Growth & Development

Root hairs more important than root exudate in rhizosheath formation

Exudates did bind soil to the roots, but were unable to compensate for the absence of root hairs.

When a plant is pulled from the ground, the soil left clinging to the roots is known as the rhizosheath. Formed primarily by the action of root hairs and root exudates, the rhizosheath improves water retention, protecting against drought, facilitates nutrient uptake, and helps protect against other abiotic stresses such as extremes of pH. Despite the importance of the rhizosheath, little is known about the relative contributions of its physical, chemical, and microbiological elements.

In a new article published in Annals of Botany, lead author Emma Burak and colleagues compared the rhizosheath-forming abilities of three wild-type species – barley, maize, and Lotus japonicus – and their respective root hairless mutants. The researchers also examined the influence of root hair traits such as length and density, differences in between axile and lateral roots, and exudate adhesiveness for barley and maize.

Rhizosheath: exudates versus root hairs. Image Burak et al. 2021.

For a given length of root, barley had the greatest rhizosheath size, followed by Lotus japonicus, and then maize. This is likely due to the longer root hairs of barley plants. In all cases, plants with root hairs had larger rhizosheaths than their root hairless counterparts. The greatest difference was seen in barley, where the increase was nearly fourfold in individuals with root hairs. Maize exudates were by far the most adhesive, but were unable to compensate for shorter root hair length. Corrected for differences in length, the type of root – axile or lateral – did not appear to have a significant effect on rhizosheath formation.

In terms of carbon cost to the plant, root hairs were found to be less costly than exudates, though the authors note that secondary effects of exudates may complicate this calculation. “While root hairs may be seen as a more carbon-efficient mechanism of enhancing rhizosheath formation than exudation, diffusion of the latter into the bulk soil can cause secondary adhesion (beyond the physical dimensions of the root hairs) and stimulate microbial activity to substantially extend rhizosheath diameter,” they write.