Deconstructing the root system of grasses through an exploration of development, anatomy and function

Abstract

Well-adapted root systems allow plants to grow under resource-limiting environmental conditions and are important determinants of yield in agricultural systems. Important staple crops such as rice and maize belong to the family of grasses, which develop a complex root system that consists of an embryonic root system that emerges from the seed, and a postembryonic nodal root system that emerges from basal regions of the shoot after germination. While early seedling establishment is dependent on the embryonic root system, the nodal root system, and its associated branches, gains in importance as the plant matures and will ultimately constitute the bulk of below-ground growth. In this review, we aim to give an overview of the different root types that develop in cereal grass root systems, explore the different physiological roles they play by defining their anatomical features, and outline the genetic networks that control their development. Through this deconstructed view of grass root system function, we provide a parts-list of elements that function together in an integrated root system to promote survival and crop productivity.

Keywords: Grasses; Oryza sativa; Zea mays; embryonic roots; environmental stress; postembryonic roots; root development; root system architecture.

Figure 1

Root system architecture in the Eudicot model (Arabidopsis) and two members of the grass family (rice and maize) at early and late developmental stages. BR, brace root; CR, crown root; LR, lateral root; PR, primary root; SR, seminal root

Figure 2

Schematics showing the emerging lateral root primordium in Arabidopsis and maize (a, b) and crown root primordium in rice (c). The dotted line represents the transverse section through the primary roots of Arabidopsis and maize, and the crown (base of the stem) of rice

Figure 3

Gene regulatory network controlling crown root initiation, meristem specification and maintenance, emergence, and outgrowth in rice. Arrows indicate positive regulatory action, and the flat‐headed arrows indicate negative regulatory action

Figure 4

Root system architecture responses to different environmental conditions. (a) Ideal condition: crown roots make up the bulk of the mature grass root system. (b) Drought: Proliferation of the primary root system branches, with steeper root angles. Crown root growth is arrested to conserve water. (c) Flooding: Shallow root system to avoid growing in deeper areas where oxygen is less available. Ethylene accumulation stimulates additional aerenchyma formation, which allows oxygen transport in roots. In addition, the formation of radial oxygen loss barriers helps to reduce the loss of oxygen transported via the aerenchyma. (d) Salinity: Reduction of root elongation. Formation of suberized barriers for salt exclusion from transport into the shoot. (e) Nutrient availability: Foraging for mobile (e.g., nitrogen) and immobile (e.g., phosphorus) nutrients either through searching far or through exploring local areas by increased branching. (f) Soil compaction: Thickening of primary root and older crown roots facilitates soil penetration but is not sufficient to allow for growth in more compacted areas. Crown roots from newer nodes are thicker from the time of emergence due to increased cortex cell area and are less sensitive to soil compaction. Increased aerenchyma counteracts the effects of hypoxia in roots growing in compacted soil