Molecular Plant Breeding 2025, Vol.16, No.1, 93-104 http://genbreedpublisher.com/index.php/mpb 99 conditions (Ertiro et al., 2022). Therefore, breeding programs must consider both genetic and environmental factors to achieve a balance between yield and protein enhancement. 6.2 Genetic diversity and maintaining resistance to diseases Maintaining genetic diversity while enhancing protein content is another significant challenge in maize breeding. Genetic diversity is crucial for the resilience of maize varieties against diseases and environmental stresses. However, the focus on specific traits like high protein content can lead to a reduction in genetic diversity, making the crops more susceptible to diseases (Abu et al., 2021; Amegbor et al., 2022). For example, breeding programs that concentrate on high-lysine inbreds may inadvertently reduce the genetic pool, limiting the ability to introduce new traits such as disease resistance. Additionally, the genetic loci associated with high protein content must be carefully managed to ensure that they do not compromise other important traits. Studies have identified multiple genetic loci that control protein synthesis factors, but these loci must be integrated into breeding programs without negatively affecting disease resistance or other agronomic traits (Tandzi et al., 2017; Sethi et al., 2023). Therefore, maintaining a broad genetic base while selecting for high protein content is essential for the long-term success of breeding programs. 6.3 Cost and scalability of breeding programs The cost and scalability of breeding programs present another set of challenges. Advanced breeding techniques, such as molecular breeding and genome editing, offer the potential to accelerate the development of high-protein maize varieties. However, these technologies are often expensive and may not be accessible to breeding programs in developing countries (Cooper and Messina, 2022; Tandzi et al., 2017). The high cost of these technologies can limit their widespread adoption, making it difficult to scale up breeding efforts to meet global demand. Furthermore, the scalability of breeding programs is also affected by the need for extensive field trials and evaluations. Breeding high-protein maize varieties requires rigorous testing across multiple environments to ensure that the new varieties perform well under different conditions. This process is time-consuming and resource-intensive, adding to the overall cost of breeding programs (Jaradat and Goldstein, 2013; Amegbor et al., 2022b). Therefore, finding cost-effective and scalable solutions is crucial for the successful development and dissemination of high-protein maize varieties. 7 Case Study Results 7.1 Summary of protein content improvements achieved The breeding of maize varieties with high protein content has shown significant advancements. For instance, the introgression of the opaque16 (o16) gene into opaque2 (o2)-based parental inbreds resulted in hybrids with up to 91% more tryptophan and 76% more lysine compared to their recurrent parents (Sarika et al., 2018; Wang et al., 2024). This demonstrates a substantial improvement in the protein quality of maize, particularly in terms of essential amino acids. Additionally, Quality Protein Maize (QPM) lines have been developed to enhance the nutritional value of maize. These QPM lines, although having a slightly lower overall protein content (7% lower) than non-QPM lines, exhibit a higher quality index due to increased levels of tryptophan and lysine (Amegbor et al., 2022a). This indicates that the focus has been on improving the quality rather than the quantity of protein. 7.2 Yield outcomes compared to non-bred varieties The yield outcomes of high-protein maize varieties have been mixed. Some studies have reported that QPM hybrids tend to yield less than commercial checks. For example, QPM hybrids evaluated in the southern U.S. environments yielded less than commercial checks, although they showed significant improvements in agronomic and kernel-quality traits (Bhatnagar et al., 2004). This suggests a trade-off between yield and protein quality in some cases. However, other studies have shown that it is possible to achieve high yields along with improved protein content. For instance, the reconstituted hybrids with the o16 gene showed similar grain yield and attributing traits to their original versions, indicating that high protein content can be achieved without compromising yield (Sarika et al., 2018). This highlights the potential for breeding strategies that do not sacrifice yield for nutritional quality.
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