Growth & Development

Seminal roots help maize grab nutrients

When humans selected for bigger seeds in maize, they may have also unintentionally improved its root system.

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Around 9000 years ago, people domesticated teosinte, a grass found in Mexico. The result was maize. The two plants are very different. Teosinte can have hundreds of ears with a few grains each. Maize, in contrast, tends to have just two ears with hundreds of grains. Do differences below the ground help contribute to differences above it? Alden Perkins and Jonathan Lynch examined the number of seminal roots in maize seedlings and their effect on nutrient uptake.

Young maize, but not as young as in this paper. Image: Canva.

Botanists call the differences between cultivated plants and their wild relatives the domestication syndrome. Some of these differences are deliberate, like bigger fruits. Other effects might be unintentional but necessary to support the plant. The roots of maize are an example. Maize forms seminal roots, secondary roots that lateral roots from the primary root. Perkins and Lynch refer back to earlier work by Lynch and others that found maize had 3.9 seminal roots, on average. Its wild relative teosinte had 0.5.

These roots may be part of what has made maize so successful as a crop. People originally domesticated the plant in the tropical soils of southern Mexico. Here, nitrates leach out of the soil easily, and there is not a lot of phosphorus. When farmers took the plant to the Mexican highlands, they had another problem. The volcanic soils should be fertile, but the soils of the Mexican highlands have high phosphorus fixation.

The increased number of seminal roots improves phosphorus uptake, say Perkins and Lynch, but it’s not clear what effect the roots have on nitrogen. It’s not just a case of grabbing some maize and teosinte and comparing how they take up nutrients. Because it’s not only the roots that vary, so do many other features.

“Understanding the influences of domestication on seminal root number in maize requires the consideration of plant performance in diverse environments and phenotypes intermediate to those of maize and teosinte,” write Perkins and Lynch. “Since maize and teosinte differ in several respects, including vigour, tillering and growth habit, it is challenging to understand how individual components of their phenotypes contribute to stress adaptation. Simulation modelling can be a useful approach to understanding maize and teosinte root architectures because it allows traits to be experimentally modified in isolation while other components of the phenotype remain constant. The functional–structural plant model OpenSimRoot includes a detailed root architectural model that accounts for root construction costs, respiration and nutrient uptake at the level of individual root segments (Postma et al., 2017). It also allows for the simulation of low-phosphorus soils and for soil nitrate leaching and depletion to be simulated in three dimensions.”

“The results suggest that seminal roots are beneficial for both nitrogen and phosphorus acquisition during the development of maize seedlings, and seminal roots can improve nitrogen acquisition in environments with several different precipitation regimes, fertilization rates and soil textural classes. High numbers of seminal roots may not be beneficial to teosinte because its lower growth rates mean that it has lower nutrient requirements as a seedling and because its small seeds have smaller carbohydrate reserves to support seedling growth.”

It might seem that maize is simply ‘fitter’ with its bigger seeds able to support more growth. However, Perkins and Lynch point out that maize grows in an artificial system. Teosinte’s smaller seeds allow it to travel further and establish populations. Teosinte also doesn’t get doused in pesticides regularly. That means it has to produce its own defences against herbivory, which can slow its growth rate. That means it can invest the limited carbon reserves in the seed into the radicle (root) and coleoptile (first leaf and shoot).

Understanding the differences between maize and teosinte could have valuable lessons for the future, say Perkins and Lynch. “While the importance of seminal roots for nutrient acquisition in maize decreases as the plant matures, seminal root number may still have important agronomic implications. Nitrogen and phosphorus fertilizers are costly inputs for maize growers, and <60 % of the nitrogen fertilizer applied is commonly recovered by the crop… Improving nitrogen acquisition efficiency at all stages of growth, including the seedling stage, has the potential to reduce the pollution from fertilization and increase yields in low-input systems.”

RESEARCH ARTICLE

Perkins AC, Lynch JP. 2021. Increased seminal root number associated with domestication improves nitrogen and phosphorus acquisition in maize seedlings. Annals of Botany 128: 453–468. https://doi.org/10.1093/aob/mcab074

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