1 Introduction

The role of corn (Zea mays L.) is far more than just a crop in the field. As the staple food for millions of people, it provides a crucial source of nutrition. In the fields of animal husbandry and industry, its use is equally extensive, almost supporting the agricultural economy of many regions (Postma and Lynch, 2011; Rao et al., 2016; Cheng and Wang, 2024). In developing countries, corn not only grows stably in diverse climates, but is also highly anticipated for its high-yield potential, which can be used to combat food security issues and support family livelihoods.

People often say that deep roots lead to lush leaves, and this is especially true for corn. The root system is not only responsible for stabilizing the plant in the ground, but also for efficiently absorbing water and nutrients, interacting with microorganisms in the soil, and affecting the overall health and yield of the plant. Different root characteristics, such as length, density, structural distribution, etc., exhibit different strategies when facing soil differences and external pressures (Ito et al., 2006). Sometimes, the formation of root cortex stomata (RCA) can save energy under low nutrient conditions and help plants persist. And the secretion of organic acids or acid phosphatases, these "chemical tools", allows corn to survive in poor phosphorus resource environments without starving (Gaume et al., 2004).

The agricultural environment is not always ideal. Soil degradation, climate change, nutrient loss, and other issues are becoming increasingly common. At this point, relying solely on traditional breeding methods may no longer be sufficient. We must have a clearer understanding of the interaction mechanism between roots and soil in order to cultivate varieties that can cope with these challenges. Compaction of soil can directly hinder the rooting of roots, which requires the selection of root types that can penetrate obstacles and maintain functionality (Shao et al., 2018). In addition, under conditions of insufficient water or high soil resistance, the adaptive response of roots directly determines the stability of crop yield (De Moraes et al., 2019; Clark et al., 2023). At present, some new root phenotype techniques and breeding explorations of key traits are providing possibilities for more resilient maize and opening a window for sustainable agricultural development (Hajabbasi and Schumacher, 2004).

The starting point of this study is precisely: How can corn live better in imperfect soil? We don't want to stop at describing the phenomenon, but to further understand which root characteristics play a key role in adversity and whether they can be strengthened through breeding methods. The goal is to provide more practical theoretical support for the development of stress resistant and high-yield corn varieties, enabling them to better adapt to increasingly complex agricultural environments.

2 Overview of Corn Root System Development

2.1 Corn root system structure (RSA)

We usually refer to the spatial arrangement of corn roots as Root System Architecture (RSA), which is like a "net" laid underground, determining how quickly and effectively crops can extract resources from the soil. This structure includes both the main root and seed root, as well as different types such as crown root and lateral root. The shape, density, and even direction of RSA are actually products shaped by both genetic characteristics and environmental conditions (Karnatam et al., 2023).

In order to more accurately characterize these structural differences, researchers have introduced new tools such as 3D imaging and time series modeling. These technologies allow us to 'see' how the root system starts from a simple main root and gradually builds complex networks. During this process, differences in branch density and root expansion speed between different genotypes were observed, and these differences were not limited to specific regions, but were reflected globally (Song et al., 2016; Jiang et al., 2019). In other words, the root system is not static, but constantly evolves to adapt to various soil conditions.

2.2 Growth stages of maize root system

Roots do not grow overnight, but gradually unfold in multiple stages. Shortly after germination, corn first grows the main root (PR) and seed sheath root (SSR), which act as the initial "explorers" responsible for laying the foundation. Next up are crown roots (CR) and support roots (BR), which not only provide additional support but also play an important role in nutrient absorption. Finally, lateral roots (LR) continue to emerge from the main root and these additional roots, further expanding the absorption area and capacity (La Cruz et al., 2015; Karnatam et al., 2023) (Figure 1). The entire development process of the root system is both orderly and influenced by environmental feedback, like an "underground battle" that constantly adjusts strategies.

The root development process of corn is not linear and stable, and its changes are particularly evident at certain developmental stages. Taking Zhang et al.'s (2020) study as an example, they observed and measured the maize root system at the 6th leaf stage, 12th leaf stage, tasseling stage, and milk ripening stage, and analyzed several key traits including main root length (PRL), root number (NRN), and root fresh weight (RFW). From the results, it can be seen that the changes in these indicators from V6 to V12, and then to VT stage are very significant (P<0.01); However, once entering the male withdrawal period, the difference from VT to R3 stage becomes less significant. In other words, the rapid expansion of maize root system is mainly concentrated in the middle and early stages, rather than throughout the entire growth cycle (Figure 2).

But the role of genes is only one aspect, and the regulation of hormones cannot be ignored. For example, plant hormones such as auxin not only regulate the number of roots, but also affect the spatial distribution and growth rate of roots. These hormones regulate different types of roots through a complete signaling network, making the root structure more adaptable to changes in the external environment.

2.3 Factors affecting the development of maize root system

The formation and development of corn root system are influenced by multiple factors such as genetics, environment, hormones, etc. These factors are not independent of each other, but interact and constrain each other repeatedly during the developmental process. For example, through integrated analysis of root system architecture (RSA) related QTLs, researchers have identified 68 meta QTLs, many of which exhibit high conservation in crops such as rice and sorghum (Wang et al., 2021; Karnatam et al., 2023). This also indicates that some regulatory mechanisms may have universality among different crops. Based on these findings, comparative genomics is becoming an important tool for improving root traits and guiding breeding.

However, environmental pressure is often a more direct variable. For example, as the planting density increases, the root volume of a single plant will significantly decrease, and the morphology of the roots will also change. Corn will actively adjust its strategies in this situation, such as reducing the number of internode roots and the growth of lateral roots, but will try to maintain the length of internode roots as much as possible in order to extend further outward and strive for more soil resources (Shao et al., 2018). This regulation is not done out of thin air, plant hormones play a crucial role in it. Signal molecules such as auxin, cytokinin, and gibberellin participate in the regulation of root systems in conjunction with environmental signals, ultimately forming a multi-level and dynamically changing developmental pattern (La Cruz et al., 2015; Sharma et al., 2021).

3 Soil Characteristics and Their Impact on Maize Root System

3.1 Soil physical characteristics

The soil that the root system needs to penetrate is not a homogeneous medium, and its physical state often determines whether the root "dares" to descend. The proportion of sand, powder, and sticky particles constitutes the soil texture, which has a substantial impact on the growth mode of roots. Generally speaking, sandy soil is looser and easier for root systems to penetrate, but has poor water retention capacity; Although clay has sufficient moisture, it may be too compact, which is not conducive to root expansion. Intermediate textures like sandy loam often become the "preferred soil" for maize root growth, with good aeration and low penetration resistance (Malik et al., 2023). In addition, the structural heterogeneity of soil is also crucial. When there are many pores and moderate bulk density, roots can grow smoothly in it without being stagnant due to spatial limitations (Phalenpin et al., 2021).

However, not all plots of land in modern agriculture are in an ideal state. The problem of soil compaction caused by heavy machinery and improper cultivation is becoming increasingly common. It makes the soil hard, oxygen cannot enter, water is not easy to penetrate, and roots are "unable to take root". Especially under humid conditions, this compaction has a more significant limiting effect on maize roots, directly manifested as a significant reduction in root length and surface area (Xiong et al., 2020). Over time, the biomass and final yield of the aboveground part will also be affected, reminding us to pay attention to the shaping effect of soil physical conditions on the root environment (De Moraes et al., 2019).

3.2 Soil Chemical Characteristics

Whether a piece of soil is "palatable" depends not only on its elasticity, but also on whether it is "nutritionally comprehensive". Soil pH, nutrient distribution, and the presence of harmful substances often determine the upper limit of root development. For example, overly acidic soil can "lock in" phosphorus, which is a key nutrient for root development; Alkaline conditions may cause trace elements such as iron and zinc to lose their activity. In response to these issues, some regions have improved acidic soils with lime, which not only adjusts pH but also promotes symbiosis between maize roots and arbuscular mycorrhizal fungi (AMF), indirectly enhancing plant nutrient absorption efficiency.

Fertilizer management is also a technical activity. If the level of nitrogen application is not well controlled, it will either lead to insufficient root development, waste or even damage to roots. Studies have shown that reasonable nitrogen application can significantly improve root quality, while zero or excessive nitrogen application reduces root biomass by 33% and 17%, respectively (Ord ó ñ ez et al., 2021). In addition, organic fertilizers such as farm manure have shown good performance in improving nutrient availability in sandy loam soil, indirectly promoting root development.

Another easily overlooked issue is toxicity. Soil with excessive levels of certain heavy metals or salts can cause root poisoning, leading to impaired cellular function and a decrease in the efficiency of water and nutrient absorption. Therefore, improving the soil chemical environment is not only to ensure that plants are fully fed, but also to prevent them from spoiling.

3.3 Soil Biological Factors

Like humans, roots underground also have 'neighborhood relationships'. The composition of soil microorganisms, including bacteria, fungi, etc., has a profound impact on the growth of roots. These microorganisms are mostly related to soil types, and the impact of variety differences is not as significant. For example, studies have shown that rhizosphere microbial communities exhibit significant differences in different soils, with each type of soil "inhabiting" a unique microbial ecosystem, which is a latent variable that affects plant performance (Chen et al., 2017).

Among them, the role of AMF is particularly noteworthy. This type of fungus can form a symbiotic relationship with corn roots, greatly improving the efficiency of phosphorus absorption, especially in soils where phosphorus resources are already scarce. The colonization efficiency of AMF itself is also affected by factors such as soil texture and pH, which are believed to be the most critical for symbiotic formation, while the influence of soil organic matter is relatively small (Carrenho et al., 2007). In addition, probiotic bacteria that can secrete plant hormones while inhibiting pathogens also play a crucial role in promoting root health and overall growth (Ganther et al., 2020).

4 Genetic and Molecular Regulation of Root Development

4.1 Key genes and pathways affecting maize root growth

The genetic factors that affect the root structure of maize are not limited to a single gene, but rather an interwoven and dense network. In this network, some key factors such as the LOB domain (LBD) and Aux/IAA proteins begin to play a role in the early stages of root formation, especially in the initiation process of seed roots, lateral roots, and stem roots (Hochholdinger et al., 2018). They belong to the components of the auxin signaling pathway, and the regulatory process is not simple, nor is a single mechanism applicable to all root types.

In addition to the signal regulatory pathways mentioned above, some genes related to cell wall relaxation, vesicle transport, and cellulose synthesis are also involved in root hair extension. Although these "peripheral" mechanisms do not directly dominate root formation, they play a supporting and extending role in the overall development of the root system.

Mutation analysis also provides some more direct evidence. For example, RTCS is an important gene that regulates the initiation of stem rooting, while RTH1 and RTH3 are crucial for root hair growth (Hochholdinger et al., 2009). QTL research further expands our understanding, suggesting that seed roots, lateral roots, and root hairs are not only closely related to nutrient uptake (especially phosphorus), but may also directly affect yield under different environmental conditions. This indicates that genetic control not only affects the morphology of roots, but also leaves clear traces at the level of crop productivity.

4.2 Progress in genomics and transcriptomics

Relying solely on gene function research is not enough. A deeper understanding of the key to root development requires a more comprehensive molecular perspective. In recent years, genome and transcriptome research has brought many new breakthroughs (Zhou and Xu, 2024). Technologies such as RNA seq have revealed the link between root development and the synthesis pathways of auxin, phenylalanine, and flavonoids (Wang et al., 2021). Through these data, researchers constructed a co expression network and identified several important modules and genes that may be involved in root development, such as rtcs and Zm00001d012781.

These molecular data often cover multiple root segments and involve multiple developmental time points. For example, in the study by Stelpfrug et al. (2016), researchers found nearly 29000 annotated genes in different parts of maize roots and recorded their expression differences along the longitudinal and transverse positions. This type of analysis not only provides rich information resources, but also provides specific clues for future gene function verification, targeted editing, and other related activities.

4.3 Application of CRISPR and other genetic tools

In the past, deciphering the function of a gene could take years, but now, the emergence of CRISPR technology has greatly accelerated this process. In maize root system research, the precise editing ability of CRISPR/Cas9 has been used to validate some key candidate genes related to root structure, such as Zm00001d005892 (AtERF109 homologous gene) and Zm00001d027925 (AtERF73/HRE1 homologous gene), which have also been shown to be closely related to root development in other plants (Zhang et al., 2020).

The role of transcription factors is gradually being revealed. Some members of the AP2-EREBP family not only play a role in root growth regulation, but also participate in nitrogen stress response (He et al., 2016). These achievements emphasize the potential of combining CRISPR with big data screening to optimize root traits more targetedly in different environments. These new technologies are not intended to replace traditional breeding, but to expand more effective improvement paths in collaboration with traditional methods.

5 Adaptation Mechanisms of Maize Roots to Soil Constraints

5.1 Coping with drought and water scarcity

Under drought conditions, corn does not sit idly by. It usually enhances water absorption capacity by increasing the number of fine roots and thinning the main roots, especially under moderate water stress, where this morphological transformation is more pronounced (Yan et al., 2022). Some regulatory genes are also involved in this process. For example, DRO1 is activated under abscisic acid (ABA) induction, which can promote root extension to deeper soil layers, allowing plants to access deeper water even if the surface dries up (Feng et al., 2022).

Sometimes, human intervention is also a necessary means. Soil amendments such as Leonardite and humic acid, which are rich in sulfate ions, can enhance the water acquisition capacity of roots and improve yield and physiological performance under drought and phosphorus deficiency conditions (Kaya et al., 2020). Especially when multiple stresses occur simultaneously, the combined use of these materials is more effective than a single improvement method.

5.2 Coping with nutrient deficiency

When facing nitrogen, phosphorus, or potassium deficiencies, maize roots do not passively respond. Structures such as root cortex air chambers (RCA) are an energy-saving adaptation method that can reduce the respiratory metabolic burden of roots and improve their adaptability to poor soil conditions (Postma and Lynch, 2011). By reducing redundant organization, this structure actually enhances the efficiency of root exploration.

In terms of improvement strategies, humic acid fertilizer and earthworm compost have been proven to improve soil conditions and increase nutrient utilization efficiency, especially suitable for "problematic soils" such as saline alkali soil. In addition, controlling the amount of nitrogen fertilizer used is also an effective strategy. Research has shown that reducing basal nitrogen input can optimize root distribution, allowing it to penetrate deeper into the ground and improve resource acquisition capacity in times of water scarcity (Wang et al., 2019).

5.3 Coping with saline alkali and acidic soils

Saline alkali soil not only brings about ion concentration issues, but also involves a series of chain reactions such as root cell function and oxidative stress. Studies have shown that beneficial microbial pretreatment of seeds can stimulate the expression of root related genes, thereby improving the efficiency of sodium, potassium, and calcium transport and alleviating salt stress (Singh et al., 2021). In addition, humic acid and earthworm compost are often used as physical environmental regulation methods to indirectly support healthy root growth by reducing salt concentration and increasing nutrient supply (Liu et al., 2019).

As for acidic soils, corn roots often respond to challenges by locally altering the rhizosphere environment. For example, increasing local pH, releasing chelated aluminum compounds, and enhancing acid phosphatase activity have to some extent alleviated the adverse effects of aluminum toxicity and low phosphorus stress. For these issues, the breeding direction is also moving towards enhancing aluminum tolerance and improving phosphorus acquisition efficiency (Rao et al., 2016).

6 Agricultural Operations and Their Effects on Root Development

6.1 Cultivation and no tillage operations

The choice of cultivation method often inadvertently determines the direction of corn root development. Comparing conventional tillage (CT) and no tillage (NT), the differences in soil environment shaping between the two cannot be ignored. Conventional tillage provides lower mechanical resistance for root extension by loosening the soil and reducing bulk density. Research has shown that this approach helps to increase root length density (RLD) and is often accompanied by finer root types, while root development in no tillage plots is slower, possibly due to lower surface temperatures or soil compaction (Guan et al., 2014). In addition, the increase in root biomass brought about by conventional tillage is often associated with better nutrient absorption.

However, this does not mean that no tillage has no advantages. Long term observation has found that although the root system of no till fields is initially limited, over time, its root system gradually extends deeper and adapts to a denser soil structure. In the surface soil, root dry weight and root length density actually increased (Himmelbauer et al., 2012). Fiorini et al.'s study also reminds us that no tillage should be judged solely based on short-term performance. By forming more biological pores and improving soil structure, no tillage can ultimately promote sustained root growth (Fiorini et al., 2018).

6.2 The effect of fertilizer application on root system

If the root system is the "arm" of a crop, then fertilizer is its essential "source of nutrients". However, different fertilization methods have significant differences in inducing root growth patterns. Taking lateral application of nitrogen and phosphorus fertilizers as an example, it can concentrate roots around the fertilization zone, greatly improving local RLD performance. Regardless of which tillage method is used, the effect is relatively stable (Chassot et al., 2001).

But having fertilizer application points alone is not enough, and the amount of fertilizer application also needs to be carefully controlled. Liu et al. (2017) found that under the premise of nitrogen fertilizer application not exceeding 184.5 kg N ha ⁻¹, root dry weight, absorption area, and root to stem ratio all showed positive growth; On the contrary, excessive fertilization actually inhibits the development of roots, indicating that "more is better" is not suitable for nitrogen application.

In addition to conventional fertilization, long-term soil improvement cannot be ignored. For example, the use of lime and phosphogypsum not only alleviates acidic soil, but also enhances the availability of nutrients such as calcium and magnesium, creating a more friendly environment for root growth (Bossolani et al., 2021).

6.3 Promoting effect of crop rotation and intercropping on root system

Changing the planting combination structure is another way of indirectly affecting the root system. Crop rotation and intercropping management strategies often indirectly enhance the growth potential of maize roots by improving soil quality. The introduction of wheat or crop rotation systems can alleviate water pressure and increase soil organic matter, which have a positive impact on root development, especially in terms of improving drought resistance.

In terms of intercropping, the introduction of leguminous crops is clearly not just for nitrogen fixation. Research has shown that under the maize soybean intercropping system, microbial biomass and the activity of phosphate solubilizing bacteria are significantly enhanced, which can positively feedback to the root system's nutrient acquisition and growth vitality (Bolo et al., 2021). Renwick et al. also pointed out that incorporating leguminous crops into crop rotation systems not only increases root biomass, but also helps improve soil organic matter, further supporting the continuity of root function (Renwick et al., 2021).

7 Case Studies: Development of Corn Roots in Marginal Soil

7.1 Overview of the research area

This case study selects the typical marginal agricultural area in southern Africa - central Malawi. There, cultivation is mainly carried out by small-scale farmers, and agricultural production is affected by multiple limiting factors, including poor soil and unstable water (Wang et al., 2019). Moreover, the rampant infestation of parasitic weeds such as Striga asiatica further exacerbates the difficulty of crop root development.

7.2 Challenges faced by farmers

In such an environment, it is difficult for farmers to improve farmland conditions through conventional means. The high cost of fertilizers and high-quality seeds often keeps improvement measures at the "ideal" stage (Mafouasson et al., 2020). Even if adjustments are made, labor shortage remains a challenge. And irregular rainfall also makes water acquisition one of the bottlenecks in root development.

Another issue that cannot be ignored is the intense resource competition between parasitic weeds and crops. Striga and other plants compete for nitrogen, phosphorus, and water, making it difficult for corn roots to effectively expand. The combination of poor soil and biological stress has caused farmers' corn yields to remain low for a long time.

7.3 Response plan and technical attempt

Faced with these complex problems, a single technology is often ineffective, so modular solutions are gradually being pushed to the forefront. One of the breeding schemes is to improve the root structure, indirectly increasing grain yield by enhancing nitrogen absorption after pollination (Mu et al., 2015). GWAS and genome prediction methods have also been introduced to identify maize varieties that perform well in low nitrogen environments in advance (Ertiro et al., 2020; Guo, 2024).

In addition to genetic improvement, there have also been attempts at the agronomic level. The Integrated Crop Soil Management Strategy (ISSM) emphasizes precise matching between crop demand and nitrogen supply. Chen et al. (2011) designed a fertilization strategy that maximizes the utilization of light and temperature windows, improving the synchronous absorption capacity of roots (Figure 3).

At the same time, some microbial intervention methods are also being explored. The use of rhizosphere growth promoting bacteria (PGPR) is believed to reduce dependence on synthetic fertilizers to some extent and enhance root absorption capacity by improving the soil microenvironment (Bounaffaa et al., 2018).

We have seen several impressive insights from the practice in central Malawi. Although the local soil conditions are not ideal, the yield and nutrient absorption efficiency of maize have still been significantly improved through genetic improvement of root traits (Mu et al., 2015; Ertiro et al., 2020). This means that focusing on underground structures in the breeding process is not just about adding icing on the cake, but a key breakthrough when facing marginal land resources.

In addition to variety improvement, the comprehensive application of agricultural management measures has also played a role. For example, the combination of rational allocation of crop types, optimized fertilization schemes, and dense planting strategies not only enhances root development, but also provides practical and feasible farming methods for small farmers (Liu et al., 2017). They do not rely on high cost inputs, but can gradually achieve stable or even increased yields by improving soil conditions.

It is worth mentioning that in the context of constantly fluctuating fertilizer prices and limited supply, biological soil improvement methods such as PGPR have also begun to be valued. They can not only reduce fertilizer dependence and save costs, but also improve soil activity and nutrient cycling (Bounaffaa et al., 2018). For farmers who have long faced resource bottlenecks, this may be a more sustainable alternative path.

8 Future Prospects of Corn Root System Research

8.1 Exploration of the combination of phenotype and genotype

Traditional root research is often trapped between laboratories and greenhouses, making it difficult to truly relate to field conditions. With the development of high-throughput phenotype (HTP) technology, especially the application of unmanned aerial vehicle (UAV) platforms in the field, we are beginning to be able to capture various manifestations of root development under more realistic conditions. The indicators such as canopy temperature, plant height, and spectral reflectance also provide a more three-dimensional data basis for the relationship between genotype and phenotype (White et al., 2012; Adak et al., 2023).

For example, some studies have used drone platforms to monitor the flowering time and growth rate of corn, in order to infer the differences in growth rhythm among different varieties. With the advancement of sensor technology, not only can the status of roots be recorded, but also the surface growth status can be modeled through multidimensional data (Tracy et al., 2019). This transformation is expected to deepen our understanding of the genetic mechanism of root traits and provide breeders with more scientific judgment tools when selecting materials.

8.2 The role of precision agriculture in root system optimization

In recent years, precision agriculture has continuously expanded its boundaries in crop management. It is no longer just about targeted sowing or quantitative fertilization, it has penetrated into the monitoring of plant root development. For example, with the help of AI and computer vision algorithms for image analysis, researchers can not only identify pests and diseases, but also evaluate root activity and determine soil conditions (Patr í cio and Rieder, 2018).

Drones equipped with multispectral and RGB cameras have become powerful tools for conducting large-scale phenotype analysis. They can extract key indicators such as NDVI and canopy coverage, providing first-hand materials for us to understand the interaction between environmental variables and root structure (Han et al., 2018; Wang et al., 2021). Combining these data with crop models or genome prediction tools can outline how genotypes adjust their root performance in specific environments (Cooper et al., 2016).

In the future, such integration methods will no longer be just scientific research tools, but may become a part of daily field management, helping farmers make precise, efficient, and low-cost agricultural decisions.

8.3 Improvement of sustainable practices and root soil synergy mechanisms

Discussing root optimization cannot bypass soil. The relationship between roots and soil is both close and subtle. Sustainable agriculture attempts to enable roots to absorb water and nutrients more efficiently without sacrificing the long-term capacity of the land. The changes in "slow variables" such as crop coverage, rotation systems, and reduced tillage intensity often show significant improvements in soil structure and root health after several years.

At the same time, with the help of emerging phenotype and simulation tools, researchers are trying to understand these interactions from a more detailed perspective. How can root system structure maximize its effectiveness in specific soil types? Is there a trade-off between carbon cost and nutrient uptake under different soil pore structures and humidities? This type of problem is gradually being revealed through imaging, modeling, and big data methods (Bucksch et al., 2014; Garc í a-Flores et al., 2015).

The significance of sustainable agricultural practices lies in the fact that they do not exist in isolation, but together with breeding, data tools, and soil science, form a more complex but resilient agricultural ecosystem.

Acknowledgments

The authors sincerely thank their colleague Anita Wang from the research team for the assistance provided in the collection of literature and materials for this study.

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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