1 Introduction

It is widely recognized that the supply of fossil fuels has obvious limitations (Zhou and Yan, 2024). The development of biofuels, especially the production of bioethanol, has received much attention from researchers in recent years. This renewable energy has a dual benefit, which can not only ease the tight supply and demand situation of oil resources, but also reduce greenhouse gas emissions. The various biomass raw materials are converted through the microbial fermentation process, and the final product is an alcoholic substance that can be used as a fuel. A sustainable alternative to traditional fossil fuels is thus formed. This fuel can be used in combination with gasoline or alone, reducing carbon emissions while improving energy security (Byrt et al., 2011). The balance between economic growth and environmental impact is well addressed by the potential shown by bioethanol.

Maize (Zea mays L.) is widely cultivated around the world and is the crop with the highest global production. Its high starch content makes it the preferred raw material for bioethanol production. The largest component of maize kernels is starch, accounting for about 70% to 80% of the dry weight. This property makes it particularly suitable for fermentation to produce bioethanol (Semenčenko et al., 2015). maize biomass, including straw, provides an important source of raw materials for second-generation biofuels such as cellulosic ethanol, which shows that the alleviation of food security issues has been achieved (Infante et al., 2018). The dual functional characteristics of grain production and biomass supply have significantly increased the application value of maize as an energy crop (Munaiz et al., 2021).

Despite these significant advantages, the optimization of maize varieties for bioethanol production still faces multiple challenges. The main obstacle is the recalcitrant nature of lignocellulosic biomass (Torres et al., 2015; Choudhary et al., 2019), which severely limits its conversion efficiency into fermentable sugars. It is crucial to increase the level of genetic diversity through the implementation of specialized breeding programs, which involves the improvement of key traits such as biomass output, cell wall digestion efficiency, and lignin composition content (Voorend et al., 2015). The trade-off between food output and biomass quality also needs to be considered, as examples show that the environmental and economic impacts of maize cultivation must be carefully managed (Slegers et al., 2017).

This study is dedicated to the development of high-fiber maize varieties, with the ultimate goal of adaptability to bioethanol production, and is centered around the problem of lignocellulosic biomass conversion. The core task is to focus on the screening of maize genotypes with higher biomass output characteristics and better quality, while the improvement of cell wall component digestion efficiency is also taken into consideration. The dual value of these genetically improved maize lines in terms of environmental friendliness and economic benefits needs to be systematically evaluated. The comprehensive use of modern genetic genomics technology and phenotypic genomics analysis methods shows that this study plays an important role in promoting the sustainable preparation of biofuels. The realization of the strategic goal of renewable energy will thus obtain new crop genetic resources to support it.

2 High-Fiber Maize

2.1 Definition and characteristics of high-fiber maize

Maize strains that have been specially selected or genetically modified have significantly increased fiber content in their biomass, which is the definition of high-fiber maize. The main substances that make up the cell wall are composed of hemicellulose, lignin, and cellulose. The increase in maize fiber content can be achieved by increasing the concentration of cellulose. Neutral detergent fiber (NDF) and acid detergent fiber (ADF), as core indicators for evaluating fiber composition, are usually measured at higher values in these strains (Choudhary et al., 2019). The genetic differences between local germplasm resources and hybrid combinations provide a rich genetic material basis for fiber-enhanced breeding projects (Munaiz et al., 2021).

2.2 Role of fiber content in enhancing bioethanol yield and production efficiency

Due to the development of second-generation biofuels based on lignocellulose, corn with high fiber content is needed as raw material in the bioethanol industry. This plant material containing a high proportion of polymers such as cellulose and hemicellulose is conducive to the subsequent cellulase hydrolysis process, can provide more abundant fermentable monosaccharides, and then converted into ethanol; at the same time, the higher the proportion of cellulose in the total dry weight, the more conducive to reducing costs. Corn kernel fiber is mainly composed of hemicellulose (about 40%), and its complex and dense sugar components and structure lead to its strong resistance to degradation. Therefore, through systematic genetic improvement, the degree of corn fiberization can be improved and the content of cellulose and hemicellulose can be increased, significantly improving ethanol yield (Slegers et al., 2017); in addition, obtaining cellulosic materials from the discarded part of the crop after harvest does not occupy limited food resources, which to a certain extent alleviates the global food security problem (Semenčenko et al., 2015).

2.3 Potential advantages of high-fiber maize in sustainable agriculture

The application of high-fiber corn has many advantages for sustainable agriculture. First, it can make full use of the two products of corn plants, namely, grains and stalks (Chen et al., 2017; Skoufogianni et al., 2019), so that the entire crop can obtain the maximum economic benefits; second, fermenting high-fiber corn as a raw material to produce ethanol can effectively reduce the greenhouse effect by developing new biomass ethanol fuels that can replace petrochemical products such as gasoline and diesel (Gao and Zhao, 2015); third, introducing high-fiber corn into the rotation system is conducive to increasing soil organic matter content and improving soil aggregate structure (Agegnehu et al., 2016); fourth, the application of high-fiber corn populations is conducive to the development of agriculture based on the bio-based market, promoting a more sustainable and diversified agricultural economy (Maitra and Singh, 2021).

3 Breeding Strategies for High-Fiber Maize

3.1 Traditional breeding methods for improving maize fiber content

Traditional breeding methods have long been used to improve various traits of maize, including fiber content, but the changes are not significant. Traditional breeding methods mainly rely on phenotypic selection, that is, plants that show ideal traits are selected and hybridized for multiple generations to obtain the target traits. The cumbersome identification and screening process of this method is time-consuming and inefficient, and may also lead to unsatisfactory breeding results. In Asia and other regions, traditional breeding has successfully improved maize yields and resistance to various stresses, and traditional breeding has been the cornerstone of agricultural development (Prasanna et al., 2010). However, the complexity of fiber traits affected by multiple genes poses a major challenge to traditional breeding methods.

3.2 Applications of molecular breeding techniques

Molecular breeding techniques, such as marker-assisted selection (MAS), have revolutionized maize improvement by marking the association between specific genes or genomic regions and target traits, improving breeding efficiency and accuracy. MAS involves the use of molecular markers associated with specific genes of interest, enabling breeders to select progeny lines with the help of molecular markers of target traits, thereby obtaining superior individual plants containing the target gene. This method has been successfully applied to improve various traits of maize, including disease resistance, insect resistance, drought resistance, waterlogging resistance, cold resistance, salinity resistance, and lodging resistance (Guan et al., 2015). For example, MAS has been used to introduce aflatoxin resistance genes into superior tropical maize lines, significantly accelerating the breeding process (Offornedo et al., 2022). Marker-assisted recurrent selection (MARS) has improved maize yield and stress resistance in tropical regions under arid and optimal conditions in sub-Saharan Africa, demonstrating the potential of MAS in improving complex traits such as fiber content (Beyene et al., 2016).

3.3 Role of genetic engineering in targeting fiber biosynthesis pathways

Genetic engineering provides a new route for directly changing the genetic composition of maize to increase fiber content. This method involves introducing foreign genes into recipient cells through in vitro recombination, so that the gene can be replicated, transcribed, translated and expressed in the recipient cells. Advances in genetic engineering, such as CRISPR/Cas9, make it possible to target and edit genes with high precision (Zhou and Hong, 2024). For example, the identification and manipulation of functional genes related to agronomic traits have paved the way for improving maize quality and yield (Ma et al., 2019). Genetic engineering can solve the problems encountered by traditional and molecular breeding methods by introducing new traits that are difficult to achieve by traditional means. This integrated approach can significantly improve the efficiency and effectiveness of breeding programs aimed at developing high-fiber maize varieties (Sethi et al., 2023).

4 Mechanisms Linking High Fiber to Bioethanol Production

4.1 Impact of cellulose and hemicellulose on ethanol yield

High-fiber maize is rich in cellulose and hemicellulose, but is wrapped in lignin. After pretreatment, the straw needs to be hydrolyzed to convert it into fermentable sugars to increase ethanol production. Cellulose and hemicellulose are key components of maize cell walls, and their decomposition releases fermentable sugars such as glucose, galactose, and xylose. Studies have shown that optimizing the fermentation process, such as through simultaneous saccharification and co-fermentation (SSCF), can increase ethanol titer. For example, by effectively utilizing 86.20% of cellulose and 82.99% of hemicellulose in pretreated maize cobs, Spathaspora passalidarum U1-58 was able to achieve an ethanol yield of 75.35% (Yu et al., 2017). The higher ethanol titer and yield demonstrate the potential of high-fiber maize as an effective raw material for bioethanol production.

4.2 Reduction in processing costs with high-fiber maize

The use of high-fiber maize can reduce the feedstock and processing costs associated with bioethanol production. The increase in the cellulose and hemicellulose content of the feedstock means that more fermentable sugars can be obtained per unit of biomass, thereby reducing the need for additional feedstock. Advances in genetic modification and breeding strategies have made it possible to develop maize varieties with improved cell wall characteristics that are easier to process. By modifying maize traits to optimize the pretreatment and hydrolysis processes, the energy and enzyme costs required for the decomposition of lignocellulosic biomass can be reduced (Torres et al., 2015). Therefore, high-fiber maize can not only increase ethanol production, but also make the production process more cost-effective.

4.3 Improvements in fermentation efficiency using high-fiber feedstock

High-fiber maize improves fermentation efficiency by providing a more stable and abundant supply of fermentable sugars. Genetically modifying maize to increase its cellulose and hemicellulose content has been the focus of recent research, aiming to accelerate the removal of lignocellulose in pre-fermentation treatments and improve processing convenience. Efficient fermentation processes, such as SSCF, have been optimized to effectively utilize fermentable sugars, resulting in higher ethanol titers and yields. For example, the SSCF process using Spathaspora passalidarum U1-58 achieved an ethanol titer of 53.24 g/L, demonstrating high fermentation efficiency when using high-fiber feedstocks. This highlights the potential of high-fiber maize to simplify the bioethanol production process, making it more efficient and higher yielding.

5 Case Study: Developing High-Fiber Maize for Bioethanol

5.1 Selection of target traits for high-fiber maize development

Developing high-fiber maize for bioethanol production requires the identification and selection of specific traits that increase biomass yield and fiber content, thereby improving maize morphology. Key traits include high straw yield, increased fiber content, and good fiber composition. Studies have shown that the genetic diversity of maize germplasm can be used to improve these traits. For example, European landraces have a lower harvest index but higher fiber concentration, and have traits such as dense tolerance and lodging resistance, making them suitable candidates for breeding programs aimed at improving the quality of residues for bioethanol production (Munaiz et al., 2021). Identification of quantitative trait loci (QTLs) associated with grain quality and yield-related traits can also help in the selection of high-fiber maize varieties (Figure 1) (Sethi et al., 2023).

5.2 Integration of breeding techniques in pilot programs

In order to effectively integrate breeding technologies, it is imperative to combine traditional and modern methods. Multi-trait selection and genomic selection (GS) are particularly useful for the selection of varieties that require complex traits. By evaluating hybrids under different environments, multi-trait selection can improve multiple desirable traits simultaneously, such as grain yield and fiber content (Ruswandi et al., 2023). On the other hand, whole genome selection uses genotype data to predict and select individuals with target traits, thereby accelerating the breeding process. This approach has been successfully applied to tropical maize breeding programs. Tropical maize has the advantages of rich genetic diversity, strong resistance, and good green retention. Its excellent genes can be used to improve grain yield and other agronomic traits (Beyene et al., 2021). High-throughput phenotyping methods, such as near-infrared spectroscopy (NIRS), can also be used to more effectively evaluate complex traits such as crop growth and yield, and provide timely feedback on various stress conditions encountered by crops (Cabrera-Bosquet et al., 2012).

5.3 Evaluation of the field performance and bioethanol yield

Evaluation of field performance and bioethanol yield involves multi-environment testing to identify superior hybrids by exploring the effects of the environment on the variety and the adaptability of the variety to different environments. This includes evaluating the adaptability of maize hybrids to different planting densities and environmental conditions. For example, newly developed maize hybrids show clear genotype-environment interactions, with some hybrids showing stable high yields under a variety of conditions (Figure 2) (Omar et al., 2022). Managed environment techniques can be used to simulate drought conditions and select plants that are more adaptable under drought stress, thereby selecting drought-tolerant hybrids that can maintain high biomass and fiber yields (Cooper et al., 2014). This approach has identified hybrids that not only perform well in the field but also produce high bioethanol yields from biomass.

6 Environmental and Economic Impacts

6.1 Reduction in greenhouse gas emissions from bioethanol production

Producing bioethanol from high-fiber maize can significantly reduce greenhouse gas (GHG) emissions compared to traditional fossil fuels, with maize ethanol reducing GHG emissions by approximately 12% to 20% compared to gasoline. Studies have shown that maize ethanol, especially when produced through sustainable practices such as double cropping with soybeans, can significantly reduce GHG emissions (9.3 to 13.2 million tons CO₂). For example, in Brazil, maize ethanol as a second crop to soybeans was found to reduce GHG emissions compared to gasoline and reduced land use emissions, highlighting the environmental benefits of this approach (Moreira et al., 2020). The use of biochar in maize cultivation has been shown to reduce nitrous oxide (N₂O) emissions, further reducing overall GHG emissions (Zhang et al., 2019).

6.2 Economic benefits for farmers cultivating high-fiber maize

The production of cellulosic ethanol from crop straw has increased farmers' enthusiasm for growing crops and their income from growing crops. Intensifying maize production, especially through sustainable practices, can increase yields and economic returns. One ton of cellulosic ethanol consumes five tons of straw, and each ton of straw can increase farmers' income by 300 yuan in the "collection, storage and transportation" stage. For example, small farmers in the North China Plain who adopted high-yield methods increased maize yields by 34.9% and economic benefits by 14.4% (Ren et al., 2020). In addition, the production of hybrid maize seeds in Northwest China showed that optimizing nitrogen fertilizer inputs and irrigation can reduce costs and increase yields, thereby improving the economic feasibility of maize cultivation (Liu et al., 2021a). These economic benefits encourage farmers to adopt sustainable practices and contribute to the bioethanol supply chain, optimize farmland management measures, and enable China to achieve intensive but sustainable agricultural production at a lower environmental cost.

6.3 Addressing sustainability concerns in bioethanol production

Sustainability is a key issue in bioethanol production, and the development of high-fiber maize can help achieve green, low-carbon sustainable development goals. Sustainable crop management practices, such as balanced fertilization and the use of biochar, can improve soil quality, promote maize production, and protect the environment (reduce greenhouse gas emissions) (Agegnehu et al., 2016). Optimizing the rotation mechanism of maize and soybeans can ensure sufficient feed while reducing nitrogen and carbon footprints. The CDSI of the bioethanol scenario has a strong carbon emission reduction capacity and contributes to more sustainable agricultural practices (Liu et al., 2021b). Using perennial biomass crops such as Silphium perfoliatum as an alternative to maize can also provide environmental benefits by reducing greenhouse gas emissions in the soil and providing multiple ecosystem services (Kemmann et al., 2021). Together, these strategies help address the sustainability issues associated with bioethanol production, making it a more viable and environmentally friendly alternative to fossil fuels.

7 Challenges and Limitations

7.1 Breeding trade-offs between high fiber and grain quality

Breeding high-fiber corn for increased bioethanol production often involves a trade-off with grain quality. The increased ethanol yield of high-fiber corn comes at a cost, often at the expense of lower grain quality and grain yield. For example, European landraces that exhibit fiber traits favorable for biofuels tend to have lower harvest indices compared to hybrids, suggesting that grain yield may suffer (Munaiz et al., 2021). There are exceptions, however, and the genetic diversity exhibited by dual-purpose corn (grain and biofuel) suggests that some types can sacrifice grain yield for fiber traits, so a balance needs to be maintained in breeding programs.

7.2 Economic and logistical barriers to adoption of high-fiber maize

There are significant economic and logistical challenges to adopting high-fiber maize for bioethanol production. The costs of developing and deploying new maize varieties with higher fiber content can be prohibitive, especially in developing regions with limited resources (Tandzi et al., 2017). The infrastructure required to process lignocellulosic biomass into bioethanol is not as extensive or developed as that for grain ethanol, which creates logistical barriers (Choudhary et al., 2019). Specialized equipment and facilities are required to handle and process high-fiber maize, which adds an economic burden and makes it less attractive to farmers and biofuel producers.

7.3 Regulatory and public perception challenges in deploying engineered maize

Deploying GM high-fiber maize varieties also faces regulatory and public perception challenges. Regulatory frameworks for genetically modified organisms (GMOs) vary by region, and some countries have implemented strict regulations that may delay or prevent the adoption of engineered crops (Infante et al., 2018). Public perception of GMOs remains a controversial issue, with concerns about safety, environmental impacts, and ethical considerations influencing acceptance. These challenges require comprehensive regulatory strategies and public engagement efforts to ensure the successful deployment of high-fiber maize for bioethanol production.

8 Future Perspectives

8.1 Advances in genome editing for next-generation biofuel crops

With the application and development of genome editing tools such as CRISPR/Cas9, the use of these genome editing tools to precisely modify the corn genome to achieve the goal of improving its biomass yield and digestibility will bring great benefits to the development of high-fiber corn for bioethanol production. Reducing the accumulation of lignin in corn plants through genetic engineering technology has been proven to effectively improve the efficiency of the bioethanol production process (Xie and Peng, 2011; Choudhary et al., 2019), that is, by making the cellulosic biomass in plant materials more accessible to cellulose-degrading bacteria. In addition, by locating the functional sites related to biomass contribution and obtaining the functional regions closely associated with them, molecular marker-based selective methods can be further promoted, thereby accelerating the breeding process of improved new corn varieties suitable for biofuel production (Andorf et al., 2019).

8.2 Potential of integrating bioethanol production with other renewable energy sources

The integration of bioethanol production with multiple renewable energy strategies opens up a promising new path for building a more sustainable and resilient energy system. For example, the comprehensive utilization of corn biomass resources to produce bioethanol and biogas simultaneously can effectively improve the energy output efficiency of a single crop resource while making full use of grain residues and lignocellulosic materials (Skoufogianni et al., 2019). Recent advances in biorefining technology have enabled corn biomass to be converted into a variety of bio-based products, covering biofuels, bioplastics, and specialty chemicals, which has not only greatly enriched the application scope of corn, but also significantly enhanced the economic attractiveness of bioethanol production (Maitra and Singh, 2021). Such integrated strategies not only optimize energy efficiency, but also effectively reduce the environmental burden of biofuel production.

8.3 Collaborative efforts for scaling high-fiber maize production globally

In order to promote high-fiber corn for bioethanol production worldwide, close collaboration between researchers, policymakers, and industry stakeholders is urgently needed. Through the implementation of international breeding programs, we can take full advantage of the genetic differences in corn germplasm from all over the world and cultivate high-quality varieties adapted to specific climate and agricultural conditions (Torres et al., 2015; Munaiz et al., 2021). At the same time, building partnerships between the public and private sectors will accelerate the dissemination of advanced breeding technologies and best management practices, ensuring that these new high-fiber corn varieties can be accessed by farmers around the world. Collaborative research initiatives should also focus on optimizing agricultural management practices and improving bioprocessing technologies to comprehensively enhance the efficiency and sustainability of bioethanol production (Semenčenko et al., 2015).

Acknowledgments

We are grateful to Mrs. Xuan for critically reading the manuscript and providing valuable feedback that improved the clarity of the text.

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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