1 Introduction

Rice (Oryza sativa) is a staple food crop that sustains over half of the world's population. The architecture of rice plants, encompassing traits such as plant height, tillering capacity, and stem strength, plays a pivotal role in determining overall crop yield and resilience. Optimal plant architecture not only enhances photosynthetic efficiency and nutrient utilization but also improves resistance to lodging and environmental stresses. As such, understanding the genetic underpinnings of rice plant architecture is crucial for developing high-yielding, resilient rice varieties that can meet the demands of a growing global population and changing climate conditions.

The objective of this study is to systematically examine the roles of two key genes, SD1 (Semi-Dwarf1) and MOC1 (MONOCULM1), in shaping rice plant architecture and enhancing yield. By exploring the genetic and molecular bases of these genes, their impacts on plant structure, and their contributions to yield improvement, this study aims to provide a comprehensive understanding of their significance and potential applications in rice breeding programs. Furthermore, the study will delve into the interaction between SD1 and MOC1, highlighting their synergistic effects and the integrated benefits they offer to rice cultivation.

SD1 and MOC1 are central to rice plant architecture and productivity. The SD1 gene is a well-studied regulator of gibberellin biosynthesis, influencing plant height and stem strength. Variations in SD1 have been associated with the development of dwarf and semi-dwarf rice varieties, which exhibit enhanced lodging resistance and yield. On the other hand, MOC1 is crucial for the regulation of tillering and branching, affecting the number of tillers and overall plant morphology. Together, these genes offer a strategic pathway for optimizing plant architecture to achieve higher yield and improved agronomic performance. This study will provide an in-depth analysis of the genetic mechanisms and practical implications of SD1 and MOC1 in rice, offering insights into future research and breeding strategies.

1 The SD1 Gene and Its Role in Rice

1.1 Genetic and molecular basis of SD1

The SD1 gene, known as the "green revolution gene", encodes the enzyme gibberellin 20-oxidase-2 (GA20ox-2), which is crucial in the biosynthesis of gibberellins (GAs), a class of plant growth hormones. The gene consists of three exons and two introns, with mutations in this gene leading to a semi-dwarf phenotype due to reduced GA levels (Ashikari et al., 2002; Monna et al., 2002; Peng et al., 2021). The most common mutations include deletions and single nucleotide substitutions that result in a nonfunctional enzyme, thereby reducing the plant's height (Monna et al., 2002; Spielmeyer et al., 2002).

The SD1 gene plays a pivotal role in the final steps of GA biosynthesis. Mutations in SD1 lead to a decrease in bioactive GA levels, which in turn affects plant height and other growth parameters. The gene's expression is regulated by feedback mechanisms involving GA levels, ensuring that GA biosynthesis is tightly controlled (Ashikari et al., 2002; Sasaki et al., 2002). Additionally, interactions with other hormones such as brassinosteroids (BR) further modulate the effects of SD1 on plant growth (Tang et al., 2017; San et al., 2020).

1.2 Impact on plant architecture

The introduction of SD1 alleles has been instrumental in creating semi-dwarf rice varieties, which are shorter and more robust compared to their wild-type counterparts. This semi-dwarfism is primarily due to the reduced levels of bioactive GAs resulting from the loss-of-function mutations in the SD1 gene (Figure 1) (Asano et al., 2007; Jia et al., 2020; Peng et al., 2021). These semi-dwarf varieties have been widely adopted in rice breeding programs to improve yield and plant stability (Sasaki et al., 2002; Asano et al., 2007).

The SD1 gene significantly influences stem length and strength. Semi-dwarf varieties with SD1 mutations exhibit shorter and sturdier stems, which are less prone to lodging (falling over) under high nitrogen fertilization and heavy grain loads (Ashikari et al., 2002; Spielmeyer et al., 2002; San et al., 2020). This enhanced stem strength is a direct consequence of the reduced GA levels, which limit excessive stem elongation and promote a more compact plant structure (Monna et al., 2002; Peng et al., 2021).

1.3 Contributions to yield enhancement

One of the most significant contributions of the SD1 gene to rice yield enhancement is its role in improving lodging resistance. The shorter, sturdier stems of SD1 mutants are less likely to lodge, even under adverse weather conditions or high nitrogen fertilization, thereby ensuring higher and more stable yields (Sasaki et al., 2002; Asano et al., 2007; San et al., 2020). This trait was a key factor in the success of the "green revolution" rice varieties, such as IR8, which dramatically increased rice production in the 1960s (Ashikari et al., 2002; Peng et al., 2021).

The SD1 gene also contributes to yield enhancement by improving grain filling and the harvest index. The semi-dwarf varieties tend to allocate more resources to grain production rather than vegetative growth, resulting in a higher harvest index (the ratio of grain yield to total biomass) (Asano et al., 2007; Jia et al., 2020). Additionally, the improved light penetration into the canopy due to the more erect leaf inclination angle in SD1 mutants further enhances photosynthetic efficiency and grain filling (Tang et al., 2017; San et al., 2020). The SD1 gene plays a crucial role in rice plant architecture and yield enhancement by regulating gibberellin biosynthesis, influencing plant height and stem strength, and improving lodging resistance and grain filling. The widespread adoption of SD1 mutants has been a cornerstone of modern rice breeding, contributing significantly to global food security.

2 The MOC1 Gene and Its Role in Rice

2.1 Genetic and molecular basis of MOC1

The MONOCULM1 (MOC1) gene plays a critical role in the regulation of rice tillering and overall plant architecture. Located on chromosome 6, MOC1 encodes a transcription factor that belongs to the GRAS protein family. This gene is essential for the initiation and development of tillers, which are the lateral branches that emerge from the base of the rice plant. Mutations or disruptions in MOC1 result in the monoculm phenotype, characterized by a single main culm with no tillers, demonstrating the gene's pivotal role in tiller formation and plant morphology (Yu et al., 2020).

The MOC1 gene functions as a master regulator of tillering and branching in rice. It is expressed in the axillary meristems, where it activates the transcription of downstream genes involved in the initiation and outgrowth of tillers. MOC1 interacts with various hormonal pathways, including those of auxin and cytokinin, to modulate the balance between tiller bud dormancy and activation. This regulation ensures optimal tillering, contributing to the plant's ability to maximize light capture and nutrient use efficiency.

2.2 Impact on plant architecture

The MOC1 gene directly influences the number of tillers a rice plant produces. By regulating the activation and growth of axillary buds, MOC1 determines the extent of tillering, which is a crucial component of rice plant architecture. Increased tillering, controlled by MOC1, leads to a bushier plant with more shoots, which can enhance photosynthetic capacity and potential grain production.

Beyond tillering, MOC1 also impacts the overall root and shoot architecture of rice plants (Figure 2) (Shao et al., 2019). The gene's regulatory functions extend to the coordination of root growth patterns, ensuring a robust root system that can support the increased shoot biomass resulting from enhanced tillering. This integrated development of roots and shoots under the influence of MOC1 is vital for maintaining plant stability and nutrient uptake, which are essential for high yield performance.

2.3 Contributions to yield enhancement

One of the primary contributions of the MOC1 gene to rice yield enhancement is through the increase in tiller and panicle numbers. More tillers typically result in more panicles, each of which can produce grains. The effective management of tillering via MOC1 ensures that these additional panicles are well-developed and capable of supporting a high grain load, thereby directly contributing to increased grain yield per plant (Deng et al., 2022).

MOC1 also plays a significant role in optimizing resource allocation within the rice plant. By balancing the growth of tillers and the development of the root system, MOC1 ensures that the plant can efficiently utilize available resources, such as water and nutrients. This optimization is crucial for sustaining high yields, especially under varying environmental conditions. Efficient resource allocation driven by MOC1 helps maintain a stable yield and improves the plant's overall fitness and resilience.

The MOC1 gene is a fundamental regulator of rice plant architecture and yield. Its influence on tillering, root and shoot development, and resource allocation highlights its importance in rice breeding programs aimed at enhancing productivity and resilience. Understanding the genetic and molecular mechanisms of MOC1 offers valuable insights into the development of high-yielding rice varieties that can meet future food security challenges.

3 Interaction Between SD1 and MOC1

3.1 Genetic interactions and pathways

The interaction between the SD1 (Semi-Dwarf1) and MOC1 (MONOCULM1) genes plays a crucial role in regulating rice plant growth and development (Zhang et al., 2020). SD1, a key gene involved in gibberellin biosynthesis, primarily influences plant height by controlling stem elongation. In contrast, MOC1 regulates tillering and branching, determining the number of tillers and overall plant morphology. When these two genes interact, they create a synergistic effect that enhances overall plant growth. The dwarfism induced by SD1 contributes to stronger stems and reduced lodging risk, while the increased tillering driven by MOC1 leads to a bushier plant structure with more potential sites for grain production. Together, these effects improve plant stability and resource allocation, resulting in robust growth.

SD1 and MOC1 are part of complex regulatory networks that involve various hormonal pathways, including gibberellins, auxins, and cytokinins (Bhuvaneswari et al., 2020). These hormones interact to balance stem elongation and tiller formation. SD1 regulates gibberellin levels, which influence not only plant height but also the activation of tiller buds modulated by MOC1. This interplay ensures that the rice plant maintains an optimal architecture for maximizing yield. Understanding these regulatory networks is essential for manipulating these genes in breeding programs to achieve desired plant traits.

3.2 Combined impact on plant architecture

The combined action of SD1 and MOC1 genes results in a balanced rice plant architecture that maximizes yield potential. SD1’s role in reducing plant height and enhancing stem strength complements MOC1’s ability to increase tillering. This balance is critical; while increased tillering can enhance grain production, it also requires sturdy stems to support the additional biomass and prevent lodging. The interaction between these genes ensures that the plant can sustain increased tiller numbers without compromising structural integrity (Jia et al., 2020).

SD1 and MOC1 together contribute to an integrated plant morphology that optimizes light capture, nutrient use, and overall plant fitness. The semi-dwarf stature conferred by SD1 allows for more efficient light penetration to lower leaves, while the increased tillering promoted by MOC1 ensures a fuller canopy that can maximize photosynthetic capacity. This integration of traits leads to a more efficient use of resources and enhances the plant’s ability to thrive in diverse environments.

3.3 Combined contributions to yield

The synergistic effects of SD1 and MOC1 on plant architecture directly contribute to maximizing grain production. SD1 improves stem strength and reduces lodging, ensuring that the plant can support a higher grain load. Meanwhile, MOC1 increases the number of tillers and panicles, leading to more sites for grain development. This combination enhances the overall grain yield per plant, making these genes valuable targets in rice breeding programs aimed at improving productivity.

Beyond grain production, the interaction between SD1 and MOC1 enhances overall plant fitness (Figure 3) (Zhang et al., 2021). The optimized plant architecture resulting from these genes improves resource allocation, water use efficiency, and nutrient uptake. This holistic improvement in plant performance ensures that rice varieties can maintain high yields under a range of environmental conditions, including stress factors such as drought or nutrient deficiency. By enhancing overall plant fitness, SD1 and MOC1 contribute to the development of resilient rice varieties that can adapt to changing agricultural landscapes and climate conditions.

The interaction between SD1 and MOC1 genes plays a vital role in shaping rice plant architecture and enhancing yield. Their synergistic effects on plant growth, regulatory networks, and combined impact on plant morphology and yield underscore their importance in rice breeding strategies. Understanding and leveraging these interactions can lead to the development of high-yielding, resilient rice varieties that are crucial for global food security.

4 Breeding Strategies for Enhancing SD1 and MOC1

4.1 Traditional breeding approaches

Traditional breeding approaches for enhancing SD1 and MOC1 in rice primarily involve selection and hybridization techniques. Selection involves identifying and choosing plants with desirable traits, such as optimal plant height and improved yield, and using them as parents for the next generation. Hybridization, on the other hand, involves crossing different varieties to combine favorable traits from both parents. For instance, the study by (Zhang et al., 2020) demonstrated the effectiveness of selecting elite SD1 alleles to achieve ideal plant height and lodging resistance. The research highlighted the importance of allele variation in SD1, which was a major factor for plant height segregation in breeding populations from an indica and japonica cross. The study also identified the potential value of the SD1Jap allele for hybrid rice breeding, suggesting its compatibility with some recently developed hybrid varieties in China.

Mutation breeding is another traditional approach that involves inducing mutations to create genetic variability, which can then be exploited to develop new varieties with desirable traits. This method can be particularly useful for enhancing traits controlled by single genes, such as SD1. The study by (San et al., 2020) explored the role of the SD1 gene in regulating leaf inclination angle, which is crucial for improving canopy photosynthesis and dry matter production. By manipulating the SD1 gene, researchers were able to enhance light penetration into the canopy, thereby improving rice yield through better lodging resistance.

4.2 Modern genetic techniques

Marker-assisted selection (MAS) is a modern genetic technique that uses molecular markers to select plants with desirable traits more efficiently. This method allows for the precise identification of specific alleles associated with traits like plant height and yield. The study by (Zhang et al., 2020) utilized newly developed functional markers to characterize different SD1 alleles, demonstrating high efficiency for genotyping a large set of germplasms. This approach can significantly accelerate the breeding process by enabling the early selection of plants with favorable SD1 alleles.

CRISPR and other gene-editing technologies offer powerful tools for directly modifying the genetic makeup of plants to enhance traits like those controlled by SD1 and MOC1. These techniques allow for precise edits to be made to the DNA, enabling the creation of new varieties with improved characteristics. Although the provided studies did not specifically mention the use of CRISPR for SD1 and MOC1, the potential for such technologies in rice breeding is immense, given their ability to target and modify specific genes with high precision.

4.3 Integrating SD1 and MOC1 in breeding programs

Successful integration of SD1 and MOC1 in breeding programs can be illustrated through various case studies. For example, the study by (Zhang et al., 2020) described a breeding practice where the null SD1 allele was introgressed into the elite japonica variety Daohuaxiang, which suffers from severe lodging. The improved semi-dwarf line demonstrated enhanced agronomic performance, showcasing the practical application of SD1 alleles in breeding programs. Similarly, the study by (San et al., 2020) highlighted the use of SD1 mutants to improve leaf inclination angle and canopy photosynthesis, further emphasizing the role of SD1 in successful breeding strategies.

While there are significant opportunities for enhancing rice plant architecture and yield through the manipulation of SD1 and MOC1, several challenges remain. One major challenge is the complex epistatic interactions between different alleles and genetic backgrounds, as noted in the study by (Zhang et al., 2020). Additionally, the integration of modern genetic techniques like CRISPR requires careful consideration of regulatory and ethical issues. However, the potential benefits, such as improved yield, lodging resistance, and canopy photosynthesis, present substantial opportunities for future research and breeding programs.

In conclusion, both traditional and modern breeding strategies offer valuable approaches for enhancing SD1 and MOC1 in rice. By leveraging techniques like selection, hybridization, marker-assisted selection, and gene editing, researchers can develop new rice varieties with improved plant architecture and yield, addressing the growing demand for food security.

5 Case Studies and Practical Applications

5.1 Successful varieties featuring SD1 and MOC1

The identification and utilization of the SD1 gene have been pivotal in the development of high-yielding rice varieties. The semi-dwarf phenotype conferred by the SD1 allele has been instrumental in creating cultivars with increased lodging resistance, allowing for higher nitrogen fertilizer application and subsequently higher yields. Notable examples include Guang-chang-ai and IR8, which were among the first high-yielding varieties to capitalize on the semi-dwarf trait, significantly improving rice yields in China and other regions (Peng et al., 2021). Additionally, the fine-tuning of SPL gene expression has been shown to enhance panicle branching and grain number, further contributing to yield improvements in rice (Wang and Zhang, 2017).

The MOC1 gene, in conjunction with MOC3, plays a crucial role in regulating tiller bud outgrowth, which is essential for optimal plant architecture and yield. The interaction between MOC1 and MOC3 upregulates the expression of FON1, promoting tiller formation and enhancing overall plant resilience (Figure 4) (Shao et al., 2019). Furthermore, modifications in root system architecture, such as those involving the DRO1 homolog, have been shown to improve rice yields in saline paddy fields by enabling plants to avoid stress conditions, thereby enhancing their resilience and productivity (Kitomi et al., 2020).

5.2 Field performance and agronomic benefits

Field trials have consistently demonstrated the agronomic benefits of varieties featuring the SD1 and MOC1 genes. For instance, varieties carrying the SD1 allele have shown significant improvements in yield due to their semi-dwarf stature, which reduces lodging and allows for higher fertilizer application (Peng et al., 2021). Similarly, the cooperative action of MOC1 and MOC3 in promoting tiller bud outgrowth has been linked to increased tiller numbers and improved yield performance in various field conditions (Shao et al., 2019). These genetic modifications have been validated through extensive yield trials, confirming their effectiveness in enhancing rice productivity.

The adoption of high-yielding and stress-resilient rice varieties by farmers has been widespread, driven by the tangible benefits observed in field performance. Farmers have reported increased yields and improved crop resilience, particularly in regions prone to abiotic stresses such as salinity. The success of varieties featuring the SD1 gene, such as IR8, has been well-documented, with farmers noting the advantages of reduced lodging and higher fertilizer efficiency (Peng et al., 2021). Additionally, the practical applications of MOC1 and MOC3 in enhancing tiller formation have garnered positive feedback from farmers, who have observed better plant architecture and higher yields (Shao et al., 2019). The integration of these genetic advancements into breeding programs continues to support sustainable agricultural practices and food security.

6 Future Prospects and Challenges

6.1 Emerging research on SD1 and MOC1

Recent studies have significantly advanced our understanding of the roles of SD1 and MOC1 in rice plant architecture and yield enhancement. The restoration of wild-type SD1 in modern semi-dwarf cultivars has shown promising results, nearly doubling plant height and increasing total grain yield per panicle (Jia et al., 2020). Additionally, the identification and utilization of various SD1 alleles, such as SD1Jap and SD1Ind, have demonstrated their potential in breeding programs to optimize plant height and improve yield (Zhang et al., 2020a). The discovery of novel alleles like GNP6 of MOC1, which regulates panicle and tiller development, further enriches the genetic resources available for rice breeders (Zhang et al., 2020b).

6.2 Potential for further yield enhancement

The potential for further yield enhancement lies in the fine-tuning of gene expression and the strategic combination of beneficial alleles. For instance, the manipulation of SPL gene expression has been shown to optimize plant architecture, thereby increasing grain number (Wang and Zhang, 2017). Combining alleles such as sdt and SD1 has resulted in a significant yield increase, suggesting that pyramiding multiple beneficial alleles could be a viable strategy for future breeding programs (Zhao et al., 2015). Moreover, the weak functional allele SD1-EQ from japonica rice has been identified as a valuable genetic resource for improving indica rice lines, indicating the potential for cross-subspecies breeding to enhance yield (Yu et al., 2020).

6.3 Addressing climate change and sustainability

Addressing climate change and sustainability is crucial for the future of rice cultivation. The development of rice varieties with improved lodging resistance and nitrogen utilization, as conferred by the SD1 allele, is essential for maintaining high yields under varying environmental conditions (Peng et al., 2021). Additionally, understanding the molecular basis of plant architecture and grain quality can lead to the development of elite varieties that are both high-yielding and resilient to climate change (Chen et al., 2018). The integration of modern biotechnological tools, such as CRISPR/Cas9, can accelerate the development of climate-resilient rice varieties (Chen et al., 2018).

6.4 Policy and regulatory considerations

The successful implementation of advanced breeding techniques and the introduction of new rice varieties require supportive policy and regulatory frameworks. Policymakers must ensure that regulations facilitate the adoption of genetically modified and genome-edited crops while addressing public concerns about food safety and environmental impact. Furthermore, international collaboration and knowledge sharing are essential to maximize the benefits of research advancements in SD1 and MOC1 for global food security. Encouraging public and private sector partnerships can also drive innovation and investment in sustainable rice production practices. The emerging research on SD1 and MOC1, combined with advanced breeding strategies and supportive policies, holds great promise for enhancing rice yield and sustainability in the face of global challenges.

7 Concluding Remarks

The role of the SD1 and MOC1 genes in rice plant architecture and yield enhancement has been extensively studied, revealing significant insights into their functions and applications. The SD1 gene, encoding the GA20ox oxidase involved in gibberellin biosynthesis, has been pivotal in the development of semi-dwarf rice varieties that contributed to the Green Revolution. Restoration of the wild-type SD1 gene in modern semi-dwarf cultivars has been shown to nearly double plant height, increase total grain yield per panicle, and elongate the second-leaf length. This restoration also affects gene expression profiles, particularly in the gibberellin pathway and related metabolic networks, defense responses, and catabolic processes. The identification and utilization of SD1 mutants have significantly improved rice yields by conferring lodging resistance and enabling high nitrogen fertilizer use.

Future research should focus on further elucidating the molecular mechanisms by which SD1 and MOC1 influence plant architecture and yield. This includes exploring the pleiotropic effects of these genes and their interactions with other metabolic and defense pathways. Additionally, the identification of new alleles of SD1 through mutagenesis and genome editing holds promise for enhancing the genetic diversity and resource base for semi-dwarf breeding programs. Investigating the balance between gibberellin and jasmonic acid pathways in relation to SD1 could provide deeper insights into optimizing growth and defense responses in rice.

The advancements in understanding the roles of SD1 and MOC1 in rice have not only contributed to historical yield improvements but also offer pathways for future innovations in rice breeding. By leveraging genetic insights and modern biotechnological tools, it is possible to develop new rice varieties that combine high yield, robust growth, and resilience to environmental stresses. Continued research in this area will be crucial for meeting the global food demand and ensuring sustainable agricultural practices.

Funding

Science and Technology Project of State Administration of Science, Technology and Industry for National Defense, Seed Innovation and Industrialization Project of Fujian Province (zycxny2021003) Science and Technology Innovation Special Fund Project of Fujian Agriculture and Forestry University (KFB23198).

Acknowledgments
The CropSci Publisher appreciates the feedback from two anonymous peer reviewers on the manuscript of this study,  whose careful evaluation and constructive suggestions have contributed to the improvement of the manuscript.

Conflict of Interest Disclosure

The author affirms 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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