4.1. Breeding for Stress Tolerance
Breeding techniques have traditionally been used to produce resilient cotton cultivars. Despite its importance in the textile industry, cotton productivity is constantly challenged by abiotic factors such as drought, salinity, and high [
152]. Because of breeding operations, Cotton’s abiotic stress tolerance has significantly improved recently. This review summarizes the findings of significant studies focusing on genetic analyses, molecular markers, and physiological responses related to abiotic stress tolerance in cotton.[
153]. Using amplified fragment length polymorphism to investigate the genetic diversity and connections between diploid and tetraploid cotton. This seminal work paved the way for the following studies, highlighting the relevance of genetic variety in breeding programs.[
154]. A genetic investigation of drought tolerance in Egyptian cotton
(Gossypium barbadense L.) yielded important insights into the heritability of drought tolerance features. A thorough exploration of the genetic foundations underlying tolerance to drought and salinity and resistance to Verticillium wilt and thrips has been undertaken. Integrating these diverse stress resistances is imperative in developing robust cotton varieties [
155]. The investigation into PEG-induced osmotic stress has been a central focus in comprehending cotton’s reaction to abiotic stress. The genetic analysis and mapping of quantitative trait loci (QTL) related to PEG-induced osmotic stress in cotton have been conducted. This study has illuminated distinct genomic regions linked to osmotic stress tolerance, offering valuable targets for subsequent breeding endeavors [
156]. The creation of molecular tools is crucial for the success of breeding programs. Shukla et al. have successfully established the inaugural linkage map for a recombinant inbred line population in cotton, employing it to investigate PEG-induced dehydration tolerance. This map is a valuable asset, facilitating the identification of genomic regions associated with dehydration tolerance [
157]. Comprehensive genetic analyses were undertaken to enhance the overall performance of cotton, with a specific emphasis on yield, fiber quality, and abiotic stress tolerance in Pima cotton [
79]. QTL mapping has played a pivotal role in unraveling the genetic underpinnings of stress tolerance. Specifically, QTL mapping was employed to investigate drought and salt tolerance in an introgressed recombinant inbred line population of Upland cotton in greenhouse and field conditions. This research pinpointed genomic regions linked to tolerance, providing potential avenues for marker-assisted selection in breeding programs [
158]. Genome-wide association studies (GWAS) have played a crucial role in identifying markers linked to stress tolerance. In this context, a GWAS was conducted to explore tolerance to biotic and abiotic stresses in a MAGIC (Multi-parent Advanced Generation Inter-Cross) population of upland cotton. This study aimed to identify significant genomic markers associated with stress tolerance in the cotton population under investigation [
159]. Numerous studies have concentrated on pinpointing drought and salt-tolerant cotton germplasm. Identifying such germplasm within the U.S. upland cotton pool has been achieved, making notable contributions to the conservation and utilization of genetic resources endowed with stress tolerance [
160]. Water stress responses in cotton, revealing changes in photosynthesis and leaf structure. Investigated the regulation of stomatal and non-stomatal water use in cotton, corn, and sorghum to enhance crop water use efficiency [
161]. Compared to corn, cotton and sorghum have more efficient transport systems, conferring drought tolerance to these species [
162].
Mapping studies of (QTL) offer valuable insights into the genetic foundations of stress tolerance, providing potential avenues for marker-assisted breeding strategies. Identifying QTLs for salt tolerance through the interspecific cross of
Gossypium tomentosum with
Gossypium hirsutum illustrates the potential of incorporating wild cotton species into breeding programs [
163,
164]. A genome-wide analysis of the calcium-dependent protein kinase gene family in
Gossypium raimondii, reveals a network involved in stress signaling [
165]. Similarly,
GhABF2, a
bZIP transcription factor, is pivotal in conferring drought and salinity tolerance in cotton [
166]. The significance of overexpressing rice
NAC gene
SNAC1 in enhancing drought and salt tolerance.[
167]. Highlighting the potential of cross-species gene transfer to improve stress resilience.
Understanding the physiological responses of cotton to abiotic stress is crucial for targeted breeding. Overexpressing the
Thellungiella halophila H+-PPase gene enhances salt tolerance and improves growth and photosynthetic performance [
168]. The effects of water stress on photosynthesis, root activity, and cotton yield under drip irrigation, offering insights into irrigation strategies for stress mitigation [
169].
Water use efficiency (WUE) in cotton breeding programs provides important selection traits in selecting varieties to cope with water scarcity [
170]. Cotton’s response to drought stress, studying growth reactions of an interspecific breeding line and its parents under controlled drought conditions with an automated irrigation system, provides valuable insights and a practical technique for drought tolerance studies in cotton [
171]. The calcium sensor
GhCaM7 modulating reactive oxygen species (ROS) production promotes cotton fiber elongation [
172]. The
GhCDPK1 gene in cotton regulates proline content and ROS levels and enhances drought tolerance [
173]. The functional genomics of stress-responsive genes provides further molecular insights into drought response mechanisms, stress tolerance mechanisms, including zinc-induced antioxidative defense and osmotic adjustment, along with the role of genes like
GhNHX1 in salt stress [
174,
175].
4.2. Transgenic Approaches
Recent advancements in genetic engineering approaches aimed at bolstering abiotic stress tolerance in cotton, utilizing key references to understand and evaluate these strategies. To overcome these challenges, researchers have employed various genetic engineering strategies. This comprehensive review provides an in-depth analysis of genetic engineering approaches for breeding abiotic stress-tolerant cotton. Overexpression of genes associated with drought tolerance improved drought resistance in transgenic cotton. Targeting critical genes involved in stress response pathways holds promise for developing cotton varieties resilient to water scarcity. Drought poses a significant threat to cotton production [
161,
176]. Variability for drought tolerance in cotton genotypes. Genetic engineering approaches to enhance drought tolerance include expressing drought-responsive genes and transcription factors. Overexpression of
AtHDG11 in tall fescue resulted in enhanced tolerance to drought and salt stress [
177]. Salt tolerance mechanisms through somatic embryo initiation and germination in diploid cotton [
178]. Comparative microarray analysis to identify salt-responsive genes in upland cotton, providing a valuable resource for unraveling the molecular basis of salt tolerance [
179]. The augmentation of glycine betaine synthesis,), exemplifies a genetic engineering strategy enhancing drought tolerance in cotton [
180]. Genes associated with heat and salinity tolerance offer potential targets for genetic engineering [
181,
182]. Salt tolerance in cotton emphasizes the need to develop resistant varieties. In cotton, the overexpression of vacuolar Na+/H+ antiporters and (AVP1) Arabidopsis vacuolar H+-pyrophosphatase gene improves drought and salt tolerance while increasing fiber yield [
183].
The balance between reactive oxygen species and antioxidant defenses is vital for enhancing stress tolerance in cotton [
184]. Increased glycine betaine synthesis in transgenic cotton lines, improving salinity tolerance. Understanding osmotic regulation and maintaining a balance in reactive oxygen species (ROS) are pivotal aspects of stress adaptation—Osmo protectants and antioxidant defences in conferring stress tolerance [
185]. The overexpression of the Arabidopsis vacuolar H+-pyrophosphatase gene (
AVP1) in cotton further exemplifies the potential of genetic engineering in augmenting osmotic regulation [
186]. As explored, understanding and manipulating root traits would be crucial for improving drought resistance in cotton [
187]. Genetic engineering approaches focusing on root architecture and function may provide an optimization of root traits is essential for improving water-use efficiency in cotton [
188]. Overexpression of rice
NAC gene
SNAC1 in transgenic cotton enhanced drought and salt tolerance by improving root development and reducing transpiration rate for developing cotton plants with increased water-use efficiency [
161].
Transcription factors such as
NAC and
MYB are pivotal in regulating stress-responsive gene expression. Overexpression of
OsMYB2 [
189] and
GhHOX3 illustrates the potential of transcription factors in conferring stress tolerance.[
190] Signal transduction pathways, including those mediated by
ABA, are explored for their relevance in stress response [
191]. The cotton’s Arabidopsis vascular sodium/proton antiporter gene has improved photosynthetic performance under salt conditions [
192]. Transcription factors, including NAC, MYB, WRKY, bZIP, and ERF/DREB, have vital roles in conferring abiotic stress tolerance [
193,
194]. Signal transduction pathways, such as those mediated by ABA, enhance photosynthetic performance under salt conditions [
195].
4.3. CRISPR/Cas in Cotton: Challenges and Solutions
The revolutionary CRISPR/Cas technology, which stands for clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas), offers precise capabilities for genome editing (GE) [
196,
197,
198]. Recent applications of CRISPR/Cas in cotton, specifically in whole genome-sequenced cotton species, have opened up new possibilities for targeted genetic modifications related to stress response pathways [
199,
200]. In the post-genomic era, developing efficient biotechnological tools for studying gene functions, such as GE, reverse genetics, or omics-based approaches, has become necessary [
201,
202]. Since the early 2000s, gene silencing technologies have been widely used to uncover gene function, revealing regulatory mechanisms and metabolic pathways [
203]. However, the limitations of existing gene silencing technologies, like RNA interference, have led to the exploration of CRISPR/Cas-based GE tools, which have diverse applications beyond the introduction of stable and heritable mutations [
204,
205].
Various approaches are available for target-specific GE, such as CRISPR/Cas, zinc finger nucleases (ZFNs), and transcription activator-like effector nucleases (TALENs)[
206]. The CRISPR/Cas system has transformed the field of plant GE with its simple design and high efficiency, which uses short RNA molecules called single guide RNAs (sgRNAs). This system has overshadowed the complexity and limitations associated with ZFNs and TALENs in designing and cloning desired gene constructs [
207,
208]. Its cost-effectiveness and ease of use have made CRISPR/Cas the preferred choice for GE in animals and plants, transforming the landscape of genetic research. Cotton, a vital fiber, biofuel, and oil crop, requires efficient GE tools for large-scale gene functional studies [
209].
While CRISPR/Cas has been successful in GE for various crops and model systems, its applications in cotton have faced challenges, partly due to the time-consuming transformation process and polyploidy [
210]. Several studies targeting genes, such as
MYB25, GhVP, GhCLA1, or
GhARG, have demonstrated the potential of CRISPR/Cas in cotton GE [
211,
212,
213,
214]. In a recent study, the CRISPR/Cas system was efficiently used to generate knockout plants for the
GhNAC3 gene [
215]. The
GhNAC3 gene encodes an NAC family transcription factor involved in drought stress tolerance through abscisic acid-related and independent pathways [
216]. Mutant lines exhibited phenotypic variations, confirming the role of GhNAC3 in average growth and development, apart from drought stress tolerance. CRISPR/Cas-based base editing allows for precise nucleotide substitutions, enabling accurate base modifications and directed evolution of intended genetic loci [
217]. Different research groups have already shown the potential use of base editing tools for cotton GE [
218,
220]. For example, Wang and coworkers [
220] utilized the recent version of ABE (GhABE8e) to install A-to-G base changes in GhTFL1, an anti-florigen encoding gene. The evolved
GhTFL1 led to the generation of new cotton germplasm with unique plant architectural features, such as compact size, shortened breeding cycle, and moderate height, which are beneficial for harvesting and potentially suitable features to evade drought stress situations. This study demonstrates the potential of base editing technology for directed evolution studies in the cotton genome. It can be applied to produce weak or strong alleles in abiotic stress-related genes, which would have been considered an unattainable task in the past.
A rapid method for validating sgRNAs for CRISPR/Cas GE was devised to tackle the challenges associated with time-consuming transformation methods in cotton [
221]. This method involves transient expression in cotyledons, enabling experimental validation in cotton. Successfully applied for multiple purposes, including validation of sgRNAs for individual genes (
GhPDS, GhCLA1, and GhEF1), simultaneous editing of homologous genes, and genomic fragment deletions, this new method showcased its versatility. CRISPR/Cas-induced mutations in stably transformed cotton plants targeting
GhCLA1 also resulted in typical albino phenotypes [
222,
223,
224,
225,
226,
227].
The integration of reverse genetics methodologies and targeted GE technologies has propelled significant advancements in understanding gene function [
228]. CRISPR/Cas-based techniques enable the production of transgene-free cotton plants through selfing or backcrossing, aligning with current guidelines for genetically modified organisms (GMOs) [
229]. Prospective application of CRISPR/Cas-based methods across the entire cotton genome provides a novel avenue to bolster cotton productivity, enhance genetic traits, confer pathogen resistance, and optimize agronomic characteristics, as reported for other crops [
230,
231,
232]. CRISPR/Cas, with its simplicity and efficiency, stands out as a powerful tool for large-scale gene functional studies in cotton [
233]. Creating a swift validation method for sgRNAs tackles the hurdles linked with prolonged transformation processes, thus rendering CRISPR/Cas more accessible for extensive applications in cotton GE [
234]. As the GE field continues to evolve, advanced GE technologies like base editing and prime editing hold immense promise for unravelling the complexities of gene function and driving new breeding innovations in cotton crop improvement for developing abiotic stress-tolerant varieties (
Figure 3).
Illustration of abiotic stress resilient cotton (Gossypium hirsutum) crop by using various biotechnological approaches. Utilize a combination of omics technologies (genomics, transcriptomics, proteomics, metabolomics, etc.) to gain comprehensive insights into the molecular mechanisms underlying stress responses in cotton. Genome editing techniques like CRISPR/Cas9 allow for targeted manipulation of the cotton genome to introduce stress-resistant traits. RNA Interference (RNAi): Silences specific genes that may hinder the plant’s stress tolerance. Genetic Engineering: Introduces genes from other organisms or modifies existing genes within cotton to enhance stress resistance.