Alun SaltAround the world, at river mouths lie dead zones, areas where oxygen concentrations are so low that little life can survive around the world. The cause is an influx of nitrogen. This nutrient spurs the growth of algae but, as the algae die, oxygen is depleted from the water – causing life to suffocate. Stopping nitrogen from getting into the rivers would improve the situation, but how can you do that? Dr Patompong Saengwilai and colleagues have found that roots may have the answer. Their research, published in the Annals of Botany, shows how roots’ hairs can influence the take-up of nitrogen. Plants with better roots could reduce the need to add so much fertilizer to fields.
“Nitrogen is a major constraint for crop production worldwide,” Dr Patompong told Botany One, “and thus farmers tend to apply an excessive amount of nitrogen fertilizers. However, several studies have shown that more than half of the nitrogen applied to the fields is not taken up by plants, and it leaches away from the root zone. Some of the nitrogen becomes nitrogenous gases, causing air pollution and some enters groundwater and water sources.”
Ideally, farmers would use less nitrogen in their fertilizer. They could do that if the plants were better at picking it up, but it hasn’t been clear how to achieve this. Looking at the roots might seem obvious; they’re how plants collect potassium and phosphorus. But botanists had thought nitrogen was different, Dr Patompong explains.
“The benefit of root hairs for the uptake of nutrients that are diffusion-limited or relatively immobile in the soil, such as potassium and phosphorus, is largely predictable because root hairs help expand soil exploration volume, allowing the plants to access more immobile nutrients surrounding the root surface. However, their roles for nitrogen uptake have been neglected because the common perception that nitrate, the dominant form of nitrogen in most agricultural soils, is readily available to plants and not limited by diffusion. This is true in many cases, but we know from our observations and few studies in the past that the availability of nitrogen can be limited by diffusion, particularly when ammonium fraction in the soil is high such as in paddy fields, or when transpiration rate and soil nitrogen content is low. This information urged us to ask whether root hairs can benefit plants’ nitrogen uptake from low N soils.”
To test the role of roots, Dr Patompong and colleagues used three approaches. First, they used the functional–structural model SimRoot. The team compared these results with experiments with maize genotypes with variable root hair length (RHL) in greenhouse and field environments.
“We chose to study maize not only because it is one of the major food sources for animals and humans, but it is also a model plant that has been intensively studied. Thus, many resources and information are available and accessible by researchers and breeders worldwide. Furthermore, our study is special in that we use recombinant inbred lines (RILs) descend from the same two parents (B73 and Mo17), hence represent distinct genotypes sharing the same genetic background, thereby reducing the risk of confounding effects from genetic interactions, epistasis, and pleiotropy. Additionally, these RILs had comparable root and shoot characteristics in normal conditions but contrasting in root hair length, so they are very suitable for our kind of study.”
The results from SimRoot provided targets to look for in the other experiments. The authors wrote in their article, “Consistent with SimRoot results, a positive relationship between root hair length and N acquisition was observed in both greenhouse experiments…, as well as the field trial… This general agreement among results from in silico environments, the field and greenhouse is noteworthy, as each of these environments are distinct.”
Lowering nitrogen availability to stress the plants had the effect of reducing root hair length. This shortening is important as the botanists found a significant relationship between root hair length and plant performance. The greenhouse plants tended to have the best root hairs, and this might be because they had the best growing medium. “It is possible that under field conditions the presence of clay and hard particles may limit root hair expansion, and thus root hair length in natural soil is shorter than in artificial sand-based media,” write the authors.
Getting longer root hairs is important for the plants, but the scientists wanted to understand why this was so. The SimRoot model suggested that the reason was surface area. When transpiration is reduced, lowering the water flow through a plant, the increased surface area of roots can offset the reduced mobility of nitrogen.
Dr Patompong said that while the team’s results are based in maize, they should be relevant to other plants. “We expect that this knowledge can be applied to other crop species, but certainly more research is needed to be done. For example, rice also forms root hairs, but they are typically much shorter than maize, and rice is grown with different cultivation methods in different agroecosystems, which may affect the formation and the utilities of root hairs.”
This article isn’t the first work Dr Patompong has done on roots. Previous publications have examined the root systems of rice and cassava. “My passion for root research stems from the fact that we do not really know much about them. Looking back at the human history of plant domestication thousands of years ago, we have been selecting crops based on their above-ground traits; big fruits, large scented colorful flowers but roots have been neglected despite the fact that we know all along that the key of success in growing any plants is to have a good root system.”
“There are not many people studying plant roots, particularly in a field scale because it is a “dirty” and very labor-intensive work. My experience as a researcher in USA, South Africa, Japan, and Thailand has shown me that selecting a good root trait can enhance the growth of plants in harsh environments and help farmers reduce fertilizer application rates, which improve their incomes and save the environment.”
Dr Patompong runs Root Lab Thailand at Mahidol University. Here they not only look at improving plants but also the soils the plants are in. Another recent article from the lab is Responses of oil degrader enzyme activities, metabolism and degradation kinetics to bean root exudates during rhizoremediation of crude oil contaminated soil. He recommends that any students interested in phytoremediation should “go for it”.
“Compared to other technologies, phytoremediation will take some time, but it is one of the most environmentally friendly and sustainable ways to clean up the environment. My advice to any students is to find inspiration and motivation from the real-world issues. If possible, go visit the place, see the land, talk to the people whose lives are affected by the pollutions and use those as a motivation for your study. I found that when you start your career with a goal to contribute to society by knowledge, technologies, or any means that you can with passion, you will find joy and success.”
For Dr Patompong, his passion is clearly with roots, and it may be part of what drives his academic collaborations.
“What I personally find most fascinating about plant roots is that they teach me a philosophy of life. Roots are not just there in the soils, but they interact with other roots, elements, microbes, and other soil biota. They compete and yet help each other to sustain lives to create a balanced ecosystem. Every single root is created for a purpose and a small change even in root anatomy can cause a significant change in overall plant growth and the ecosystem around it. Everyone is a part of the planet, and we can make a significant positive impact to the world we live in.”
RESEARCH ARTICLE
Saengwilai P, Strock C, Rangarajan H, Chimungu J, Salungyu J, Lynch JP. 2021. Root hair phenotypes influence nitrogen acquisition in maize. Annals of Botany. https://doi.org/10.1093/aob/mcab104
Fi Gennu
AJ Cann