Root problem in compact soil

When soil is tough and compact enough to make using a shovel difficult, roots are going to have trouble too, but not for the same reasons

An international team of researchers, led by Bipin Pandey from the University of Nottingham, UK – and including researchers from the University of Adelaide’s Waite Campus – found that it isn’t the physical toughness of the ground that stops roots from growing properly, but a hormone signal pathway that acts as a warning signal.

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Key Research Points

• Roots respond to hormone ethylene in tough soil
• Gaseous ethylene cannot diffuse in tough soil and accumulates at root tips
• This signaled plant roots to stop growing, leading to poor yield
• Mutant plants responded less to ethylene and kept growing

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Soil quality is essential for good plant growth. Compact soil is a big problem for farmers and slows down crop breeding and growth, so this research could greatly help plant breeders overcome slow root growth.

Plants release many hormones, which largely affect how, when and where they grow. One of these is ethylene, produced as a gas, which regulates growth and aging of plant organs.

It would make sense to think that roots in compact soil would stop growing because it’s physically too hard, but the team found the roots were actually responding to the amount of ethylene in the soil.

“Interestingly, being gaseous in nature, ethylene is easily diffused away from root tips through soil pores,” says Pandey. “Noncompacted soils have large air-filled pores which allow the gaseous exchange from roots to soil.

“When the soils are compacted, these large soil pores become very narrow and collapsed, thus blocking the diffusion of ethylene from the root tips. Gradually, a large amount of ethylene is trapped near root tips.”

The ethylene instead “pools” in a more concentrated batch near the root tissue, which sends a feed-back message to the roots to tell them to stop growing so quickly. This means, the authors report, that the concentration of ethylene acts like an early warning signal to the roots to inform them of tough soil.

“Eventually, poor root growth and thus poor nutrient and water uptake results in severe yield reduction,” says Pandey

In their study, published in Science, they found that mutant Arabidopsis (a common model plant used in research) and rice roots were better at growing in tough, compact soil than wild-type – “normal” – plants. Instead, the mutant plants did not respond to this ethylene warning signal as much; they just toughed it out and kept growing.

“These ethylene insensitive mutants keep pushing the hard soils as if there are no physical barriers,” says Pandey. “One striking phenotype which these ethylene insensitive mutants exhibit is that the root tips do not swell upon encountering strong soils, which make them easier to penetrate through hard soil.”

Compact soils are common in many parts of Australia, which makes farming the land arduous.

“This research will enable the breeders and scientists to create new crop varieties having higher penetrative capacity in harder soils and potentially can enhance the nutrient and water uptake capacity,” says Pandey.

“This seminal discovery can also be utilised to create and discover futureproofing crops equipped with deep rooting capacity.

These deep-root varieties can also help tackle mild drought conditions having higher capacity to search deeper profiles of soils for water.”