Back in the 1990s, the term “wood-wide web” was coined to refer to the mycorrhizal fungi thought to connect plants underground and provide chemical communication within the subterranean ecosystem. The term has gained new importance as our understanding of just how pervasive and important fungi are in all ecosystems across the globe has grown.

However, plants also have their own chemical communication tools. Dabing Zhang, Shanghai Jiao Tong University, China, and University of Adelaide, Australia, Malcolm J. Bennett, University of Nottingham, UK, and colleagues have found that plants can use one of the simplest of olefins, ethylene, as a probe to determine the level of soil compaction around their roots by sensing the level of diffusion of this gas.

Ethylene as a Simple Hormone

Ethylene (C₂H₄), or ethene in IUPAC nomenclature, is well-known as a ripening hormone. It can be used to rapidly ripen green tomatoes and bananas to get them ready for the supermarket shelves. Conversely, stifling it can allow fruit and vegetables to be transported or stored for longer before sale. But what else might this small molecule do for plants?

The researchers explain that plant roots find it just as hard as gardeners to dig their way through highly compacted soil. However, where a gardener may experience a straightforward physical barrier to their spade, something far more subtle is being experienced by a growing plant and its exploratory roots. The team has demonstrated that an ethylene signaling pathway modulates root growth so that the plant does not even attempt to penetrate highly compacted soil, which would not be particularly suitable for growth. The phenomenon that leads to the modulation is fairly straightforward: Well aerated, loose soil will allow the easy diffusion of ethylene produced by root tissues, whereas the flow of gas through compacted soil will be impeded. This difference is sensed by the plant’s roots.

The team points out that soil compaction can be detrimental to any plant, but it becomes important for humans when those plants are crop plants and the soil compaction is negatively affecting crop yields. The researchers found that Arabidopsis (rockcress) and rice plants that are rendered insensitive to ethylene diffusion through genetic engineering will push out penetrating roots regardless of the degree of soil compaction—different from the wild-type plants.

Warning Signal for Roots

The team used the EIN3-GFP (green fluorescent protein) Arabidopsis ethylene response reporter to investigate the effect of covering root tips with a barrier that is impermeable to ethylene. The results suggest that impeded ethylene diffusion (as in compacted soil) in wild-type plant specimens leads to an accumulation of the hormone in the root tips of plants. This, in turn, leads to growth inhibition. “The ethylene acts as an early warning signal for roots to avoid compacted soils,” the researchers explain. This could have implications for engineering or breeding crops that can cope well with compacted soils. This finding was applicable regardless of whether the compacted soil in question was heavy clay or sandy loam.

Given the growing problem of poor soil quality in many parts of the world—partly down to changes in agricultural practice, but also caused by climate change and other factors—such crops might become essential in increasingly marginalized agricultural land. They might, coupled with other modifications, even allow crops to be grown on land previously considered beyond farming.

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• Plant roots sense soil compaction through restricted ethylene diffusion,
  Bipin K. Pandey, Guoqiang Huang, Rahul Bhosale, Sjon Hartman, Craig J. Sturrock, Lottie Jose, Olivier C. Martin, Michal Karady, Laurentius A. C. J. Voesenek, Karin Ljung, Jonathan P. Lynch, Kathleen M. Brown, William R. Whalley, Sacha J. Mooney, Dabing Zhang, Malcolm J. Bennett,
  Science 2021, 371, 276–280.
  https://doi.org/10.1126/science.abf3013