Science International  Volume 5 Issue 3, 2017

Research Article

Role of Expressed Sequence Tags in Cotton Improvement
Viralkumar B Mandaliya
Gujarat National Law University, Attalika Avenue, Knowledge Corridor, Koba, 382007 Gandhinagar, Gujarat, India

Vrinda S Thaker
UGC-CAS-Department of Biosciences, Saurashtra University, 360 005 Rajkot, Gujarat, India

ABSTRACT:
Cotton is a very economically important crop and cotton scientist, breeders and farmers are in a quest to improve the cotton quality. Genetic marker system will aid a tool to improve cotton crop. Expressed sequence tags (ESTs) are short DNA sequences reverse-transcribed from the cellular mRNA population. The ESTs in cotton had used to study comparative genetic mapping, to study genes involved in cotton fiber development and to develop various PCR-based molecular markers. The highest number of ESTs was reported in Gossypium hirsutum (G. hirsutum) i.e., 337,811 and lowest in Gossypium hirsutum (G. herbaceum) i.e., 247. Cotton ESTs in future by locating and detecting agronomic important genes will lead to open up new rise in cotton genomics and transcriptomics research.
 
    How to Cite:
Viralkumar B Mandaliya and Vrinda S Thaker , 2017. Role of Expressed Sequence Tags in Cotton Improvement. Science International, 5: 127-132
DOI: 10.17311/sciintl.2017.127.132
 


INTRODUCTION
Cotton belongs to the genus Gossypium of the family Malvaceae1 and exhibits a very important natural textile fiber source and cotton seed is a significant food source for humans and livestock2. A mankind, in his quest to improve cotton quality3, is looks for variant forms i.e., varieties and hybrids. Plant breeders and farmers have created new varieties through traditional plant breeding. About 150 varieties and hybrids have been released during the last 50 years. Out of these about 30-40 are under large scale cultivation although about 20 varieties and hybrids account for more than 50% of production4.

In traditional plant breeding crosses between plants are performed. Sexual crossing of such nature are done in an uncontrolled manner and this often leads to a random combination of genes which results in new traits, some of which may be undesirable. Selection and careful evaluation of the offspring is therefore necessary. Traditional plant breeding has gone through many phases, from the era of cross pollination between varieties of the same species to hybridization between different species. However, traditional plant breeding is costly and time consuming and moreover, the selection and evaluation of the new varieties can take several years to achieve5. The advancement in DNA technology had leads to a new area of modern plant biotechnology. The application of DNA technology in agricultural research has progressed rapidly over the last 20 years6.

Nowadays, the cotton scientists have been emphasized on the utilization of public and private sector made cotton hybrids7. Numbers of studies have demonstrated that plants exhibiting great phenotypic and genomic variability. Cotton genus (Gossypium L.) includes approximately 45 diploid species (2n = 2x = 26) differentiated cytogenetically into 8 genome groups (A-G and K) and 5 allotetraploid species (2n = 4x = 52)8. In order to exploit this diversity an efficient molecular marker system is required9. In plant breeding the development of molecular marker systems facilitated the selection and evaluation process greatly. These molecular tools have increased the speed and precision for achieving desired agronomic traits.

Restriction fragment length polymorphism (RFLPs) belongs to the first generation of hybridization-based markers that developed in humans in the 1980s10 and thereafter used in plant biotechnology11. The RFLP where size based variation in DNA fragments produced by a digestion of DNA12. The RFLPs have been used extensively to compare genomes in the cotton plants1. The advantages of RFLPs include detecting unlimited number of loci, codominant and the use of probes from other species. However, RFLPs are expensive, time consuming and labour intensive. The PCR based marker systems are more rapid and requires less plant material. The first of PCR based marker was known as rapid amplified polymorphic DNAs (RAPDs) and are produced by PCR using genomic DNA and arbitrary primers13-15. However, the results from RAPDs were not reproduced in different laboratories. The RFLPs and RAPDs have been used to map or tag agronomically important genes including resistance genes against viruses, bacteria, fungi, nematodes and insects16,17.

Amplified fragment length polymorphism (AFLPs) combines both PCR and RFLP and it is generated by digestion of PCR amplified fragments using restriction enzymes18. For example, AFLPs have been used to assess the levels of genetic diversity within and between cotton hybrids1. The AFLPs are highly reproducible and this enables rapid generation and high frequency of identifiable AFLPs, making it an attractive technique for identifying polymorphisms and for determining linkages by analyzing individuals from a segregating population17.

Another class of molecular markers which depends on the availability of short oligonucleotide repeat sequences in the genome of plants is the simple sequence repeat (SSR) polymorphism or microsatellites19,20. The SSR markers are fairly cheap and no sequence information is required for their detection. The SSR gives good polymorphism as well as requiring only a small quantity of DNA to start with. However, similar to RAPDs the major problems encountered with SSR that its reproducibility in different laboratories were low.

Currently plant biologists are exploiting the use of expressed sequence tags (ESTs) as markers in gene discovery research5. For example, a recently described set of ESTs from cotton fiber21 provides a valuable new resource for developing PCR-based DNA markers for fiber genes.


WHAT ARE ESTs?
The ESTs are short DNA sequences corresponding to a fragment of a complimentary DNA (cDNA) molecule and which may be expressed in a cell at a particular given time. The ESTs are currently used as a fast and efficient method of profiling genes expressed in various tissues, cell types or developmental stages22.

The concept of using cDNAs as a route to expedited gene discovery was first demonstrated in the early 1980s23. In 1990, Sydney Brenner proposed that an obvious method for characterizing the ‘important’ part of the human genome would involve looking at messengers from the expressed genes-thus advocating the application of high-throughput methods for transcriptome sampling. Mark Adams first used the term EST in relation to gene discovery and the human genome project in 199123.

Figure 1: EST sequencing

ESTs are typically unedited, automatically processed, single-read sequences produced from cDNAs (small DNA molecules reverse-transcribed from the cellular mRNA population). Gene discovery via ESTs is comprised of four steps which include (i) The construction of cDNA libraries and single-pass sequencing of (randomly) selected clones, (ii) EST quality check the removal of vector and low quality sequences, (iii) The alignment of ESTs to identify the number of represented genes and (iv) The annotation of these genes or the partial sequences which are available thereof Fig. 1 summarizes EST sequencing.

EST sequencing initially favored the 5’ end of directionally cloned cDNAs because the 5’ sequences are likely to contain more protein coding sequence than the 3’ ends, which often contain significant untranslated regions (UTRs). Improvements in the techniques for cDNA preparation and the arrival of capillary-based sequencing have driven the evolution of high-throughput sequencing for ESTs and especially plant ESTs, while the 3’ end of the cDNA clone is often preferred because it is likely to offer more unique sequence (in many cases, the UTR) and can be used to distinguish between gene paralogues. The EST sequencing strategies in which both ends of the cDNA are sequenced are also becoming widespread. Thus, EST is a sequence representative of the corresponding cDNA clone and can be used for its characterizing with various bioinformatics tools and softwares24.

Needles in the EST haystack exploited using bioinformatics tools and software packages: Some specific plant EST databases with their websites were enlisted in Table 1. The majority of databases are generally contains ESTs from various plants. Cotton Gen is specific for cotton crop. There are several bioinformatics tools and software packages that useful for EST annotation and protein sequence prediction, database creation, to facilitates the analysis of large volume of ESTs or to predict t he transcript from ESTs, etc. are enlisted into Table 2.

Udall et al.31 reported approximately 185,000 Gossypium EST sequences comprising >94,800,000 nucleotides were amassed from 30 cDNA libraries constructed from a variety of tissues and organs under a range of conditions, including drought stress and pathogen challenges.

The number of cotton ESTs available in NCBI dbEST database were comparatively complied in Table 3 (accessed on primariliy on 20th January, 2014 and then on 17th February, 2017). The highest number of ESTs was reported in G. hirsutum and lowest in G. herbaceum. There were no change in records observed in G. raimondii and G. herbaceum i.e., 63,577 and 247, respectively in 2014 and 2017. The number of ESTs were computationally aligned and trimmed to remove unessential sequences from ESTs because vector and low-quality sequences need to be removed from the raw sequence data as well as bacterial sequences or other contamination should be removed32.


APPLICATIONS OF COTTON ESTS
Development of molecular markers: ESTs allow the efficient development of highly valuable molecular markers, because genes often represent single or low-copy sequences. Often EST-based RFLP markers allow comparative mapping across different species. The ESTs also allow a computational approach to the development of SSR (simple sequence repeat) and SNP (single nucleotide polymorphism) markers24.

Tabe 1: Some specific plant EST databases

Tabe 2: Bioinformatics tools and software packages to exploit ESTs

Tabe 3: Number of ESTs in cotton (Gossypium sp.)

The available sequence information allows the design of primer pairs, which can be used to screen cultivars of interest for length polymorphisms.

Chee et al.33 had developed PCR-based markers from known function EST sequences for the cultivated tetraploid cotton species Gossypium barbadense and Gossypium hirsutum. Here, outcomes suggested that digestion of PCR-amplified sequences offers one means by which cotton genes can be mapped to their chromosomal locations more quickly and economically than by RFLP analysis. Zhang et al.34 and Qureshi et al.35 reported that cotton EST-SSR markers were derived from Gossypium arboreum and Gossypium hirsutum and Gossypium barbadense and their study effectively proved that EST-SSRs are valuable for genetic diversity analysis and genetic mapping.

High-throughput transcript profiling: ESTs also provide the main resource for the construction of cDNA arrays in plants. The construction and use of such EST arrays for high-throughput transcript profiling can be divided in four general steps: (i) Identification of a non-redundant set of cDNA clones, (ii) Synthesis and deposition of hybridization targets on an appropriate surface, (iii) Preparation of mRNA from the tissue of interest, labelling of the hybridization probe and the hybridization to the array and finally (iv) Data acquisition and evaluation. Arpat et al.21 reported that genetic characterization of rapid cell elongation in cotton fibers and approximately 14,000 unique genes were assembled from 46,603 expressed sequence tags (ESTs) from developmentally staged fiber cDNAs of a cultivated diploid species (Gossypium arboreum L.).

Biological interpretation of expression data: Biological experimentation are carried out in vitro, ex vivo, in lab or in field conditions36-49. Expression data are expected to yield insights into regulatory processes during plant development and stimulus response. To reach that goal, it is necessary to compare the pre-processed array data with known models of metabolic and regulatory networks as depicted in databases or the general literature and to confirm or reject specific hypotheses. Many successful examples have been provided already, for example, Gossypium hirsutum derived EST-SSRs can be used in identification of quantitative trait loci (QTLs) and comparative genomics studies of diploid and allotetraploid cotton50.


CONCLUSION
The application of molecular markers in cotton plants has tremendous utility to cotton scientists for improving the plant. EST based gene discovery will leads to advance our understanding of the complexity of biological and cellular processes that are required for cotton fiber development and important genes. The EST based markers like SNP and EST-SSR that able to locate and detect agronomic traits and obtain a transcript map, which can be directly compared with earlier etected quantitative trait loci. From this, it could be foresee that cotton ESTs in future will provide the new horizons in cotton genomics and transcriptomics research.


SIGNIFICANCE STATEMENTS
This study will help cotton scientist, breeders and farmers to improve the cotton quality. In this study, genetic marker system analysed to improve cotton crop. The ESTs in cotton had used to study comparative genetic mapping which helps in future.


ACKNOWLEDGMENT
The authors would like to thank the Government of Gujarat for financial support in form of Centre for Advanced Studies in Plant Biotechnology and Genetic Engineering (CPBGE) programme and funding research project GG-R1/RP/VM-1.


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