1 Introduction

Drought stress is a significant challenge in cotton cultivation, affecting both the growth and productivity of this vital fiber and cash crop. Water scarcity, exacerbated by climate change, poses a severe threat to cotton production worldwide, impacting approximately 45% of the world’s agricultural land (Abdelraheem et al., 2019). In regions where cotton is a major economic driver, such as China and the United States, prolonged drought conditions can lead to substantial yield losses and reduced fiber quality (Ul-Allah et al., 2021). The negative effects of drought on cotton include impaired photosynthesis, disrupted carbohydrate metabolism, and poor reproductive success, all of which culminate in inferior fiber quality and reduced yields.

Given the critical role of cotton in the global economy and its susceptibility to drought, there is an urgent need to develop drought-tolerant cotton varieties. Genetic improvement offers a promising solution to this problem. By leveraging genetic variation and advanced breeding techniques, researchers aim to enhance the drought tolerance of cotton cultivars. Studies have shown that traits such as root length, shoot length, and stomatal conductance are key indicators of drought tolerance and can be targeted in breeding programs (Zahid et al., 2021). Additionally, the identification of drought-responsive genes and quantitative trait loci (QTL) has paved the way for marker-assisted selection (MAS) and transgenic approaches to improve drought resistance in cotton (Sun et al., 2023a). The integration of these genetic strategies is essential for developing resilient cotton varieties capable of withstanding water-deficient conditions.

This study reviews the current research status of genetic improvement of drought resistance in cotton, explores various morphological, physiological, biochemical, and genetic traits related to drought tolerance, as well as methods for identifying and selecting these traits, and discusses the progress made in breeding drought resistant cotton varieties, including the use of advanced genomic tools and transgenic methods. This article aims to emphasize the potential of genetic improvement to mitigate the adverse effects of drought on cotton production and ensure the sustainability of this important crop in the face of increasingly severe water scarcity.

2 Physiological and Genetic Basis of Drought Tolerance in Cotton

2.1 Key physiological mechanisms

Drought tolerance in cotton involves several key physiological mechanisms, including osmotic adjustment, stomatal regulation, and root development. Osmotic adjustment helps maintain cell turgor and water uptake under drought conditions by accumulating solutes like proline and sugars (Hu et al., 2022). Stomatal regulation is crucial for controlling water loss through transpiration. For instance, the overexpression of GhMKK3 in cotton enhances drought tolerance by promoting ABA-induced stomatal closure, thereby reducing water loss (Wang et al., 2016). Root development is another critical factor, as deeper and more extensive root systems can access water from deeper soil layers. The GhMKK3 gene also promotes root growth, which is essential for drought resistance.

2.2 Genetic traits associated with drought tolerance

Several genetic traits are associated with drought tolerance in cotton, including genes involved in ABA biosynthesis, stress-induced proteins, and transcription factors. The NAC transcription factor GhirNAC2 plays a significant role in drought tolerance by regulating ABA biosynthesis. It directly binds to the promoter of GhNCED3a/3c, key genes in ABA biosynthesis, enhancing ABA content and promoting stomatal closure (Shang et al., 2020). Another important gene is GhWRKY17, which modulates ABA signaling and reactive oxygen species (ROS) production, thereby affecting drought tolerance (Yan et al., 2014). Additionally, the bZIP transcription factor GhABF2 enhances drought tolerance by regulating genes related to ABA and drought response, increasing proline content and antioxidant enzyme activities (Liang et al., 2016).

2.3 Importance of understanding genotype-environment interactions

Understanding genotype-environment interactions is crucial for developing drought-tolerant cotton varieties. Drought tolerance is a complex trait influenced by multiple genetic and environmental factors. For instance, the expression of drought-responsive genes like GhNAC4, which regulates secondary cell wall biosynthesis and ribosomal protein homeostasis, is significantly induced by abiotic stress and plant hormones (Jin et al., 2023). Moreover, the interaction between GhROP3 and GhGGB proteins negatively regulates drought tolerance by affecting ABA and IAA signaling pathways. These interactions highlight the importance of considering both genetic makeup and environmental conditions in breeding programs aimed at improving drought tolerance in cotton. Integrating functional genomic approaches and genome-wide studies can facilitate the identification and characterization of novel genes, providing a better understanding of the complex stress cellular biology of plants (Mahmood et al., 2019).

3 Molecular Breeding Approaches for Drought Tolerance

3.1 Marker-assisted selection (MAS) for drought-related traits

Marker-Assisted Selection (MAS) is a powerful tool in the breeding of drought-tolerant cotton genotypes. MAS involves the identification and use of molecular markers linked to drought tolerance traits, which significantly enhances the efficiency and accuracy of selecting superior genotypes (Wang and Li, 2024). This method leverages the linkage between markers and quantitative trait loci (QTL) associated with drought tolerance, allowing breeders to select plants with desirable traits even at the seedling stage (Shiri and Akhavan, 2014). Despite its potential, the application of MAS in cotton breeding has been limited due to the complexity of drought tolerance traits and the need for high-resolution mapping of QTL using genome-wide SNP markers (Abdelraheem et al., 2019).

3.2 Genomic selection (GS) and its applications in cotton

Genomic Selection (GS) is an advanced breeding approach that uses genome-wide markers to predict the performance of genotypes. This method has shown promise in improving drought tolerance in crops by enabling the selection of superior genotypes based on their genomic estimated breeding values (GEBVs). GS models, such as Bayes B, have demonstrated high prediction accuracy for drought-related traits, making them valuable for breeding programs (Shikha et al., 2017). In cotton, GS can accelerate the development of drought-tolerant varieties by integrating genomic data with phenotypic data, thus overcoming the limitations of traditional breeding methods (Rasheed et al., 2023).

3.3 CRISPR-Cas9 and other genome editing tools for targeted improvement

CRISPR-Cas9 and other genome editing tools have revolutionized the field of plant breeding by enabling precise modifications of specific genes associated with drought tolerance (Figure 1). The CRISPR-Cas9 system, in particular, has been widely adopted due to its simplicity, adaptability, and efficiency (Han, 2024). This technology allows for the targeted editing of genes involved in drought response pathways, such as those regulating stomatal closure, root development, and osmolyte accumulation (Joshi et al., 2020; Núñez-Muñoz et al., 2022). In cotton, CRISPR-Cas9 has been used to edit genes like GmHdz4, which play a crucial role in enhancing drought tolerance by improving root architecture and antioxidant enzyme activity (Zhong et al., 2022). The application of CRISPR-Cas9 in cotton breeding holds great potential for developing varieties that can withstand prolonged drought periods, thereby ensuring stable yields under water-limited conditions (Zafar et al., 2020; Park et al., 2022).

Figure 1 Current gene editing tools available to generate drought-tolerant crops (Adopted from Núñez-Muñoz et al., 2022)

4 Biotechnological Interventions in Cotton Improvement

4.1 Transgenic approaches (e.g., introduction of drought-tolerant genes)

Transgenic approaches have been pivotal in enhancing drought tolerance in cotton. For instance, the overexpression of the Arabidopsis Enhanced Drought Tolerance1/Homeodomain Glabrous11 (AtEDT1/HDG11) gene in cotton has significantly improved drought and salt tolerance, leading to better agronomic performance and higher yields under both normal and drought conditions (Yu et al., 2016). Similarly, the Gossypium arboreum universal stress protein (GUSP1) has been genetically modified and introduced into cotton, resulting in increased leaf relative water content, chlorophyll content, and net photosynthesis under drought stress (Hassan et al., 2021). Another notable example is the overexpression of the GhABF2 gene, a bZIP transcription factor, which has been shown to enhance drought and salinity tolerance in cotton by regulating genes related to ABA, drought, and salt response.

4.2 Role of RNA interference (RNAi) in enhancing stress tolerance

RNA interference (RNAi) has emerged as a powerful tool for enhancing stress tolerance in cotton. By silencing specific genes, RNAi can modulate the plant's response to drought stress. For example, silencing the GhNAC4 gene, which is involved in secondary cell wall biosynthesis and ribosomal protein homeostasis, has been shown to impair drought resistance, indicating its crucial role in stress tolerance (Jin et al., 2023). Additionally, the silencing of GbMYB5, an R2R3-type MYB transcription factor, compromised the drought tolerance of cotton plants, highlighting the importance of this gene in the plant's adaptive response to drought stress (Chen et al., 2015).

4.3 Application of omics technologies

Omics technologies have provided comprehensive insights into the molecular mechanisms underlying drought tolerance in cotton. Transcriptomic analyses have identified numerous drought-responsive genes, such as those coding for transcription factors and regulatory proteins, which are differentially expressed under drought conditions (Hasan et al., 2018). Proteomic studies have revealed the upregulation of proteins involved in stress response pathways, such as antioxidant enzymes and osmolyte biosynthesis enzymes, in drought-tolerant cotton varieties (Liang et al., 2016). Metabolomic analyses have shown increased levels of proline, soluble sugars, and other metabolites that contribute to osmotic adjustment and stress tolerance in transgenic cotton plants.

5 Case Study: Drought-Tolerant Cotton in Practice

5.1 Background and objectives of the case study

This case study focuses on the development and implementation of drought-tolerant cotton varieties in regions severely affected by water scarcity. The primary objective is to enhance cotton production under drought conditions by employing advanced breeding strategies and biotechnological interventions. The study region includes areas with significant agricultural dependence on cotton, where water scarcity poses a critical challenge to crop yield and quality. The breeding strategies are centered on identifying and utilizing genetic variations and traits associated with drought tolerance to develop resilient cotton cultivars.

5.2 Implemented breeding and biotechnological strategies

To address the challenge of drought stress in cotton, several breeding and biotechnological strategies have been implemented. Traditional breeding methods involved crossing drought-tolerant genotypes with high-yielding varieties to combine desirable traits. For instance, ten cotton genotypes, including three drought-tolerant and seven susceptible ones, were evaluated under different water regimes to assess their drought tolerance mechanisms. The study revealed that traits such as days to boll opening and proline content had significant genetic control and heritability, indicating their potential for improving drought tolerance in cotton (Mahmood et al., 2021).

In addition to traditional breeding, biotechnological approaches have been employed to enhance drought tolerance. Transgenic techniques, such as the overexpression of specific genes, have shown promising results. For example, the overexpression of the com58276 gene from Caragana korshinskii in cotton plants conferred enhanced drought tolerance without affecting growth and fiber content (Figure 2) (Pu et al., 2023). Similarly, the rice gene OsSIZ1 was overexpressed in cotton, resulting in higher net photosynthesis, better growth under drought conditions, and improved fiber yield in field trials (Mishra et al., 2017). Another notable approach involved the overexpression of the Arabidopsis EDT1/HDG11 gene, which significantly improved drought and salt tolerance in cotton, leading to better agronomic performance and higher yields under drought conditions.

Figure 2 Physiological indexes of TM-1, OE1, OE3, and OE4 plants at various growth periods. (A) Number of sections. (B) Number of fruits. (C) Plant height. (D) Weight of lint. (E) Weight of unginned cotton. (F) Length of fiber. (G) Specific strength of fiber. (H) Micronaire of fiber (Adopted from Pu et al., 2023)

5.3 Outcomes and lessons learned

The implementation of these breeding and biotechnological strategies has yielded significant outcomes. The identification and utilization of drought-tolerant genotypes have led to the development of cotton varieties with improved resilience to water scarcity. For instance, genotypes such as NIAB-135, NIAB-512, and CIM-554 were identified as potential candidates for breeding programs aimed at enhancing drought tolerance (Zahid et al., 2021). The use of transgenic approaches has also demonstrated substantial improvements in drought tolerance and yield. The overexpression of genes like com58276, OsSIZ1, and EDT1/HDG11 has not only enhanced drought tolerance but also maintained or improved fiber quality and yield under stress conditions (Yu et al., 2016).

Lessons learned from these efforts highlight the importance of integrating traditional breeding with modern biotechnological tools to achieve sustainable improvements in crop resilience. The success of these strategies underscores the need for continued research and development to identify and characterize novel genes and traits associated with drought tolerance. Additionally, the importance of field trials and real-world testing cannot be overstated, as they provide critical insights into the performance and adaptability of new cultivars under actual growing conditions.

6 Challenges and Limitations in Achieving Drought Tolerance

6.1 Genetic bottlenecks and limited genetic diversity

One of the primary challenges in achieving drought tolerance in cotton is the limited genetic diversity within cultivated varieties. Over the years, intense selection and inbreeding have narrowed the genetic base of upland cotton, making it difficult to introduce new traits such as drought tolerance (Magwanga et al., 2019). The erosion of genetic diversity limits the availability of beneficial alleles that could enhance drought resistance. To overcome this bottleneck, researchers have suggested the introgression of traits from wild cotton progenitors, which possess unique characteristics that can be beneficial for drought tolerance (Magwanga et al., 2018). However, the process of introgression and subsequent breeding is complex and time-consuming, requiring extensive genetic mapping and phenotyping to identify and incorporate the desired traits effectively (Abdelraheem et al., 2019).

6.2 Environmental variability and testing conditions

Environmental variability poses a significant challenge in the development and testing of drought-tolerant cotton varieties. Drought stress is not uniform and can vary greatly in intensity, duration, and timing, making it difficult to standardize testing conditions (Mahmood et al., 2021; Zahid et al., 2021). This variability complicates the assessment of drought tolerance, as genotypes may respond differently under varying environmental conditions. Additionally, the interaction between genotype and environment (G × E) further complicates the selection process, as traits that confer drought tolerance in one environment may not be effective in another (Sun et al., 2023b). Reliable and high-throughput screening methods are needed to improve the consistency and accuracy of phenotyping under diverse environmental conditions (Çelik, 2023).

6.3 Economic and regulatory constraints in commercialization

The commercialization of drought-tolerant cotton varieties faces several economic and regulatory constraints. Developing new varieties involves significant investment in research and development, including extensive field trials and regulatory approvals (Yu et al., 2016). The cost of these processes can be prohibitive, especially for smaller breeding programs. Additionally, regulatory frameworks for genetically modified organisms (GMOs) vary by region and can be stringent, requiring comprehensive safety and environmental impact assessments (Mahmood et al., 2019). These regulatory hurdles can delay the introduction of new drought-tolerant varieties to the market. Furthermore, the economic viability of these varieties depends on their performance relative to existing cultivars, which may already be well-adapted to local conditions (Mishra et al., 2017). Ensuring that new drought-tolerant varieties offer a clear advantage in terms of yield and resilience is crucial for their adoption by farmers.

7 Future Directions for Genetic Improvement

7.1 Integration of multi-omics data in breeding programs

The integration of multi-omics data, including genomics, proteomics, and metabolomics, is essential for advancing drought tolerance in cotton. By combining these large-scale datasets, researchers can uncover the genetic architecture and molecular networks that contribute to drought tolerance. This approach allows for a comprehensive understanding of the complex interactions between genes and environmental factors, facilitating the identification of key genetic markers and pathways involved in drought response (Joshi et al., 2016). The use of meta-quantitative trait loci (QTL) analysis across multiple environments and diverse genetic backgrounds can further enhance the precision of these findings, enabling more effective breeding strategies (Singh et al., 2015).

7.2 Advances in precision breeding and phenotyping technologies

Precision breeding and advanced phenotyping technologies are revolutionizing the development of drought-tolerant cotton varieties. High-throughput genotyping (HTG) and high-throughput phenotyping (HTP) platforms are critical for improving the accuracy and efficiency of QTL detection and marker-trait associations (Bhat et al., 2020). These technologies enable the rapid and precise characterization of germplasm and breeding material, facilitating the selection of superior genotypes with enhanced drought tolerance. The integration of digital phenotyping tools allows for the large-scale, accurate measurement of complex traits, thereby accelerating the breeding process and improving the reliability of phenotypic data (Tuberosa, 2012).

7.3 International collaboration for germplasm exchange and knowledge sharing

International collaboration is vital for the exchange of germplasm and the sharing of knowledge and resources in the quest for improved drought tolerance in cotton. Collaborative efforts can lead to the development of more diverse and resilient cotton varieties by incorporating genetic material from different regions and environments (Abdelraheem et al., 2019). Such partnerships can also facilitate the dissemination of advanced breeding techniques and phenotyping methods, ensuring that researchers worldwide have access to the latest tools and technologies (Sun et al., 2023b). By working together, the global scientific community can make significant strides in addressing the challenges posed by drought stress and enhancing the sustainability of cotton production.

8 Conclusion

Research into genetic improvement for drought tolerance in cotton has yielded significant insights. Studies have identified various genotypes with enhanced drought tolerance, such as DTV-9 × BT-252 and DTV-9 × DTV-10, which exhibit superior combining ability and heritability for traits like days to boll opening and proline content under drought stress. The genetic basis of drought tolerance has been explored through quantitative trait loci (QTL) mapping and the identification of drought-responsive genes, although these have yet to be widely adopted in commercial breeding programs. Additionally, transgenic approaches, such as the overexpression of genes like com58276 from Caragana korshinskii and OsSIZ1 from rice, have shown promise in enhancing drought tolerance and improving fiber yields under stress conditions.

The findings underscore the potential for developing drought-tolerant cotton varieties through both traditional breeding and modern biotechnological approaches. The identification of drought-tolerant genotypes and the understanding of their genetic mechanisms can guide the selection of parent plants for breeding programs aimed at improving drought resilience. Moreover, the integration of transgenic methods, such as the overexpression of specific genes, offers a viable pathway to enhance drought tolerance and ensure stable yields under water-limited conditions. These advancements are crucial for sustainable agriculture, as they can mitigate the adverse effects of climate change on cotton production and contribute to food security and economic stability in cotton-growing regions.

Future research should focus on the large-scale implementation of identified drought-tolerant genes and QTLs in commercial breeding programs. The development of high-throughput phenotyping methods and the creation of diverse mapping populations will be essential for the accurate identification and transfer of drought tolerance traits. Additionally, exploring the synergistic effects of combining multiple stress-tolerance traits, such as drought and salinity, could further enhance the resilience of cotton crops. Continued investment in both traditional and biotechnological research will be vital to achieving these goals and ensuring the long-term sustainability of cotton production in the face of increasing environmental challenges.

Acknowledgments

We express the profound sense of reverence to the entire research team, friends, and any other person who contributed; we have deep gratitude for you so much.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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