Root hairs are the most important root trait for rhizosheath formation of barley (Hordeum vulgare), maize (Zea mays) and Lotus japonicus (Gifu)

Abstract

Background and aims: Rhizosheaths are defined as the soil adhering to the root system after it is extracted from the ground. Root hairs and mucilage (root exudates) are key root traits involved in rhizosheath formation, but to better understand the mechanisms involved their relative contributions should be distinguished.

Methods: The ability of three species [barley (Hordeum vulgare), maize (Zea mays) and Lotus japonicus (Gifu)] to form a rhizosheath in a sandy loam soil was compared with that of their root-hairless mutants [bald root barley (brb), maize root hairless 3 (rth3) and root hairless 1 (Ljrhl1)]. Root hair traits (length and density) of wild-type (WT) barley and maize were compared along with exudate adhesiveness of both barley and maize genotypes. Furthermore, root hair traits and exudate adhesiveness from different root types (axile versus lateral) were compared within the cereal species.

Key results: Per unit root length, rhizosheath size diminished in the order of barley > L. japonicus > maize in WT plants. Root hairs significantly increased rhizosheath formation of all species (3.9-, 3.2- and 1.8-fold for barley, L. japonicus and maize, respectively) but there was no consistent genotypic effect on exudate adhesiveness in the cereals. While brb exudates were more and rth3 exudates were less adhesive than their respective WTs, maize rth3 bound more soil than barley brb. Although both maize genotypes produced significantly more adhesive exudate than the barley genotypes, root hair development of WT barley was more extensive than that of WT maize. Thus, the greater density of longer root hairs in WT barley bound more soil than WT maize. Root type did not seem to affect rhizosheath formation, unless these types differed in root length.

Conclusions: When root hairs were present, greater root hair development better facilitated rhizosheath formation than root exudate adhesiveness. However, when root hairs were absent root exudate adhesiveness was a more dominant trait.

Keywords: L. japonicus; Rhizosheath; barley; maize; root hairs; root mucilage.

Fig. 1.

The barley (A, B), maize (C) and L. japonicus (D) mutant genotypes without root hairs and their WTs with root hairs. The L. japonicus genotypes (D) have been directly removed from the soil and possess their intact rhizosheaths.

Fig. 2.

A freshly excavated WT barley root system soaking in a metal dish of water (A). Once the soil had been gently removed the roots were spread out on a flatbed scanner (B). The resulting images were processed in WinRHIZO (C); here the axile (blue) and lateral (red and yellow) roots are easily distinguishable by a 253.6-µm diameter threshold.

Fig. 3.

Rhizosheath weight plotted against total root length for barley (A), maize (B) and L. japonicus (C). Black symbols represent individual plants of the root-hairless mutants brb (A), rth3 (B) and Ljrhl1 (C), while grey symbols represent their respective WT. Each panel shows all plants from all four harvests. A linear model was fitted to each genotype, represented by the dashed lines and corresponding equation. All trend lines have a P value <0.001 and R² > 0.57. P values are from ANCOVA.

Fig. 4.

Total root length apportioned into axile (black) and lateral (grey) contributions per harvest for barley (A), maize (B) and L. japonicus (C). Error bars are equal to 1 standard error for each root type.

Fig. 5.

Estimated effect on rhizosheath formation of a 1-unit increase in root type as calculated by a linear regression model for barley (A), maize (B) and L. japonicus (C). The unit for root hairs is a binary increase from absence to presence and the units for axile and lateral roots are for a 1-m increase in root length. Error bars are equal to 1 standard error. *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 6.

Root hair length density (A, C) and length (B, D) versus distance from the root tip in barley (A, B) and maize (C, D). Grey symbols represent lateral roots and black symbols represent axile roots. Error bars are equal to 1 standard error. Linear regressions were fitted when the root types differed significantly.

Fig. 7.

Scanned images from the soil adhesion assay. The drops are distributed in a 10-mm square grid. The dilution of exudates in DI water descends from left (50 µg/5 µL) to right (1 µg/5 µL), with a single droplet comprising 5 µL.

Fig. 8.

Amount of soil adhering to a nitrocellulose membrane spotted with root exudates collected from the axile and lateral roots of the barley (A) and maize (B) genotypes. The bars are coded in shades of grey to reflect the mucilage saturation of the droplet, from darkest, 50 µg/5 µL, through 25 and 10 µg/5 µL, to lightest, 1 µg/5 µL. Error bars are equal to 1 standard error.

Fig. 9.

A conceptual representation of the contributions that root hair and mucilage traits make to rhizosheath formation. For both images the soil particles that are not faded represent the rhizosheath soil, bound to the root. For roots without any root hairs (A) only soil particles that are in direct contact with the main root are bound as a rhizosheath. If the root produces a more adhesive mucilage, then it binds more soil particles (represented by the particles with a graduated fade). The presence of root hairs (B) increases the root surface area for the soil particles to bind to, resulting in a more prominent rhizosheath. Longer and denser root hairs (right side of B) increase the radial extent of the rhizosheath more than when root hairs are shorter and less dense (left side of B).