Roots Withstanding their Environment: Exploiting Root System Architecture Responses to Abiotic Stress to Improve Crop Tolerance

Abstract

To face future challenges in crop production dictated by global climate changes, breeders and plant researchers collaborate to develop productive crops that are able to withstand a wide range of biotic and abiotic stresses. However, crop selection is often focused on shoot performance alone, as observation of root properties is more complex and asks for artificial and extensive phenotyping platforms. In addition, most root research focuses on development, while a direct link to the functionality of plasticity in root development for tolerance is often lacking. In this paper we review the currently known root system architecture (RSA) responses in Arabidopsis and a number of crop species to a range of abiotic stresses, including nutrient limitation, drought, salinity, flooding, and extreme temperatures. For each of these stresses, the key molecular and cellular mechanisms underlying the RSA response are highlighted. To explore the relevance for crop selection, we especially review and discuss studies linking root architectural responses to stress tolerance. This will provide a first step toward understanding the relevance of adaptive root development for a plant's response to its environment. We suggest that functional evidence on the role of root plasticity will support breeders in their efforts to include root properties in their current selection pipeline for abiotic stress tolerance, aimed to improve the robustness of crops.

Keywords: abiotic stress tolerance; crop breeding; drought; flooding; nutrient limitation; root system architecture (RSA); salinity; temperature stress tolerance.

FIGURE 1

An overview of the different root types that together form the root system. A dicot root system consists only of one primary root and several orders of lateral roots. In addition, dicots can produce special stress-induced shoot-born roots called adventitious roots. A monocot root system produces additional axial roots, which can be separated in embryonic seminal roots and non-embryonic shoot-born roots. There are several types of shoot-borne roots, such as nodal and crown roots, often distinguished by the exact place they develop and their increasing thickness. In monocots, the primary and seminal roots are especially important during early seedling establishment, but shoot-born roots soon take over and are responsible for most of the water and nutrient uptake. All axial root types can produce several orders of lateral roots.

FIGURE 2

RSA is defined as the spatial configuration of root components and determines the soil volume that can be explored by the roots. Dicot roots consist of a main root and several orders of lateral roots. Monocot roots contain in addition seminal roots and shoot-borne roots. Each plant species has genetically defined limits to its RSA. Within these limits, the RSA is plastic and external (abiotic stress) factors modulate the length, number, positioning and angle of root components. The RSA plasticity varies strongly among and within plant species. This figure illustrates the modulations in RSA for a typical dicot root system.

FIGURE 3

The RSA responds to abiotic stress in different ways. This figure illustrates for dicots how length, angle and number of primary (blue) and lateral roots (grey) change in response to phosphate deficiency (A), nitrate deficiency (B), drought (C) and salinity (D). The arrows indicate an either positive (to the right) or negative effect (to the left).

FIGURE 4

Schematic overview of the effect of root-zone temperature on plant performance and underlying general changes in RSA (brown). To broaden the temperature range for optimal plant performance (yellow), plants should invest in lateral root formation (suboptimal temperature range) and/or axile root length (supraoptimal temperature range). The adaptive value of these RSA changes are, respectively, an increased root surface area to improve resource uptake capacity and drought adaptation by penetration to lower soil layers.