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, 112 (2), 317-30

A Conceptual Model of Root Hair Ideotypes for Future Agricultural Environments: What Combination of Traits Should Be Targeted to Cope With Limited P Availability?

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A Conceptual Model of Root Hair Ideotypes for Future Agricultural Environments: What Combination of Traits Should Be Targeted to Cope With Limited P Availability?

L K Brown et al. Ann Bot.

Abstract

Background: Phosphorus (P) often limits crop production and is frequently applied as fertilizer; however, supplies of quality rock phosphate for fertilizer production are diminishing. Plants have evolved many mechanisms to increase their P acquisition, and an understanding of these traits could result in improved long-term sustainability of agriculture. This Viewpoint focuses on the potential benefits of root hairs to sustainable production.

Scope: First the various root-related traits that could be deployed to improve agricultural sustainability are catalogued, and their potential costs and benefits to the plant are discussed. A novel mathematical model describing the effects of length, density and longevity of root hairs on P acquisition is developed, and the relative benefits of these three root-hair traits to plant P nutrition are calculated. Insights from this model are combined with experimental data to assess the relative benefits of a range of root hair ideotypes for sustainability of agriculture.

Conclusions: A cost-benefit analysis of root traits suggests that root hairs have the greatest potential for P acquisition relative to their cost of production. The novel modelling of root hair development indicates that the greatest gains in P-uptake efficiency are likely to be made through increased length and longevity of root hairs rather than by increasing their density. Synthesizing this information with that from published experiments we formulate six potential ideotypes to improve crop P acquisition. These combine appropriate root hair phenotypes with architectural, anatomical and biochemical traits, such that more root-hair zones are produced in surface soils, where P resources are found, on roots which are metabolically cheap to construct and maintain, and that release more P-mobilizing exudates. These ideotypes could be used to inform breeding programmes to enhance agricultural sustainability.

Keywords: Arabidopsis; Hordeum vulgare; barley; cost/benefit; modelling; phosphorus; root anatomy; root architecture; root function; root hairs.

Figures

Fig. 1.
Fig. 1.
(A) Geometrical representation and parameterization of the root hair distribution on a unit length of root. (B) Representation of the fraction of soil available to roots as a function of root-hair-length density.
Fig. 2.
Fig. 2.
Specific P uptake and calculated cost associated with root-hair (RH) traits. (A) Influence of root-hair density, root-hair length and root-hair longevity on specific P uptake. The x-axis indicates the fraction of the change in density, length and longevity with respect to wild type (WT). The y-axis indicates the corresponding changes in specific P uptake. (B) Estimated cost function for root-hair density, root-hair length and root-hair longevity. The x-axis indicates the fraction of the change in density, length and longevity with respect to wild type (WT). The y-axis is the cost of building these traits.
Fig. 3.
Fig. 3.
Shape of the cost function on a subset of the trait space. The z-axis represents total cost and colourmap indicates variations from optimal P acquisition. The x-axis indicates root-hair length, the y-axis indicates root-hair longevity and the z-axis indicates the cost of making these traits. The vertical line indicates the optimal set of root-hair traits under a 15 % reduction in P availability. The new optimal root-hair traits will require a 3·5 % increase in root-hair density, a 6·9 % increase in root-hair length, and root-hair longevity would have to be increased by 6·2 %. In this new configuration, specific P uptake would be reduced by 0·7 %.
Fig. 4.
Fig. 4.
Conceptual model of root hair contribution to P acquisition: a synthesis of experimental results, conclusions taken from literature and hypotheses suggesting targets for future breeding programmes for improved P acquisition in barley. Modifications to root hairs in barley (Hordeum vulgare ‘Optic’) resulting from breeding pressure or natural selection are highlighted in modifications 1–3. Root-hair zones are identified by straight graduating light brown lines. Modification 1 represents an increase in root-hair numbers and length and the inset demonstrates the resulting overlap of P-depletion zones which are defined by red dotted lines. Modification 2 represents the optimum status, which in the case of the Optic cultivar, means no change. Here the inset demonstrates a zone of contiguous root-hair zones with the extent of the P-depletion zone defined by the red dotted lines. Modification 3 represents a scenario where no root hairs are present and the roots are colonized by AM fungi which produce a P-depletion zone identified by the pink highlighted area surrounding the roots in the inset. The associated yield for each modification is represented by the green barley heads, the greater the number the greater the yield. Potential target traits or ideotypes for improving P acquisition are divided into the three categories. (1) Architectural – (a) intensive: increasing the total root-hair surface area (more red root-hair zones on more roots); (b) extensive: increasing the number of red root-hair zones in the surface area of soil where the greatest amount of P is typically found. (2) Anatomical – (a) intensive: longer-living root hairs represented by green root-hair zones; (b) extensive: red root-hair zones on low-cost green roots. (3) Biochemical – (a) intensive: root-hair zones delivering exudates represented by yellow dots; (b) extensive: root-hair zones recruiting beneficial microbes represented by blue circles.

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