2. Introduction
Grain cereals are an essential food supply, representing more than 78% of daily human calories (FAO Stat). The use of grains as a food source began already during the Middle Stone Age, long before cereal domestication. With the growing world population, the global food demand is also increasing. In the present context of climate change and intensive agriculture that leads to soil degradation, higher desertification and salinization of cultivated areas are expected in the very near future. In this regard, the sustainability of cereal yield has become a challenge for food security. To safeguard environmental quality, usage of fertilizers and watering has to be reduced. One has to exploit the intrinsic ability of plants to respond and adapt to adverse conditions, notably to a temporary period of drought/flooding or suboptimal nutrient supplies. Historically, research in the field has focused on the aboveground plant parts, neglecting the underground organs. This hidden status renders difficult access to intact root systems for analysis, particularly field experimentation. In the last decades, methods to study roots have evolved from destructive to non-destructive imaging techniques.
Roots play critical plant functions, serving not only for anchorage but also for water and nutrient uptake and transport, storage, communication and interaction with the soil microbiome and other plants. The root system architecture (RSA) describes the 3D organization of the roots in the soil and refers to root morphology, topology and distribution. The RSA is shaped by the combination of the genetic background, the availability of water and nutrients whose distribution is heterogeneous in the soil, and the stress response. Among nutrients, nitrate (N) and phosphorus (P) probably have the highest impact on plant growth and crop yield. Whereas N is highly soluble and will leak in the deep soil strata, P is quickly immobilized in the surface soil layers. Therefore, deep rooting will ensure N acquisition from deep soil layers, whereas a shallow RSA enables plants to use P accumulated in the topsoil. RSA reflects the soil volume that plants can explore. Its description includes branch number, branching pattern, length, orientation, angle or deepness, root diameter, and surface area. Several ideotypes have been described in the literature.
In cereal crops, the root system consists of the embryonically formed primary and seminal roots and the post-embryonically developed nodal roots. These latest arise from the consecutive shoot nodes below the ground (referred to as ”crown”) and are called crown roots. In adult cereal plants, crown roots form the entire rootstock, resulting in the characteristic fibrous root system. In maize, another type of nodal roots develops from aboveground; they are called brace roots and serve as anchorage. Both embryonically and post-embryonically formed roots share the ability to produce highly branched lateral roots.
In recent years, the genetics of RSA has been revealed while using mutants in different crops species affected in their RSA. Using quantitative genetic and transcriptomic approaches, quantitative trait loci (QTL) and genes involved in RSA in crops have been discovered. Although lateral and crown roots have different origins (root-to-root and shoot-to-root, respectively), conserved molecular mechanisms have been described. In rice, a detailed picture of the genes involved in root elongation and lateral and crown root initiation and emergence has appeared in the last few years. Phytohormones have been described for their role in the process, with auxin being the central actor. Approaches to improve cereals’ RSA include classical transgenic manipulation, either overexpression or silencing, of genes involved in phytohormone synthesis and signalling. However, pleiotropic effects on the overall plant development were reported. This highlighted the need to improve the precision of transgene expression. With the new area of CRISPR-mediated gene editing, one can expect that this goal will be reached.