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.