1 Introduction

Rice is a staple food for more than half of the world's population, making it one of the most critical crops globally. With the ever-increasing global population, there is a growing need to enhance rice yields to ensure food security. Improving rice yields not only supports the growing population but also contributes to the economic stability of rice-producing regions. Rice is integral to global food security, providing more than one-fifth of the calories consumed worldwide. In many developing countries, rice is the primary source of sustenance and income for millions of people. The importance of rice yield improvement cannot be overstated, given the challenges posed by climate change, decreasing arable land, and the need for sustainable agricultural practices.

Increasing rice yield is essential to meet the food demands of a growing global population, projected to reach 9.7 billion by 2050. Traditional rice-growing methods are insufficient to meet this demand due to their lower efficiency and susceptibility to environmental stressors. Thus, there is an urgent need to adopt advanced breeding techniques and agricultural practices that enhance rice productivity sustainably.

In addition to food security, higher rice yields can lead to economic benefits for farmers and rice-producing countries. Improved yields translate to higher incomes for farmers, better livelihoods, and economic growth in rural areas. Enhanced productivity also reduces the need for expanding agricultural lands, thus preserving natural ecosystems and biodiversity. Heterosis, commonly referred to as hybrid vigor, is a phenomenon where the offspring of two different parental lines exhibit superior qualities compared to their parents. In the context of agriculture, heterosis is particularly significant as it can lead to higher yields, greater resistance to diseases and pests, and improved stress tolerance in crops. The concept of heterosis has been exploited in various crops, including maize, sorghum, and rice, to achieve substantial yield improvements.

The genetic basis of heterosis involves the interaction of alleles from the parent lines, which can result in increased vigor, growth rate, and productivity in the hybrid offspring. This genetic interplay often leads to the expression of desirable traits that are not present or are less pronounced in the parent lines. Understanding the genetic mechanisms underlying heterosis is crucial for developing effective breeding strategies aimed at maximizing crop yields. Heterosis is particularly significant for rice breeding programs as it offers a viable solution to the limitations of traditional breeding methods. Hybrid rice varieties have been shown to yield 15%~30% more than conventional varieties under optimal conditions (Virmani, 1996). This yield advantage is critical for addressing food security challenges and improving the economic viability of rice farming.

The objective of this study is to explore the genetic basis of heterosis in rice and to identify effective breeding strategies to maximize rice yields. The study aims to provide a comprehensive understanding of the genetic and molecular mechanisms underlying hybrid vigor in rice. By synthesizing current research findings, it offers insights into how heterosis can be harnessed to improve rice productivity. This study promotes the understanding and application of heterosis in rice breeding. By exploring the genetic basis of hybrid vigor and identifying effective breeding strategies, the study supports efforts to maximize rice yields and enhance global food security. The findings are intended to provide important insights for researchers, breeders, and policymakers involved in rice production and agricultural development.

2 Understanding Heterosis in Rice

2.1 Historical perspectives on heterosis

Heterosis, commonly known as hybrid vigor, was first observed by Charles Darwin and later formally described by George Shull and Edward East in the early 20th century. The concept revolutionized plant breeding, particularly in maize, where hybrids showed remarkable improvements in yield and resilience. In rice (Oryza sativa L.), the application of heterosis has been more complex due to the crop's predominantly self-pollinating nature. However, the potential benefits of hybrid rice-such as higher yields, better stress tolerance, and improved resistance to pests and diseases-have driven extensive research and breeding efforts since the 1970s.

The success of hybrid rice in China, initiated by the pioneering work of Yuan Longping, marked a significant milestone. Yuan’s development of the first commercial hybrid rice variety in the 1970s demonstrated the feasibility of exploiting heterosis in rice, leading to widespread adoption and significant yield gains. Today, hybrid rice accounts for over 50% of the total rice cultivation area in China and has contributed to food security and agricultural sustainability.

2.2 Theories and mechanisms of heterosis

Understanding the underlying mechanisms of heterosis is crucial for effective hybrid breeding. Several theories have been proposed to explain heterosis, each highlighting different genetic and molecular aspects. The dominance theory posits that heterosis arises when dominant alleles mask the deleterious effects of recessive alleles. In hybrid plants, the combination of alleles from two genetically diverse parents can result in the suppression of harmful recessive alleles, leading to improved performance. This theory suggests that the greater the genetic divergence between the parents, the higher the potential for heterosis.

Dominance theory has been supported by numerous studies in various crops, including rice. Research has shown that hybrid rice varieties often exhibit higher levels of heterozygosity, which correlates with increased vigor and yield (Li et al., 2020). However, dominance theory alone cannot fully explain all aspects of heterosis, prompting the exploration of additional theories. 

Overdominance theory, also known as the single-gene heterosis theory, suggests that heterosis results from the superior performance of heterozygous genotypes at certain loci. According to this theory, the heterozygote is more vigorous than either homozygote, leading to enhanced traits in the hybrid. This phenomenon can occur due to interactions between alleles at a single locus or the presence of advantageous alleles that function optimally when heterozygous. In rice, evidence for overdominance has been found in studies identifying specific loci where heterozygous combinations outperform both homozygous parents. For example, QTL mapping has revealed loci associated with yield and other agronomic traits that exhibit overdominant effects, contributing to the overall hybrid vigor (Luo et al., 2013).

Epistasis refers to the interaction between genes at different loci, where the effect of one gene depends on the presence of one or more modifier genes. Gene interactions can significantly influence heterosis by creating complex networks that enhance the overall performance of the hybrid. Epistatic interactions can result in non-additive genetic effects, contributing to the superiority of hybrids over their parents. Studies in rice have demonstrated that epistatic interactions play a crucial role in heterosis. By using advanced genomic tools, researchers have identified numerous epistatic QTLs that contribute to traits such as yield, stress tolerance, and disease resistance (Yu et al., 1997). These findings underscore the importance of considering gene interactions in hybrid breeding programs.

2.3 Genetic basis of heterosis

Quantitative Trait Loci (QTL) mapping is a powerful tool used to identify genomic regions associated with complex traits, including those contributing to heterosis. By crossing genetically diverse parents and analyzing the resulting progeny, researchers can pinpoint specific loci that influence hybrid vigor. QTL mapping has been instrumental in uncovering the genetic basis of heterosis in rice.

Numerous QTLs associated with yield, biomass, and other agronomic traits have been identified in hybrid rice. For example, studies have identified QTLs on chromosomes 1, 2, and 6 that significantly contribute to yield heterosis (Huang et al., 2016). These QTLs often contain multiple genes, each playing a role in the observed heterotic effects. The identification and characterization of these QTLs provide valuable targets for marker-assisted selection (MAS) and genomic selection (GS) in rice breeding.

Advances in genomic technologies have revolutionized the study of heterosis, enabling researchers to delve deeper into the molecular mechanisms underlying hybrid vigor. High-throughput sequencing, transcriptomics, and proteomics have provided insights into the gene expression patterns and regulatory networks involved in heterosis. Genomic studies have revealed that hybrids often exhibit unique gene expression profiles compared to their parents. These differences in gene expression can lead to enhanced metabolic activities, improved stress responses, and optimized growth processes in hybrids. For instance, transcriptomic analyses have shown that hybrid rice plants have upregulated genes associated with photosynthesis, nutrient uptake, and stress tolerance (Zhang et al., 2018).

Moreover, epigenetic modifications, such as DNA methylation and histone modifications, have been implicated in the regulation of heterosis. These epigenetic changes can influence gene expression and contribute to the phenotypic superiority of hybrids. Understanding the role of epigenetics in heterosis offers new avenues for manipulating and enhancing hybrid performance through targeted breeding strategies. The genetic basis of heterosis in rice is multifaceted, involving complex interactions between genes, regulatory networks, and environmental factors. QTL mapping and genomic studies have provided valuable insights into the loci and molecular mechanisms that drive hybrid vigor. By integrating these findings into breeding programs, it is possible to develop superior hybrid rice varieties that can meet the growing demands for food security and agricultural sustainability.

3 Breeding Strategies for Exploiting Heterosis

3.1 Traditional hybrid breeding

Traditional hybrid breeding has been the backbone of hybrid rice development for decades. This approach involves several key steps, including inbreeding and line development, followed by crossbreeding techniques to produce hybrid varieties. The first step in traditional hybrid breeding is the development of inbred lines. Inbreeding involves the self-pollination of rice plants over several generations to produce homozygous lines that are genetically uniform. These inbred lines serve as the parental lines for producing hybrids.

The process of inbreeding helps to stabilize desirable traits and eliminate undesirable ones. However, it also results in inbreeding depression, where the progeny exhibit reduced vigor and performance due to the accumulation of deleterious alleles. Despite this drawback, inbreeding is crucial for creating pure lines that can be used in hybrid breeding programs (Virmani, 1996). Once stable inbred lines are developed, crossbreeding techniques are employed to produce hybrid varieties. Crossbreeding involves the controlled pollination of two genetically distinct inbred lines to produce F1 hybrids. These hybrids exhibit heterosis, manifesting in superior traits such as increased yield, improved resistance to diseases and pests, and enhanced stress tolerance.

In rice, two main types of hybrid breeding systems are used: two-line and three-line systems. The two-line system involves the use of a male-sterile line and a maintainer line, while the three-line system includes a male-sterile line, a maintainer line, and a restorer line. The three-line system, pioneered by Yuan Longping in China, has been widely adopted due to its effectiveness in producing high-yielding hybrids.

3.2 Modern approaches in hybrid breeding

Advancements in molecular biology and genomics have paved the way for modern hybrid breeding approaches, such as marker-assisted selection (MAS) and genomic selection. These methods enhance the efficiency and precision of breeding programs by leveraging genetic information. Marker-assisted selection (MAS) is a technique that uses molecular markers to identify and select desirable traits in breeding populations. By associating specific markers with traits of interest, breeders can screen for these markers in early generations, accelerating the breeding process and increasing the accuracy of selection.

In rice, MAS has been successfully used to identify markers linked to key traits such as yield, disease resistance, and stress tolerance. For example, markers associated with the Sub1 gene, which confers submergence tolerance, have been used to develop flood-tolerant rice varieties (Xu et al., 2006). Similarly, markers linked to major QTLs for yield and grain quality have been employed to select superior parental lines and hybrids (Li et al., 2020).

Genomic selection is a more advanced approach that uses genome-wide markers to predict the breeding value of individuals. Unlike MAS, which focuses on a few markers linked to specific traits, genomic selection considers the combined effects of all markers across the genome. This allows for the selection of individuals with the best overall genetic potential.

Genomic selection involves the development of a training population with known phenotypes and genotypes. Statistical models are then used to estimate the effects of all markers and predict the performance of untested individuals. This approach has been shown to significantly improve the accuracy and efficiency of selection in rice breeding programs (Spindel et al., 2015).

3.3 Developing superior hybrid varieties

The development of superior hybrid rice varieties involves several critical steps, including the assessment of combining ability and parental selection, as well as rigorous testing and evaluation of hybrid performance. Combining ability refers to the ability of parental lines to produce superior hybrids when crossed. It is assessed through diallel crosses, where multiple parental lines are crossed in all possible combinations, and the performance of the resulting hybrids is evaluated. General combining ability (GCA) indicates the average performance of a line when crossed with several other lines, while specific combining ability (SCA) reflects the performance of specific hybrid combinations. The selection of parental lines with high GCA and SCA is crucial for developing superior hybrids. This involves evaluating the genetic diversity and compatibility of potential parents, as well as their performance in various environments. Advanced molecular techniques, such as genome-wide association studies (GWAS), can also aid in identifying parental lines with desirable traits and high combining ability (Figure 1) (Huang et al., 2016).

Figure 1 Two candidate genes hd3a and tac1 for heterosis for yield traits in type-A hybrids (Adopted from Huang et al., 2016) Image caption: a: Gene structure of Hd3a and hd3a alleles; b: Plots of the advantage of the heterozygous state (Hd3a/hd3a) over Hd3a/Hd3a for grain yield per plant and yield components in nine type-A populations of 3 947 F2 lines; c: Gene structure of TAC1 and tac1 alleles; d, The performances of tiller angle for three genotypes of the TAC1 gene in 438 F2 lines (mean±s.e.); The three genotypes are colour-coded in d and e; e: Computational modelling of the planting number per unit area (mu, equivalent to one-fifteenth of a hectare) for TAC1 gene; f: The allele frequencies of hd3a and tac1 in 1 063 pairs of type-A parents and 254 pairs of type-B parents (Adapted from Huang et al., 2016)

Rigorous testing ensures that only the best-performing hybrids are released to farmers. This process includes on-farm trials and farmer participatory breeding programs, where farmers' feedback is incorporated into the selection process. Successful hybrids are then multiplied and distributed as hybrid seeds to farmers, contributing to increased productivity and sustainability in rice production (Virmani, 1996).

The exploitation of heterosis in rice through traditional and modern breeding strategies has significantly enhanced rice yields and resilience. By integrating advanced genomic tools and techniques, breeders can develop superior hybrid varieties that meet the growing demands for food security and agricultural sustainability.

4 Case Studies of Successful Hybrid Rice Varieties

4.1 High-yielding hybrids

Shanyou 63, developed in China, is one of the most renowned high-yielding hybrid rice varieties. Released in the 1990s, Shanyou 63 quickly gained popularity due to its remarkable yield potential. The hybrid combines the male-sterile line Zhenshan 97A with the restorer line Minghui 63, both of which were carefully selected for their combining ability and superior traits.

Shanyou 63’s success can be attributed to its vigorous growth, high tillering capacity, and enhanced photosynthetic efficiency. Field trials demonstrated that Shanyou 63 could achieve yields up to 20% higher than conventional inbred varieties under optimal conditions. Its high yield potential made it a cornerstone of China's hybrid rice program, contributing significantly to national food security (Cheng et al., 2007). The International Rice Research Institute (IRRI) has also developed several high-yielding hybrid rice varieties. IRRI's super hybrids, such as Mestizo 1 and Mestizo 2, are notable examples. These hybrids were developed by crossing elite tropical rice lines with high yield potential and stress tolerance.

Mestizo 1 and Mestizo 2 have shown yield advantages of 15%~20% over traditional inbred varieties. Their development involved extensive field testing across multiple environments to ensure adaptability and stability. These hybrids have been widely adopted in the Philippines and other Asian countries, contributing to increased rice production and farmer incomes (Khush, 2013).

4.2 Stress-resistant hybrids

Sub1 hybrids, developed to withstand submergence stress, are a remarkable example of stress-resistant hybrid rice. These hybrids carry the Sub1 gene, which confers tolerance to prolonged flooding, a common challenge in many rice-growing regions. The Sub1 gene was incorporated into popular hybrid varieties through marker-assisted backcrossing (Xu et al., 2006).

Sub1 hybrids, such as Swarna-Sub1 and Samba Mahsuri-Sub1, have demonstrated their resilience in flood-prone areas. These hybrids can survive complete submergence for up to two weeks, allowing farmers to achieve stable yields despite adverse weather conditions. The adoption of Sub1 hybrids has been particularly beneficial in South and Southeast Asia, where flooding is a recurrent problem (Ismail et al., 2013).

Drought tolerance is another critical trait for improving rice resilience. Hybrid varieties such as Arize 6444 Gold, developed by Bayer CropScience, have been bred for enhanced drought tolerance. Arize 6444 Gold combines the traits of high yield potential and drought resilience, making it suitable for regions with erratic rainfall patterns.

Field trials have shown that Arize 6444 Gold can maintain high yields under water-limited conditions, providing a reliable option for farmers facing drought stress. This hybrid's performance highlights the importance of integrating stress tolerance traits into hybrid breeding programs to ensure food security in the face of climate change.

4.3 Quality-enhanced hybrids

In India, hybrid rice breeding has also focused on improving grain quality. Varieties such as Pusa RH10 and DRRH3, developed by the Indian Agricultural Research Institute (IARI), combine high yield potential with superior grain quality. These hybrids are characterized by long, slender grains, high amylose content, and good cooking quality, catering to consumer preferences (Singh et al., 2017).

Pusa RH10 and DRRH3 have been well-received by both farmers and consumers. Their adoption has not only increased rice production but also enhanced market value due to their premium grain quality. These hybrids exemplify the successful integration of yield and quality traits in hybrid rice breeding.

Vietnam has made significant strides in developing quality-enhanced hybrid rice varieties. Hybrids such as TH3-3 and PAC 807, developed by the Vietnam Academy of Agricultural Sciences (VAAS), are known for their high yield potential and excellent grain quality. These hybrids feature aromatic grains, desirable texture, and good cooking properties, meeting the high standards of both domestic and export markets.

TH3-3 and PAC 807 have gained popularity among Vietnamese farmers, leading to increased adoption and improved livelihoods. The success of these hybrids underscores the importance of breeding programs that address both productivity and quality to meet consumer demands (Nguyen et al., 2015).

Ihe development and adoption of high-yielding, stress-resistant, and quality-enhanced hybrid rice varieties have significantly contributed to global rice production. These case studies highlight the diverse approaches and successes in hybrid rice breeding, demonstrating the potential of heterosis to enhance food security and agricultural sustainability.

5 Challenges and Opportunities in Hybrid Rice Breeding

5.1 Biological and genetic constraints

Genetic diversity is the cornerstone of hybrid breeding, providing the necessary variability for selecting superior hybrid combinations. However, maintaining and expanding genetic diversity within breeding programs poses significant challenges. In hybrid rice breeding, the establishment of heterotic groups-genetically distinct groups of germplasm that exhibit high heterosis when crossed-is crucial. Identifying and developing these groups requires extensive germplasm characterization and evaluation.

The limited genetic diversity within rice germplasm pools can restrict the potential for heterosis. To address this, breeders must continually introduce new genetic material from wild relatives and landraces to broaden the genetic base. For instance, incorporating genes from Oryza rufipogon, a wild relative of rice, has been shown to enhance yield and stress tolerance in hybrids (Huang et al., 2016).

Inbreeding depression, the reduced biological fitness due to the mating of closely related individuals, is a significant challenge in hybrid rice breeding. Developing pure lines through successive self-pollination can lead to the accumulation of deleterious alleles, resulting in reduced vigor and fertility. Overcoming inbreeding depression requires careful management of breeding populations and the use of techniques such as recurrent selection to maintain genetic health.

To mitigate inbreeding depression, breeders often employ a strategy known as “genetic purging,” which involves selecting against deleterious alleles during the inbreeding process. Additionally, advanced molecular tools can help identify and eliminate these harmful alleles, thereby improving the overall fitness of inbred lines used in hybrid breeding programs (Li et al., 2020).

5.2 Technological and methodological advances

Recent advancements in genomics and biotechnology have revolutionized hybrid rice breeding, providing new tools and techniques to enhance breeding efficiency and precision. The advent of high-throughput sequencing technologies has enabled the rapid and cost-effective sequencing of rice genomes, facilitating the identification of genes and quantitative trait loci (QTLs) associated with important agronomic traits.

Genomic selection (GS) is one such advancement that uses genome-wide markers to predict the breeding value of individuals. This approach allows breeders to select superior candidates early in the breeding cycle, significantly reducing the time and resources required to develop new hybrids (Spindel et al., 2015). Additionally, CRISPR/Cas9 genome editing technology has emerged as a powerful tool for introducing precise genetic modifications, enabling the development of hybrids with enhanced traits such as disease resistance and stress tolerance (Zhang et al., 2018).

Biotechnological approaches, such as marker-assisted selection (MAS), have also been instrumental in hybrid rice breeding. MAS uses molecular markers linked to desirable traits to facilitate the selection of superior parents and hybrids. This method has been successfully applied to improve traits such as yield, grain quality, and biotic and abiotic stress resistance (Xu et al., 2006).

The integration of computational tools and data analysis techniques has greatly enhanced the efficiency of hybrid rice breeding programs. Advanced bioinformatics platforms and statistical software enable breeders to manage and analyze large datasets generated from genomic studies. These tools facilitate the identification of key genetic regions and the development of predictive models for trait selection.

Machine learning algorithms and artificial intelligence (AI) have also been applied to predict hybrid performance based on genetic and phenotypic data. These technologies can analyze complex interactions between genes and environmental factors, providing insights into the genetic basis of heterosis and guiding breeding decisions (Perez-Enciso and Zingaretti, 2019).

Furthermore, decision support systems (DSS) that integrate genomic, phenotypic, and environmental data can assist breeders in optimizing crossing schemes and selecting the best hybrid combinations. These systems use advanced algorithms to simulate breeding outcomes and predict the performance of hybrid progeny under different scenarios, enhancing the precision and success rate of hybrid breeding programs (Bernardo, 2014).

5.3 Socioeconomic and environmental considerations

The widespread adoption and acceptance of hybrid rice are critical for realizing the benefits of heterosis. However, several socioeconomic factors can influence the adoption of hybrid rice varieties by farmers. These include the cost of hybrid seeds, the availability of reliable seed supply systems, and the level of technical knowledge required for hybrid cultivation.

Hybrid rice seeds are typically more expensive than traditional inbred seeds due to the cost-intensive breeding and seed production processes. This can pose a barrier to adoption, particularly for smallholder farmers with limited financial resources. To address this, governments and agricultural organizations need to provide subsidies and support programs to make hybrid seeds more affordable and accessible (Spielman et al., 2012).

Moreover, the successful cultivation of hybrid rice requires specific agronomic practices and management techniques. Extension services and training programs are essential to equip farmers with the necessary knowledge and skills to grow hybrid rice effectively. Collaborative efforts between research institutions, government agencies, and the private sector can facilitate the dissemination of best practices and promote the adoption of hybrid rice (Li et al., 2012).

The environmental impact and sustainability of hybrid rice cultivation are important considerations in the development and deployment of hybrid varieties. Hybrid rice has the potential to contribute to sustainable agriculture by increasing yields and reducing the need for chemical inputs. However, the intensive management practices required for hybrid rice can also pose environmental challenges.

One of the key environmental benefits of hybrid rice is its ability to produce higher yields with less land, helping to preserve natural ecosystems and biodiversity. Additionally, hybrids with improved resistance to pests and diseases can reduce the reliance on chemical pesticides, promoting more sustainable farming practices (Peng et al., 2003).

However, the intensive use of fertilizers and irrigation in hybrid rice cultivation can lead to environmental issues such as soil degradation, water pollution, and greenhouse gas emissions. Sustainable management practices, such as integrated pest management (IPM), precision agriculture, and the use of organic fertilizers, are essential to mitigate these impacts and ensure the long-term sustainability of hybrid rice production (Tilman et al., 2011).

In conclusion, hybrid rice breeding presents both challenges and opportunities that need to be carefully managed to maximize its benefits. Advances in genomics, biotechnology, and computational tools offer promising avenues for overcoming biological and genetic constraints, while addressing socioeconomic and environmental considerations is crucial for the successful adoption and sustainability of hybrid rice. By leveraging these opportunities and addressing the challenges, hybrid rice breeding can continue to play a pivotal role in enhancing global food security and agricultural sustainability.

6 Future Directions in Heterosis Research and Hybrid Breeding

The future of heterosis research and hybrid breeding in rice is poised for transformative advancements driven by emerging trends and innovations. This section explores the cutting-edge technologies and approaches that promise to revolutionize hybrid rice breeding, including gene editing and synthetic biology, the integration of multi-omics strategies, and the role of policy and regulatory frameworks.

6.1 Emerging trends and innovations

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) technology has revolutionized genetic engineering, providing a precise and efficient tool for editing plant genomes. In rice breeding, CRISPR/Cas9 and other gene-editing technologies offer unprecedented opportunities to enhance hybrid vigor by targeting specific genes associated with yield, stress resistance, and other desirable traits.

Gene editing can be used to create novel genetic variations that enhance heterosis. Zhang et al. (2018) have successfully used CRISPR/Cas9 to knock out genes that negatively affect yield and to introduce beneficial alleles that improve agronomic performance. This technology also allows for the rapid development of hybrids with improved resistance to biotic and abiotic stresses, such as pests, diseases, drought, and salinity (Figure 2).

Figure 2 The rapid domestication of NSRPs using precise gene editing (Adopted from Zhang et al., 2018) Image caption: After the reference genome (or assembled transcriptome) of an NSRP is generated, candidate domestication genes can be identified based on their orthologous relationship with domestication genes in the closest crop relative. A CRISPR-Cas9 vector targeting multiple candidate domestication genes can then be generated. CRISPR-Cas9 can be delivered into the plants in at least two ways: (1) the plasmid can be delivered into calli (for example, through particle bombardment), with co-delivery of a plasmid for overexpression of Bbm and Wus2 orthologous genes to facilitate plant regeneration; or (2) the plasmid DNA or Cas9-sgRNA ribonucleoprotein complex can be mixed with magnetic nanoparticles (MNPs), and the coated MNPs can then be drawn by a strong magnetic field into pollen through pollen apertures. The magnetofected pollen is used to pollinate emasculated flowers to generate gene-edited plants. Edited NSRPs with desired traits can then be selected in the field or glasshouse (Adapted from Zhang et al., 2018)

One of the key advantages of CRISPR is its ability to make precise modifications without introducing foreign DNA, which can address regulatory and consumer concerns associated with genetically modified organisms (GMOs). This makes CRISPR-edited hybrids more likely to gain acceptance and regulatory approval, paving the way for their adoption in agricultural practices.

Synthetic biology, which involves the design and construction of new biological parts, devices, and systems, offers exciting possibilities for hybrid rice breeding. This interdisciplinary field combines principles from biology, engineering, and computer science to create synthetic gene networks and metabolic pathways that can enhance plant traits.

In rice, synthetic biology can be used to engineer metabolic pathways that enhance photosynthesis efficiency, nutrient uptake, and stress tolerance. Ermakova et al. (2019) exploring the synthesis of C4 photosynthetic pathways in C3 plants like rice to improve photosynthetic efficiency and yield. Additionally, synthetic biology can be applied to create novel biosensors and regulatory circuits that optimize plant growth and development under varying environmental conditions.

The integration of synthetic biology with traditional breeding methods and modern genomic tools has the potential to accelerate the development of high-performing hybrid rice varieties that are tailored to meet the challenges of climate change and food security.

6.2 Integrating multi-omics approaches

The integration of multi-omics approaches-genomics, transcriptomics, and proteomics-provides a comprehensive understanding of the molecular mechanisms underlying heterosis. By analyzing the genome, transcriptome, and proteome of hybrid rice and its parental lines, researchers can identify key genes and regulatory networks that contribute to hybrid vigor.

Genomics involves sequencing and analyzing the DNA of rice plants to identify genetic variations and QTLs associated with desirable traits. Advanced genomic tools, such as whole-genome sequencing and genome-wide association studies (GWAS), enable the identification of alleles that confer heterosis and the development of molecular markers for marker-assisted selection (Huang et al., 2016).

Transcriptomics examines the complete set of RNA transcripts produced by the genome, providing insights into gene expression patterns and regulatory networks. By comparing the transcriptomes of hybrids and their parents, researchers can identify differentially expressed genes that contribute to enhanced performance (Zhang et al., 2018).

Proteomics involves the large-scale study of proteins, including their expression, structure, and functions. Proteomic analyses can reveal post-translational modifications and protein-protein interactions that play critical roles in hybrid vigor. Integrating proteomic data with genomic and transcriptomic information provides a holistic view of the molecular basis of heterosis (Swetha et al., 2020). Metabolomics and phenomics are emerging fields that complement genomics, transcriptomics, and proteomics, offering additional layers of information on the physiological and metabolic states of hybrid rice.

Metabolomics involves the comprehensive analysis of metabolites—small molecules involved in metabolic processes. By profiling the metabolome of hybrids and their parents, researchers can identify metabolic pathways that are enhanced or repressed in hybrids, contributing to their superior performance. Metabolomic studies can also reveal biomarkers associated with stress tolerance, yield, and quality traits (Fiehn, 2002).

Phenomics focuses on the high-throughput measurement of phenotypic traits, capturing the physical and biochemical characteristics of plants. Advances in imaging technologies, such as remote sensing, hyperspectral imaging, and drone-based phenotyping, allow for precise and rapid assessment of traits such as plant height, biomass, leaf area, and stress responses. Integrating phenomic data with multi-omics information enables the identification of genotype-phenotype correlations and the selection of superior hybrids (Yang et al., 2020).

6.3 Policy and regulatory frameworks

International collaborations are essential for advancing hybrid rice research and breeding. Collaborative efforts between research institutions, government agencies, and private sector organizations facilitate the exchange of knowledge, resources, and technologies, accelerating the development and dissemination of hybrid rice varieties.

Programs such as the International Rice Research Institute (IRRI) and the Africa Rice Center (Africa Rice) have been instrumental in promoting hybrid rice research and breeding across different regions. These organizations work with national agricultural research systems (NARS) and other partners to develop and distribute improved hybrid varieties that address local agricultural challenges (IRRI, 2021).

Global initiatives, such as the CGIAR Research Program on Rice (RICE), provide platforms for collaborative research and capacity building, ensuring that advancements in hybrid rice breeding benefit farmers worldwide. Strengthening international collaborations and partnerships is crucial for addressing the complex challenges of food security and sustainable agriculture.

Intellectual property (IP) and seed distribution are critical aspects of hybrid rice breeding that require careful consideration to ensure equitable access and benefits. The development of hybrid rice involves significant investments in research and breeding, and protecting intellectual property rights (IPR) is important for incentivizing innovation.

However, IP protection must be balanced with the need for accessible and affordable hybrid seeds for farmers, particularly smallholders in developing countries. Policies that promote the sharing of genetic resources and technologies, while ensuring fair compensation for breeders, can facilitate the widespread adoption of hybrid rice.

Seed distribution systems must also be strengthened to ensure that high-quality hybrid seeds reach farmers in a timely and cost-effective manner. Public-private partnerships and innovative business models, such as community-based seed production and distribution networks, can enhance seed accessibility and support sustainable hybrid rice cultivation (Spielman et al., 2012).

The future of heterosis study and hybrid breeding in rice is bright, with numerous emerging trends and innovations poised to transform the field. Advances in gene editing, synthetic biology, and multi-omics approaches offer new opportunities for enhancing hybrid vigor and developing superior hybrid varieties. International collaborations and supportive policy frameworks are essential for ensuring that these advancements translate into tangible benefits for farmers and contribute to global food security and sustainability.

7 Concluding Remarks

This study has explored the genetic basis and breeding strategies for maximizing rice yields through heterosis. The concept of heterosis, or hybrid vigor, has been a cornerstone of plant breeding, significantly contributing to the enhancement of rice production. Key findings from this study include the identification of genetic mechanisms underlying heterosis, such as dominance, overdominance, and epistasis, which collectively contribute to the superior performance of hybrid rice varieties.

The historical development of hybrid rice, particularly in China under the leadership of Yuan Longping, has demonstrated the feasibility and benefits of exploiting heterosis in rice breeding. Hybrid varieties have consistently shown higher yields, better stress tolerance, and improved resistance to pests and diseases compared to traditional inbred varieties. Traditional breeding strategies, including the development of inbred lines and the use of two-line and three-line hybrid systems, have laid the foundation for successful hybrid rice cultivation. Modern approaches such as marker-assisted selection (MAS) and genomic selection (GS) have further revolutionized hybrid breeding by increasing the efficiency and precision of selecting superior hybrids. Advances in genomics and biotechnology, including CRISPR and synthetic biology, have opened new avenues for enhancing hybrid vigor and developing hybrids with novel traits.

The integration of multi-omics approaches, encompassing genomics, transcriptomics, proteomics, metabolomics, and phenomics, provides a comprehensive understanding of the molecular basis of heterosis. These approaches enable the identification of key genes, regulatory networks, and metabolic pathways that contribute to hybrid performance, facilitating the development of more robust and high-yielding hybrids.

The findings of this study have significant implications for rice breeding programs aimed at maximizing yields through heterosis. The successful development and adoption of hybrid rice varieties depend on several critical factors, including genetic diversity, breeding strategies, technological advancements, and socio-economic considerations. Maintaining and expanding genetic diversity is essential for sustaining heterosis in hybrid breeding programs. Breeders should continuously introduce new genetic material from diverse sources, including wild relatives and landraces, to broaden the genetic base and enhance the potential for heterosis. Establishing well-defined heterotic groups and optimizing parental selection are crucial steps in developing superior hybrids.

The integration of modern genomic tools and biotechnological approaches can significantly enhance the efficiency and precision of hybrid breeding. Marker-assisted selection and genomic selection should be widely adopted to accelerate the breeding process and improve the accuracy of selecting high-performing hybrids. CRISPR and other gene-editing technologies offer powerful tools for introducing desirable traits and creating novel genetic variations that enhance hybrid vigor. The comprehensive use of multi-omics approaches can provide deeper insights into the genetic and molecular mechanisms underlying heterosis. By integrating data from genomics, transcriptomics, proteomics, metabolomics, and phenomics, breeders can identify key drivers of hybrid performance and develop targeted strategies for improving hybrid traits. This holistic approach can also facilitate the identification of biomarkers for stress tolerance, yield, and quality traits, enabling more precise selection and breeding.

Socio-economic and environmental considerations are also critical for the successful adoption of hybrid rice varieties. The cost of hybrid seeds and the availability of reliable seed supply systems can influence the adoption rates among farmers, particularly smallholders with limited resources. Policies that promote the accessibility and affordability of hybrid seeds, coupled with extension services and training programs, can enhance the adoption and successful cultivation of hybrid rice. Environmental sustainability should be a key focus in hybrid rice breeding programs. While hybrid rice has the potential to increase yields and reduce the need for chemical inputs, intensive management practices can pose environmental challenges. Sustainable management practices, such as integrated pest management, precision agriculture, and the use of organic fertilizers, should be promoted to mitigate the environmental impact of hybrid rice cultivation.

Maximizing rice yields through heterosis represents a promising strategy for addressing the global challenges of food security and agricultural sustainability. The success of hybrid rice breeding programs hinges on a comprehensive understanding of the genetic and molecular mechanisms underlying heterosis, the adoption of modern genomic and biotechnological tools, and the consideration of socio-economic and environmental factors. To advance hybrid rice breeding, several recommendations can be made based on the findings of this study. First, breeding programs should prioritize the maintenance and expansion of genetic diversity by incorporating diverse germplasm sources and establishing robust heterotic groups. Second, the integration of advanced genomic tools and biotechnological approaches, such as CRISPR and synthetic biology, should be accelerated to enhance breeding efficiency and precision.

Acknowledgments

The authors extend sincere thanks to two anonymous peer reviewers for their feedback on the manuscript of this study, whose evaluations and suggestions have contributed to the improvement of the manuscript.

Conflict of Interest Disclosure

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

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