Narrow roots more easily penetrate hard soil, which means breeding crops to foster this type of root could help boost crop productivity
Most scientists say climate change is altering heat cycles and rainfall patterns leading to harder, drier soils that challenge crop growth and yield.
Hard soil caused by machinery compaction is compounding the problem. It can lead to yield reduction of some 25 percent and, when combined with drought, up to 75 percent.
The problem is further compounded by a lack of natural plant root penetration. Plant roots create tiny channels and macro pores in the soil that allow for air and water movement. As roots grow and penetrate deeper into soil, they open channels to allow for the spread of water.
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Efforts to mitigate the negative impact of soil compaction and hard, dry soil include reducing tillage, controlling farm traffic, and enhancing subsoil management. But these are time-consuming and sometimes costly and do not necessarily affect deeper soil layers.
By contrast, breeding soil-compaction-resistant crop lines offers a genetic solution to improving strong root growth.
Recently an international team of researchers led by the University of Nottingham in the United Kingdom and Shanghai University discovered key genes, hormone signals and plant growth processes in rice roots that control the crop’s ability to penetrate hard soils.
The study has overturned decades of conventional thinking that, in hard soils, roots swell to aid their penetration. They discovered that, in fact, it is narrow roots that more easily penetrate hard soils. Breeding crops to foster narrow roots offers a genetic solution to improve root growth and viable crop productivity.
Bipin Pandey of the Biotechnology and Biological Sciences Research Council and lead researcher at the University of Nottingham said researchers thought that by understanding how plant roots sense soil hardness, they could manipulate the plant system to penetrate hard soils.
He said that the mechanism is widespread across other crop species such as corn and wheat as well as non-commercial plants such as Arabidopsis commonly used in laboratory experiments.
“Basically, under compacted soil conditions, soil particles are compressed together, which abolishes the soil pore networks that (normally) act as passages to diffuse the gaseous hormone ethylene away from root tips.”
But in dry soil, roots use ethylene to sense soil compaction as the restricted air space causes this hormone to accumulate around root tips where it acts to slow root growth.
“We have recently discovered that ethylene acts as an early warning signal for sensing soil hardness,” he said. “In a nutshell, hard soil restricts the diffusion of ethylene out of the roots, which acts as a stop signal for root growth. We recently showed that ethylene orchestrates (two other plant hormones) auxin and abscisic acid (ABA) to regulate root tip elongation and swelling responses, respectively, in compacted soil.”
It would seem, from an evolutionary point of view, that the combined hormone response to hard soils causing roots to slow their downward growth and swell in size is counterproductive. But Pandey suggested that this is likely an adaptive response.
“In hard soil, ethylene inhibits the root growth so that the plant can put its resources into other more favourable conditions where the soil is not too hard and where it can keep foraging for nutrients and water for growth. It seems to be an adaptive response to divert its resources to non-compacted soil.”
Given that the interaction of ethylene, auxin and ABA had not been studied in roots during growth in compacted soil, the researchers initially grew plants in both non-compacted and compacted soil conditions and then measured ABA levels in root tip tissue using X-ray imaging. They found that ABA levels increased threefold in root tips growing in compacted soil compared to non-compacted soils.
They then grew both wild-type rice plants and mutant plants in soils of various compaction levels. The mutant plants were T-DNA insertion mutants. The T refers to “transfer” and it is a genetic transfer adaptation to show a particular biological process in plants, in this case ABA biosynthetic genes.
“These are T-DNA insertional mutants selected to test the ethylene response,” said Pandey. “However, we have identified that natural genotypes such as (corn) and wheat, which are fairly insensitive to ethylene, can penetrate (soil) better than ethylene sensitive genotypes.”
He wrote in the report that soil compaction stress is also known to decrease water availability that may serve to increase root ABA levels. While compact soil pores can fill with water molecules, the water is less accessible to roots because the pores are smaller and harder to reach.
Even if moisture is in the soil but is less accessible to each root tip, the condition mimics mild drying, which therefore enhances ABA levels in root tip tissues, initiating swelling. This may be part of the adaptive response designed to increase root surface area and facilitate water uptake during compaction stress.
Based on the study, the researchers have received positive feedback from crop breeders wanting to explore opportunities to develop new soil-compaction-resistant crops.
“We have collaboration with wheat breeders who are employing this new knowledge in the field conditions,” said Pandey.
Future work will explore the relationship between these hormone-driven root growth responses to compaction and structural features of the soil, such as the presence of channels and cracks, thickness of the compacted soil layer and levels of moisture. In addition, they plan to study the sensing mechanism of the rhizosphere when roots are growing in a complex soil environment.
“Our collaborators are also looking into how ethylene affects the root growth responses such as root hair elongation and root angle.”
The research was published recently in the Proceedings of the National Academy of Sciences.
