Plants growing in tough soil respond to hormones, not barriers

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 research team involving the University of Adelaide has 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.

Co-author of the study published in Science, is Professor Dabing Zhang, the leader of the University of Adelaide and Shanghai Jiao Tong University Joint Lab for Plant Science and Breeding at Waite campus.

Roseworthy wheat crop

A wheat crop at the University of Adelaide's Roseworthy campus

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 study co-author Dr Bipin Pandey from the University of Nottingham.

“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 Dr Pandey.

In their study, 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 Dr 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 Dr 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.”

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