequence with transmembrane and the other sequence outside the membrane. The CELLO version 2 [32]and Euk-mPLoc 2.0 [33] predicted that the subcellular localization of GhGASA10-1 is extracellular. YLoc [34]and BaCelLo [35] predicted that localization of GhGASA10-1 is a secreted pathway.
To verify this prediction, the full-length CDS of GhGASA10-1 was ligated with 35S-1300-GFP vector. The constructed vector was in ltrated into N. benthamiana mature leaves and visualized by confocal microscopy. The uorescence of 35S-GFP was detected in nucleus and the cytomembrance (Fig. 4). In contrast, the GhGASA10-1::GFP fusion protein was localized in cell membrane appearing green, and cell membrane presented red uorescence stained by cell membrane marker Dil. Subsequently, the cell membrane was merged into yellow by both GhGASA10-1::GFP fusion protein and cell membrane marker Dil. The above results demonstrated that GhGASA10-1 was localized in the cell membrane, which may synthesize secreted protein transport to the cell wall involved in cell wall synthesis and promote cotton bers cell wall development through the secreted pathway.
Overexpression and tissues speci city analysis of GhGASA10-1 in Arabidopsis To further con rm the gene function, GhGASA10-1 was overexpressed in Arabidopsis. Among 10 lines GhGASA10-1-overexpressing transgenic Arabidopsis of T3 generation, 3 lines were selected for further analysis. Tissue speci city expression analysis (Fig. 5A) showed that GhGASA10-1 is explicitly expressed in the roots at the seedling stage. However, GhGASA10-1 is signi cantly down-regulated in the roots and speci cally expressed in the ower buds at the owering stage.
When grown on 1/2 MS medium, wild-type and 3 transgenic lines exhibited a noticeable phenotypic difference in Arabidopsis seedling germination stages. The seedling germination after 14 days of standard cultivation (Fig. 5B), GhGASA10-1-overexpressed seedlings germinated vigorously than wild type seedlings. Statistics on seeds germination rate showed that transgenic seeds' germination rate was signi cantly higher than the wild type, especially on the third day (Fig. 5C). The length of the taproot after 14 days of vertical cultivation (Fig. 5D), GhGASA10-1-overexpressed seedlings formed more main roots than wild type seedlings. The seed from wild type and transgenic plants viz., OE1, OE2, OE3 were germinated on MS-agar medium to further verify the transgenity. The results comprehend the trasgenity, as shown in Fig. 5D, the root growth was observed with signi cantly higher expression in the transgenic lines than in the wild-type plants. Biological statistics (Fig. 5E) showed that the length of the root of transgenic lines was twice than wild-type of the length of Arabidopsis seedling stages. These results showed that GhGASA10-1 promotes seedling germination and root extension in Arabidopsis.
As mentioned above GhGASA10-1 was screened as a putative candidate gene for ber cell elongation. However, the tissue speci city of this gene in Arabidopsis suggested marked changes in root elongation. To validate our hypothesis that GhGASA10-1 play a crucial role in cell elongation, we further examined the roots of Arabidopsis and compared both the transgenic and wild type. Interestingly, we observed that overexpression of GhGASA10-1 extensively promotes root length with the elongation of root cells instead of an increase in the number of cells (Fig. 5F/G). These results strengthen our hypothesis that GhGASA10-1 may be important in ber cell elongation.

GhGASA10-1 expression level associates with cellulose synthesis
As the over-expression of GhGASA10-1 in Arabidopsis promote cell elongation resulting in root elongations, we speculated that GhGASA10-1 might promote downstream transcription factors, leading to high expression of cellulose synthase genes, further promoting cell elongation. Comparing the expression of cellulose synthase genes (AtCesAs) (Fig. 6) in wild-type and overexpressing Arabidopsis, ve members of the AtCesAs family were found to be upregulated. Among them, AtCesA5b/9 were upregulated twice, while AtCesA4/7 were upregulated three times, and AtCesA10 was upregulated more than ten times. Taken together, this data provides strong evidence that over-expression of GhGASA10-1 strongly induced cellulose synthesis associated genes and promote cell elongation.
GhGASA10-1 induced by IAA but not GA 3 It has been shown that GASA family genes being involved in the regulation of phytohormones in different plants and act as binding promoter element in upland cotton. IAA or GA3IAA might regulate GhGASA10-1 or GA3 might regulate GhGASA10-1, so we take hormone-treated ovule in vitro culture to verify GhGASA10-1 expression level. Surprisingly, qRT-PCR (Fig. 7) results showed that GhGASA10-1 was upregulated during crucial ber elongation stages. Moreover, GhGASA10-1 was regulated by IAA, but not GA3 (Fig.S4) during cotton ber development when treated with different concentrations of hormone IAA, GA3 and their inhibitors in cotton ovule.

Discussions
The whole-genome sequences (WGS) of different cotton species have been accomplished in recent years, which has boosted the research on genetic breeding and functional genes discovery of cotton [36,37].
Moreover, cotton bers are single-celled trichomes, and are excellent model materials for studying singlecelled elongations [38]. Some studies showed that phytohormones IAA, GA3, BR etc., promoted cell elongation in cotton ber development [39], and reported that the GASA family genes regulated by different phytohormones to promote or inhibit cell elongation and cell division as well as other function in many plants [12]. Therefore, it is particularly important to exploit and understand the functional mechanism of some crucial genes of the GASA family regulating ber cell elongations for different ber development stages in cotton.
In this study, we identi ed GASA genes in seven representative cotton species, including three wild allotetraploid cotton G. mustelinum, G. darwinii, G. tomentosum with 48, 45, 47 GASA genes respectively, two cultivated allotetraploid cotton G. hirsutum, G. barbadense with 45, 41 GASA genes respectively, and its two diploid ancestors, G. arboreum and G. raimondii with 24, 25 GASA genes respectively. This differential distribution of GASA family genes emphasized the loss of genes in different allotetraploid cotton species, consistent with the higher rate of gene loss in different allotetraploid cotton than in diploid species [15,40]. The GASA proteins are quite conservative in higher plants, mainly divided into three subgroups in Arabidopsis and Gossypium, which is consistent with GASA family of Zea mays [10], Oryza sativa [11,21], Vitis vinifera [12], and Glycine max [17]. GASA genes are present in lower fern plants S. moellendor i [11]. In the life tree, the number of GASA genes have hardly changed from ferns to angiosperms, rstly reported in our study. The above results showed that GASA family genes are relatively conservative in gene structure and quantity [10,12].
Promoter region elements and expression pro le of GhGASA family genes in cotton were analyzed, and ber speci c expression of GASA10 member in different cotton species. The results emphasized the involvement of phytohormones viz. GA3 and IAA in promoting cell wall and ber elongation. Most of the GASA genes regulated by GA, ABA, SA [11] are involved in the hormonal signaling pathway in different plants, speci cally in uencing many plant functions such as bud dormancy, bud germination, root length, stem elongation, seed size, and yield [12,36,41,42].
According to the expression pro le of GASA genes in three major cotton species, different GASA family genes showed tissue-speci c expression such as GhGASA1/2 was highly expressed in leaves of both cultivars, whereas GhGASA10 showed high expression in the fruit and seed of both cultivars. GASA genes have tissue-speci c expression; that is also consistent with the previous repost suggesting ower-tissuespeci c expression of CcGASA4, OsGASR1/9 genes [21,23,42]. Numerous studies have found that the function of GASA genes not only promotes cell elongation and other tissue development, but also resists various abiotic stresses i.e., salinity, drought, cold, fungal, and paclobutrazol (PBZ) [29,43,44].
GhGASA10-1 was localized in the cell membrane, which may synthesize secreted protein transform into the cell wall involved in cell wall compound for ber cell elongations. Citrus clementina CcGASA4 and Rice OsGASR9 localized to the plasma membrane and nucleus [23], while Pyrus pyrifolia PpyGAST1 localized to the cytoplasm and AtGASA5 protein localized in the cell wall/extracellular matrix [45]. This study results showed that GhGASA10-1 might be consistent with the function of AtGASA10.
The overexpression of GhGASA10-1 in Arabidopsis was analyzed, which promoted seedling germination. Interestingly, the overexpression of GhGASA10-1 remarkably promoted the main root extension, and the cellular level of Arabidopsis roots length was found that GhGASA10-1 promotes Arabidopsis roots cell elongations, which further indirectly con rmed that GhGASA10-1 promote cell and ber elongation of cotton. AtGASA10 is involved in changing the hydroxyl length facilitating cell wall growth by regulating cell elongation. The above results showed that the functions of GhGASA10-1 differ from AtGASA10 function, and might have different regulation mechanisms, which is crucial for further characterization of GhGASA10-1 gene for its involvement in ber cell elongation [30].
Most previous studies have shown that changes in plant organs may be due to the regulation of CesAs genes. different CesAs genes perform different functions of primary and secondary cell wall synthesis [46]. Our results suggested that GhGASA10-1 in Arabidopsis promotes root elongations, which lead to the hypothesis that GhGASA10-1 may promote downstream transcription factors, leading to high expression of cellulose synthase and further promoting cell elongation.
Our study found that the expression of GhGASA10-1 was not promoted by the exogenous phytohormones GA3, which is consistent with the expression of AtGASA10 being not regulated by GA3 [30]. OsGASR9 is involved in response to GA in rice [21]. Interestingly, GhGASA10-1 was upregulated by the exogenous phytohormones IAA; the result showed that IAA might play a crucial role in ber cell elongation and development, which is further researched for cotton ber quality main as ber elongations.

Conclusions
In this study, the evolutionary relationships of GASA gene family were identi ed in different Gossypium species, which were classi ed with 3 distinct subclasses and quite conservative. GASA genes might have originated in lower fern plants Selaginella moellendor i. From the origin of ferns to angiosperms, the number of GASA genes has hardly changed. The GhGASA10-1 were localized in the cell membrane, which may synthesize secreted protein transform into the cell wall involved in cell wall compound for ber cell elongations. The overexpression of GhGASA10-1 promote seedling germination in advance and promote Arabidopsis roots cell elongations in Arabidopsis, further indirectly con rmed that GhGASA10-1 might promote cell and ber elongation of cotton. GhGASA10-1 promote AtCesA4/5b/7/9/10 in Arabidopsis, especially AtCesA10 being more remarkable, which revealed that GhGASA10-1 not only promotes primary wall synthesis but also primarily promotes secondary wall synthesis. GhGASA10-1 was upregulated by IAA, IAA may play a crucial role in ber cell elongation and development. These results reveal the structural characteristics and expression patterns of GhGASA gene family and functional veri cation of GhGASA10-1 in ber elongation and provide crucial information for further regulation mechanism/ network of ber elongation.

Materials And Methods
Database search and sequence retrieval Cotton Functional Genomics Database (CottonFGD) (https://cottonfgd.org/) platform was employed to obtain the genomic datasets and protein sequences of different cotton species i.e., G. arboreum L, G. raimondii Ulbr, G. hirsutum L, G. barbadense L. The protein sequences of three wild cotton such G. darwinii, G. mustelinum, G. tomentosum were downloaded from NCBI(https://www.ncbi.nlm.nih.gov/) [47]. The protein sequences of Arabidopsis thaliana were downloaded from the Arabidopsis Information Resource (TAIR) (https://www.arabidopsis.org/). The other species protein sequences come from NCBI (https://www.ncbi.nlm.nih.gov/). Based on the sequence similarity of the protein domains, GASA protein sequences in different plant species were extracted using TBtools [48].
TBtools, with default parameters [48], was employed to search for various GASA protein sequences where the GASA domain (PF02704), obtained from pfam database (http://pfam. xfam.org/) was used a query sequence. Repeated proteins were redundant, thus deleted and only GASA protein sequences with e-value > 30 were kept and double checked through NCBI-CDD (NCBI conserved domain database, https://www.ncbi.nlm.nih.gov/cdd) for further analysis.
Phylogenetics, Gene Structure, And Motif Analysis Multiple sequence allignments of obtained GASA protein sequences, including Arabidopsis and Gossypium (as mentioned above), was done using Muscle wrapper in TBtools. Subsequently,, IQ-TREE in TBtools was utilized to generate phylogenetic tree with 1000 bootstraps [48]. A bar graph was made by the number of GASA family protein sequences of different cotton species.
Furthermore, The genes structure of GASA were analyzed using Gene Structure Shower of TBtools [48]. We also exploited motifs with conserved domains of GASA proteins using MEME (http://memesuite.org/tools/meme) with default parameters. The GASA family genes characteristic was visualized and integrated into graphics using TBtools [48].
Analysis Of CIS-elements Related To Plant Hormone 2.0 kb upstream sequences of GASA family genes in G. hirsutum and GASA10 genes from other four cotton species G. arboreum L, G. raimondii Ulbr, G. hirsutum L, G. barbadense, were extracted by TBtools, and the cis-elements were determined utilizing the PlantCARE database (http://bioinformatics.psb.ugent. be/webtools/plantcare/html/).
Col-0, ecotype of Arabidopsis thaliana, seeds were put in 4°C for vernalization and later grown on agarsolidi ed Murashige and Skoog (MS) medium, which were placed in an incubator with a 16h / 8h (light / dark) cycle at 22°C. The seedlings were transplanted in mixed soil (vermiculite: humus = 1:1). the Agrobacterium tumefaciens strain (GV3101) with constructed overexpression vector was transformed into Arabidopsis plants using the oral dip method [49].

RNA Extraction And Quantitative PCR
The total RNA of cotton stem tips and leaves were isolated by the RNAprep Pure Plant kit (Tiangen, China). Approximately 1000 ng of RNA were reversely synthesized into cDNA by MonScript RTIII Super Mix with dsDNase (Two-Step) (Monad, China).
The real-time PCR detection system (RT-PCR) utilized ABI 7500 real-time PCR system and ChamQ Universal SYBR qPCR Master Mix (Vazyme, China). The Histone3 gene which express stably in upland cotton was used as an internal control. The relative expression levels of NCED genes were calculated by the 2 −ΔΔC T method.
The cotton ubiquitin gene (Gh_A10G005800) and Arabidopsis β-actin genes (actin 2, actin 8) were used as internal references [50,51]. To further explore OE-GhGASA10-1 in Arabidopsis whether through regulation of cell wall synthesis cellulose synthase genes to promote main root elongation, and related 12 AtCesAs primer pairs [46]. The primers used in the quantitative PCR analysis are shown in Table S1. All qRT-PCR experiments were executed for at least 3 biological replicates.

Subcellular Localization Of Ghgasa10-1
The ampli ed exonic region of ChGASA10-1, using speci c primers (Table S1)corresponding Sma I and Kpn I restriction enzyme sites, was fused to the 5' terminal of the GFP gene and consequently generated GhGASA10-1-GFP fusion construct comprising CaMV 35S promoter. The GhGASA10-1-GFP vector and positive control (empty vector) were transformed into the Agrobacterium tumefaciens strain (GV3101), then transformed into tabacco (Nicotiana tabacum) leaves [52]. Leaves of the seedings were stained with cell membrane CM-Dil (10 µM, Sigma-Aldrich) and visualized using a laser confocal microscope (Zeiss LSM710, Germany).

In Vitro Cotton Ovule Culture And Hormone Treatment
Randomly selected cotton bolls (TM-1) were collected and sterilized in 0.1% (w/v) HgCl 2 and 75% (v/v) ethanol for 15 min and 5 min, respectively. Collected bolls were washed with sterilized distelled water after sterilization. Ovule samples were collected from air dried bolls under sterile conditions. Collected ovules were then cultured in BT medium as control treatment in dark environment at 28-30°C, as previously described by [53]. The ovules were also cultured for harmone treatment assay with different