1 Introduction

When it comes to corn (Zea mays L.), almost no one will deny its position in global agriculture. Whether it is for food, breeding, or for power generation, corn is almost everywhere. Its diverse uses also make it a cornerstone of global food security, especially in the context of a growing population. In fact, corn itself has very rich genetic resources. Height, growth cycle, yield performance. These have various forms in different germplasm materials.

However, "many varieties" alone are far from enough. When the climate begins to become extreme, such as droughts, heat waves, and continuous pests and diseases, traditional breeding ideas will soon be unable to keep up. At this time, the importance of genetic diversity is truly revealed. It is not just "good-looking", but the key to whether it can stabilize yields in adverse environments.

Having said that, in reality, there are still many difficulties in corn breeding. An old problem has not been completely solved-although there are many germplasm resources, only a small part of them can be used in breeding. Especially for those tropical corn resources, it is not only difficult to adapt them to temperate planting environments, but also very time-consuming and labor-intensive. In contrast, the private sector has made impressive achievements in temperate corn breeding, but is still lagging behind in the tropics. This article not only talks about the problem, but also hopes to bring the focus back to germplasm resources. We start with the resources themselves, see what problems are currently blocking progress, and then propose some possible solutions, such as introducing genome editing technologies such as CRISPR-Cas, or using molecular markers to speed up the selection process. The goal is simple: to make breeding more efficient, especially in the face of increasing environmental pressures today.

2 Overview of Corn Germplasm Resources

2.1 Types of corn germplasm

The genetic resources of corn are actually very rich, and it is not possible to describe them in just one or two ways. Local varieties cultivated by farmers for generations are the earliest type. Although they do not look as "advanced" as modern varieties, they are often more adapted to local climates and soils and have more diverse genetic traits (Nelimor et al., 2020). Some wild relatives such as teosinte, although not used much in agriculture, have become indispensable "raw materials" in research and breeding because they retain the early genetic characteristics of corn (Flint-Garcia et al., 2009). As for the improved varieties that are widely used now, most of them are the result of continuous screening by the breeding team under established goals, such as increasing yields, disease resistance and drought resistance (Nyoni et al., 2023). However, over-reliance on such improved varieties can easily make genetic diversity monotonous.

2.2 Global collection and conservation efforts

In fact, many people are not aware of how complicated the preservation and collection of corn resources are. Since the mid-20th century, many research institutions have realized that preserving these diverse germplasm resources is a kind of "insurance" for future agricultural security. Organizations such as the International Maize and Wheat Improvement Center (CIMMYT) not only collect local varieties, but also wild species and modern varieties. There is more than one way to preserve them - seed banks are the most familiar form, but it is equally important to keep varieties in their original habitats (the so-called "in situ conservation"), because this allows them to continue to evolve naturally and adapt to changing environments (Rajpal et al., 2023). These two methods have their own uses, one is stable and the other is active, and they cannot replace each other (Yu et al., 2020).

2.3 Genetic diversity and its role in breeding

If breeding only relies on a few existing improved varieties, it is actually difficult to make a breakthrough. The truly promising genetic variation is often hidden in those less noticed local varieties and wild species (Nelimor et al., 2020). For example, some local varieties may be naturally drought-resistant or disease-resistant, which happen to be lacking in modern high-yield varieties. Such genetic resources are actually of great significance in expanding the genetic basis of breeding materials (Rajpal et al., 2023). Now the technology has also caught up, such as double haploid technology, genome typing and other methods, which can make these diverse "resources" used more quickly, improve breeding efficiency, and also help to breed new varieties with stronger adaptability and more stable performance (Prasanna, 2010; Wu and Li, 2024).

3 Progress in Characterization and Evaluation of Germplasm Resources

3.1 Use of molecular technology in characterization of germplasm resources

The characterization of corn germplasm resources is no longer limited to traditional methods. The addition of molecular technology, especially SNP genotyping, has given us a more detailed means to see the genetic differences and population structure between varieties. In a study of the National Maize Inbred Line Germplasm Bank in the United States, more than 680,000 SNP markers were identified through sequencing (Romay et al., 2013). These data not only allowed researchers to see the genetic distance between different germplasms, but also exposed obvious population stratification.

Of course, SNP chips are also constantly being upgraded. The launch of Maize6H-60K chip and 55K chip has greatly improved the genome coverage and genotyping efficiency. Especially when dealing with rare variants in tropical germplasm, the latter performs more prominently (Xu et al., 2017). This type of chip not only helps identify unique alleles, but also provides considerable convenience in the construction of germplasm fingerprints and assisted selection breeding (Jones et al., 2007; Lu et al., 2009).

If SNP technology has changed the "clarity" of germplasm identification, then GWAS (genome-wide association analysis) has rewritten the way we look for trait genes. Its usefulness has become increasingly evident in recent years - more than once, it has revealed gene regions related to agronomic traits such as corn flowering period, seed coat color and even sweetness (Romay et al., 2013). Especially after combining with high-density SNP chips, GWAS can more accurately locate quantitative trait loci (QTL), providing a more powerful reference for variety improvement (Xu et al., 2017; Tian et al., 2020).

It is worth mentioning that next-generation sequencing (NGS) brings another perspective to observe genetic diversity. Analysis of 265 maize inbred lines by genotyping sequencing (GBS) revealed significant population structure among breeding materials (Ertiro et al., 2017). Compared with traditional typing methods, NGS is more efficient in marker development and high-density genetic map construction, which is necessary for marker-assisted selection (Jones et al., 2007; Chen et al., 2016).

3.2 Phenotypic evaluation and trait assessment

Not all germplasms can perform well in adversity, so phenotypic evaluation is particularly important. Faced with stresses such as drought and disease, breeders are more concerned about who can "bear it". For example, the drought resistance-related markers included in the 55K SNP chip are very valuable for maize cultivation in water-stressed areas (Xu et al., 2017). On the other hand, the double haploid lines in the Iowa BSSS population also showed good stress resistance potential, and the changes in their allele frequencies indicated that selection pressure was quietly taking effect (Ledesma et al., 2023). Of course, stress resistance is not the only breeding goal, and yield and quality are the results most often pursued by breeders. The Maize6H-60K chip is not only useful in identifying stress resistance genes, but also plays a key role in the study of the genetic basis of yield-related traits (Tian et al., 2020). For commercialization, sometimes a hybrid combination with stable high yield is more popular than a "drought-resistant" variety. SNP-based trait positioning has helped to select many excellent lines (Jones et al., 2007). However, "invisible characteristics" such as nutrition and quality have also begun to be valued. For example, SNPs related to seed oil synthesis have been identified, providing new clues for improving the nutritional content of corn (Xu et al., 2017). For example, the study of inbred lines in the mid-altitude humid ecological zone of Ethiopia also found unique genetic variation, which has the potential to be used for improving nutritional quality (Chen et al., 2016). The ultimate goal of this kind of evaluation of germplasm is to serve the needs of consumer taste and the food industry.

4 Multiple Ways to Use Maize Germplasm Resources in Breeding

In maize breeding, the role of germplasm resources cannot be overemphasized. However, there is often more than one way to use these resources. In addition to using them directly, the key is how to combine, how to select, and how to introduce them. This section will discuss the use of these resources from several common but not completely overlapping perspectives - including pre-breeding, development of new breeding lines, hybridization strategies, and some molecular methods that have been popular in recent years.

4.1 About pre-breeding and introduction of wild traits

When it comes to expanding the genetic base, people's first reaction may not be "pre-breeding", but it is indeed an inconspicuous but critical link. Many times, some wild relatives or local old varieties have useful traits, but they are not easy to use directly in modern breeding materials. This requires "pre-breeding" to make a transition. For example, the SeeD project did a thorough job. They used genomic selection methods to discover those multi-gene-controlled traits from local varieties and introduced them step by step into core breeding materials. In the final analysis, even resources that look "wild" may become part of future excellent varieties under the correct screening and introduction.

4.2 How to develop diverse germplasm into good breeding materials

Not all "diversity" can be directly realized. The value of diverse germplasm depends on whether you can transform it into a usable breeding line. This step requires strategy, such as which traits are worth retaining and which can be accurately introduced with the help of markers. Interestingly, the successful development of drought-resistant hybrids in the American Corn Belt is a typical example. The combination of drought-resistant traits is very complex, and they have used many methods, such as predicting breeding value through phenotypic and genetic information, and even using molecular markers to lock in key drought-resistant traits. The hybrids that were finally bred performed very well under drought conditions, which is inseparable from the clever integration of these "diverse germplasm" resources.

4.3 Hybridization and hybrid vigor: not a simple addition problem

Hybrid vigor sounds like a "strong combination", but the science behind it is not so straightforward. The good performance of hybrid offspring depends on the complementarity between different germplasms. You can't just pick two parents and expect their offspring to have a good harvest. A study involving 724 tropical and temperate inbred lines found that some combinations, such as Reid × SPT and Lancaster×LRC, performed well in terms of yield (Yu et al., 2020). This actually illustrates a problem: tropical germplasm is not only used in tropical areas. With the right combination, they can also help temperate varieties improve their performance. In other words, the hybridization strategy requires vision, not just pairing.

4.4 About genomic selection and marker-assisted breeding

Traditional seed selection relies on vision, while modern technology relies on computing power. Genomic selection (GS) is a typical example. It uses whole genome information to predict the genetic potential of a material and then selects seeds based on this (Crossa et al., 2017). This method is particularly suitable for multi-trait and multi-gene targets, and the effect is more obvious when combined with high-throughput phenotyping technology.

As for marker-assisted selection (MAS), it is no longer a new thing. For example, the work done by Bouchez et al. in 2002 used MAS to introduce favorable genes related to early maturity and high yield into breeding lines. Although this method is sophisticated, it requires high accuracy in the positioning of QTL (quantitative trait loci). In a 2007 study, Ribaut and Ragot further demonstrated that the yield of tropical corn was significantly improved after the introduction of favorable alleles in five key areas under drought conditions. It can be said that these methods have accelerated breeding and improved accuracy.

5 Case Study: Finding Solutions to Fall Armyworm from Germplasm Resources

5.1 Why pay special attention to fall armyworm?

Most people who grow corn have heard of the fall armyworm (Spodoptera frugiperda), especially corn farmers in Africa and Asia - it is not a new problem. It was first discovered in West and Central Africa in 2016, and then it quickly spread to many regions, causing serious damage to corn fields (Goergen et al., 2016; Wan et al., 2021). It is highly adaptable, reproduces quickly, and is easily resistant to pesticides, which makes prevention and control work more complicated.

Chemical methods are not very reliable, and breeding has become a necessary way. Especially for small farmers with limited resources, they do not have much budget to use pesticides frequently, and they need varieties that can "defend themselves" (Figure 1) (Assefa et al., 2019; Baudron et al., 2019). Although no variety can completely resist the fall armyworm, reducing losses through resistance breeding is a realistic and sustainable direction.

5.2 How to select corn resistant to fall armyworm?

Finding germplasm resources sounds like a very academic thing, but it can actually be understood as "picking out those that can withstand insects from a pile of corn materials." This process requires both testing in a controlled laboratory environment and observing in the field to see which type is less susceptible to insect damage. The judgment criteria are not complicated, such as the degree of leaf bites, the proportion of insect survival, the growth of the entire plant, etc.

Of course, not all results are determined solely by genetics. Studies have shown that simple field management measures such as frequent weeding or less tillage can actually help reduce insect damage (Baudron et al., 2019). In addition, pathogenic fungi such as Metarhizium rileyi, or some natural enemy insects, also play a role in helping to control pests (Assefa et al., 2019; Firake and Behere, 2020). So when selecting materials, these "external aids" should also be taken into account.

5.3 How to put resistance traits into good varieties?

Even if you find a few resistant materials, you can't plant them in the field immediately. Because they may be insect-resistant, but other traits may not be good. The next step is to "graft" these resistance characteristics onto high-yield and adaptable varieties through traditional breeding or biotechnology. This is usually achieved through repeated backcrossing, which sounds slow. In fact, with marker-assisted technology, breeders can track the transmission process of resistance genes faster.

It is worth mentioning that in addition to improving varieties, a method called "push-pull system" is also being explored. For example, use fake stinkweed to drive away pests, and then plant crops such as Brachi grass next to corn to attract pests away from the main crop. Combining these ideas with resistance breeding is expected to further improve overall insect resistance (Midega et al., 2018).

5.4 What are the impacts on global food and future research?

Once corn varieties can better resist fall armyworms, the most direct benefit is more stable yields, farmers use less pesticides, and the cost of farming will be reduced. This is particularly critical for high-risk areas such as sub-Saharan Africa and the Himalayas (Figure 2) (Singh et al., 2023).

But this is far from over. Subsequent research needs to clarify the genetic mechanism behind resistance, deeply understand the role of natural enemy systems, and explore how to use resistant varieties with various field management methods (Wan et al., 2021). To truly solve the problem, it is not enough to rely on scientists alone. Farmers' experience and policy coordination are equally important. These three aspects must work well together to truly make corn production more stable and long-term.

6 Challenges and Future Directions of Maize Germplasm Utilization

6.1 Stuck points in the utilization process

The utilization of maize germplasm resources has been a long-standing problem. Many people may have heard of the phenomenon of genetic erosion, which refers to the fact that the genetic diversity of maize is becoming narrower and narrower. This is difficult to avoid in breeding work, especially in the origin of maize, such as Mexico. Although farmers there still retain some local varieties, the protection efforts are obviously not enough, and it is becoming increasingly difficult to maintain diversity (Dyer et al., 2014).

Moreover, in order to pursue stability or high yield, many breeding projects repeatedly use certain excellent varieties, which further compresses the genetic resource base that was originally not abundant (Șuteu et al., 2013). This is like fishing in the same small pond over and over again. After a long time, the fish will naturally become fewer and fewer. If new varieties want to make breakthroughs, they are not strong enough and it is difficult to make differences.

In addition to genetic problems, there are also practical constraints. For example, funds.Compared with other short-term and fast-acting projects, the preservation and utilization of germplasm resources not only takes a long time to take effect, but also requires high investment, which makes it difficult for many places to continue to advance. This problem is not unique to a certain country. Many germplasm banks in developing countries are facing the dilemma of low utilization rate (Nass and Paterniani, 2000).

Another thing is the technical threshold. Some advanced tools, such as double haploid technology, are indeed effective, but the cost is too high, and small breeding units can't afford it, especially in countries with limited resources (Kleiber et al., 2012).

6.2 New opportunities for technical means

Fortunately, new technologies have emerged in recent years, which has opened up some space for the reuse of germplasm resources. Gene editing tools such as CRISPR-Cas9 are no longer new. Its power lies in that it can accurately manipulate the genome, adding what should be added and removing what should be removed. It can directly introduce some rare but useful traits into corn varieties, thus expanding the available genetic space (Andorf et al., 2019).

As for phenotype, in the past, if you wanted to know which corn plants were drought-resistant and which were disease-resistant, you had to rely on human eyes and manual records, which was surprisingly inefficient. Now it is different. With the high-throughput phenotyping platform, not only can data be collected quickly, but also more accurately. In conjunction with genomic breeding tools, high-quality germplasm can be quickly screened according to the stress environment and more targeted strategies can be formulated (Prasanna et al., 2021). To put it bluntly, it helps speed up breeding work and also gives us more confidence to cope with climate change.

6.3 Possibility of sharing and cooperation

In the final analysis, the problems of corn breeding are often not solved by a single country or institution. For example, the multinational cooperation projects led by the International Maize and Wheat Improvement Center (CIMMYT) have promoted many new stress-resistant varieties in many regions around the world, and the actual effect is quite good (Prasanna et al., 2021).

Moreover, these collaborations are not just exchanges at the research level, but also involve the joint participation of public and private institutions. Some projects, such as Seed Discovery, are aimed at genetic variation in local varieties. Through genomic screening, these resources are transformed into pre-breeding materials to guide breeding on a larger scale (Gorjanc et al., 2016). In this way, not only genetic diversity is expanded, but also cooperation becomes a real driving force, rather than a slogan at the paper level.

7 Suggestions and Outlook

Corn is not a crop that can be ignored. From food to feed to bioenergy, it covers almost every field of agriculture. Precisely because of its wide range of uses, the global demand for it will only grow. But the current problem is also obvious: the climate is becoming more and more unpredictable, the environmental pressure is increasing, and the challenges of breeding are more difficult than before. Although the corn germplasm bank is rich in resources, it is not so smooth to use, especially in multi-target breeding tasks, the utilization rate is not high.

If you want to break through the status quo, some directions are still worth trying. For example, combine the protection of germplasm resources with breeding practice, not just "storage", but also "use". If genetic diversity can be brought into play more effectively, breeding efficiency will naturally keep up. Regarding the future improvement direction, this article proposes some ideas. Although they may not be comprehensive, I hope they can bring some inspiration to corn breeding.

Cooperation has actually been mentioned many times, but it is indeed the key. In particular, platforms such as the International Maize and Wheat Improvement Center (CIMMYT) have brought together many global resources, not only developing many corn varieties with strong adaptability and good resistance, but also making technology exchanges and germplasm sharing more efficient. It is difficult to advance this matter by fighting alone. Countries should be more open and the mechanism should be more flexible. Only in this way can resources and technologies be truly circulated and serve more farmers.

Of course, the role of the private sector cannot be ignored. In some places, it is precisely because of public-private cooperation that drought-resistant corn varieties can be truly implemented, farmers can plant them, and the yield has also increased. Just like some projects promoted by CIMMYT, it is through these methods that varieties that adapt to climate change are sent to the places where they are most needed.

In addition to cooperation, another variable is technology. The emergence of new technologies has indeed brought many possibilities to breeding. For example, gene editing-especially CRISPR-Cas9 - can directly introduce or eliminate certain target traits, which is much more efficient than traditional breeding. There is also high-throughput phenotyping technology, which makes large-scale screening more realistic. The problem is that the application of these technologies is uneven in different countries. In particular, some developing countries with insufficient resources still find it difficult to fully use these methods. Future research should perhaps focus more on how to use these new tools in the most practical scenarios, so that improved varieties can really enter the fields.

After all, the protection and efficient use of corn germplasm resources is not the responsibility of any country, nor can any institution accomplish it alone. This is a global matter. Looking to the future, breeding goals will not become simpler, but more and more complex. Climate change, land degradation, population growth, none of these problems can be avoided. Therefore, if you really want to do well, in addition to using new technologies and strengthening cooperation mechanisms, you must also have policy guarantees and stable financial support. Only when all aspects keep up can corn breeding be more and more stable on the road to sustainable development. Corn is not just a crop in the field, it is also the support for global food security. In the future, it will continue to be one of the most core roles in the agricultural system.

Acknowledgments

The authors extend sincere thanks to two anonymous peer reviewers for their invaluable feedback on the manuscript.

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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