Additionally, maize genotypes with reduced lateral root branching density have superior water capture, growth, and yield under drought Zhan et al. Thus, a phenotype with a few but long lateral roots is another selection target Zhan and Lynch, Maize leaf architecture may also need to be shaped with the changing climate in mind.
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In maize leaves, a larger number of stomata and shorter distance between the vascular bundles in the leaf blade are beneficial characteristics for drought tolerance de Souza et al. Moreover, leaf curl, which helps conserve water for use in other functions, is an anatomical characteristic and can potentially be used to enhance plant tolerance to drought and heat.
In maize, leaf curl is found to be controlled by few genes Entringer et al. Thus, maize lines with rapid leaf-curling could be a useful resource for tolerance improvement. Globally, new land for maize production is limited. Thus, it is essential to increase maize productivity per unit area to cope with the increased demand for grains. High-density planting is a practical approach for increasing maize yield per unit area.
However, maize varieties that are suitable for high-density planting are still lacking in some developing countries due to specific climates and environments; thus, breeding such varieties is an important task in maize improvement. Multiple leaf-related traits of maize are related to high-density planting. Leaf angle is an important agronomic trait Li et al. A smaller leaf angle results in a more upright leaf orientation. This is beneficial for increasing the leaf area index, reducing maize shade syndrome and improving photosynthetic efficiency Sakamoto et al. The quantitative trait loci for leaf angle were recently mapped Ku et al.
The level of mechanized maize production is relatively lower in developing countries than in the United States. In China, the Yellow River Valley is the primary maize production area and often encounters rainy weather during the maize harvest season.
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Manual harvesting is time consuming and labor intensive. Furthermore, China has undergone rapid urbanization in recent years, and there were more people living in urban areas million than in rural areas million at the end of As a result, rural labor is extremely lacking.
Therefore, the future development of maize production relies largely on the full implementation of agricultural mechanization in developing countries e. To meet the demands of mechanized harvest, several maize shoot traits should be shaped via both genetic improvement and molecular markers selection breeding. Maize kernels with a rapid filling speed, rapid dehydration and easy threshing characteristics are needed. Uniform and moderate plant and ear height, as well as lodging resistance upon maturity will also be beneficial characteristics for maize production mechanization.
In China, a few promising maize varieties for mechanized harvest have been developed and are being evaluated in field conditions prior to large-scale planting. Roots provide the interface between plants and the complex soil environment. Roots extract water and minerals contained in soils that are required for plant growth. Therefore, root architecture is particularly critical in water and nutrition acquisition. Over the past years, maize breeding has focused on yield and shoot traits and phenotypes York et al. Maize root architecture can now be modified for efficient water and nutrition use as well as to accommodate high-density planting.
Water uptake by roots is essential for plant growth and for high yields, particularly in water-deficit conditions. In an ideal maize root system, the primary roots run straight into the soil, the brace roots and crown roots are steep, and the angles of the brace roots are shallower than those of the crown roots. Such root traits can enhance water uptake from a drying soil, reduce root lodging and allow high-density planting and better growth of maize.
Moreover, anatomical and molecular traits contribute to root performance. For example, reduced xylem vessel diameter can save soil water for later use by reducing the root hydraulic conductance Meister et al. The water uptake of the above-mentioned root architecture traits must be intensively evaluated in drought-prone regions.
Advances in Maize Genomics and Their Value for Enhancing Genetic Gains from Breeding
Additionally, QTL mapping analysis has been used to reveal regions of the maize genome that control root architecture in water uptake Landi et al. Increasing nutrient uptake efficiency from soils is a major challenge for roots because of the wide range of soils in which maize is cultivated.
The universally limiting nutrients in agricultural soils include N, P, and K. In many developing countries, a common practice in maize production is to apply excess chemical fertilization for high yield. This inevitably results in environmental degradation. This system integrates architectural, anatomical, and physiological phenes Lynch, N and water are highly mobile resources and enter the deeper soil strata. A root system with a rapid exploitation of deep soil would optimize N capture and water uptake in drying and N-deficient soils Lynch, Other ideal root traits for N acquisition under N deficiency conditions include reduced maize lateral root branching Zhan and Lynch, and smaller crown root number Saengwilai et al.
P is immobile and concentrates in the topsoil. Thus, the elite maize root traits and phenotype associated with enhanced P acquisition include shallow axial root growth angles, short but many laterals, and long root hairs Lynch, These approaches could provide substantial genetic resources for the improvement of maize root architecture.
At present, the genes underlying root architecture and root adaptation to nutrition deficiency have not been characterized and cloned, and the molecular mechanism remains unclear. In spite of the availability of multiple advanced breeding tools and identified elite trait-related genes, it is still difficult for maize breeding, because current genetic modification technology lacks the power to resolve drought tolerance as a single, broadly applicable solution.
In many cases, maize yield will not synergistically increase with stress resistance. Thus, substantial work is needed before the genes can be used in maize breeding, particularly, the phenotypes of transgenic maize needs to be evaluated under filed and stressed conditions. An ideotype of maize plants can be proposed based on recently relevant studies and the above discussion Figure 2.
A maize ideotype includes elite shoot traits e. Moreover, an ideotype root has reduced living cortical area, reduced cortical cell file number, and increased cortical cell size in response to drought stress. Importantly, stress-related genes will be brought in transgenic maize plants for biotic and abiotic stress resistance and synergic high yield.
The maize ideotype with above traits would allow to plant more plants per unit area, facilitate mechanized harvest, and improve water and nutrient uptake especially in drying and nutrient-deficient soils. Figure 2. A diagram of proposed maize ideotype. Maize ideotype plants will have improved shoot and roots traits and phenotype, enhanced stress resistance and maintain a high productivity in a changing climate.
Globally, maize production is entering a pivotal period when modern biotechnology is sufficiently powerful to make better maize plants for sustainable grain production in a changing climate. The maize ideotype proposed here may shed light on targeted maize improvement in the near future.
Increasingly promising genetic resources for elite maize traits are being discovered using high-throughput omics approaches, particularly genomics, proteomics and metabolomics. The application of these genetic resources in breeding practices can significantly increase the gene pools and allow for the modification of many important agronomic traits in maize. Conventional breeding programs combined with molecular modification techniques e. The creation of maize ideotypes and their subsequent application in production may play a vital role in ensuring sustainable grain production in a changing climate.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Abdel-Ghani, A. Association analysis of genes involved in maize Zea mays L. Plant Mol. Alter, S. DroughtDB: an expert-curated compilation of plant drought stress genes and their homologs in nine species. Database , bav Abiotic stress responses in legumes: strategies used to cope with environmental challenges. Plant Sci. Ashraf, M.