ROOT SYSTEM ARCHITECTURE (RSA)
Similar to shoot traits, root traits are also important for maintaining
crop yield (Ogura et al., 2019; Uga et al., 2013) and its stability
(Sandhu et al., 2016) under moisture stress conditions. Drought stress
leads to changes in root structure to improve uptake of water and
nutrients from soil. For example, when there is heterogenous
distribution of moisture in soil profile, roots are known to exhibit
hydro-patterning, where auxin signaling facilitates distribution of
lateral roots towards zone of higher water content (Orosa-Puente et al.,
2018). In maize, root growth have been shown to play a significant role
in partitioning of lateral roots in order to extract more water from
depth (Robbins & Dinneny, 2018). Modification of root system has been
shown to increase drought tolerance in plants (Vivek et al., 2017). Deep
root systems confers high drought tolerance to plants by increasing
mineral and water uptake (Mahmood et al., 2022).
Plant root systems have their own complexity which is crucial to their
function. Plant root systems can be categorized into four different
types: (1) tap roots (coarse roots), first roots to emerge from seeds
providing anchorage, deciding root depth and overall root system
architecture (RSA), controlling overall penetration into soil layers
(Henry et al., 2011; Wasaya et al., 2018), (2) lateral roots (fine
roots), comprising majority of the total root system playing most active
role in water uptake (Comas et al., 2013; Rewald et al., 2011) (3) basal
roots, which originate from hypocotyl (Weinhold 1967), and (4)
shoot-borne roots, are the ones that originate from shoot tissues (Esau
1965).
Studies on RSA are usually done at macro and microscale; at macroscale
studies include occurrence of primary and lateral roots, their
organization, and role in uptake of water and nutrient while at
microscale studies include role of root hairs in increasing surface area
and their assistance in uptake of water and nutrients (Bates & Lynch,
2000; Smith & De Smet, 2012). Generally, roots show plasticity in their
architecture as a response to environmental factors such as availability
of water and nutrient, soil salinity soil temperature, soil density and
microorganism interactions (Smith and Smet 2012).
In both dicots and monocots primary roots are the first to arise from
the embryonically formed meristematic tissues. At the tip of both mature
and primary roots, a zone of meristematic cells (a.k.a root apical
meristem) is present which keeps on dividing; giving rise to other cell
types in root. Root consists of three zones in both monocot and dicot:
distal root apex zone, consisting of root apical meristem (RAM);
elongation zone; and differentiation zone (Jovanovic et al., 2007). A
protective layer of tissues is also present at the tip of roots in both
monocots and dicots known as root cap which plays a crucial role in root
gravitropic responses. In the center of root cap, columella cells are
present and some columella cells (statocytes), are occupied by
specialized starch containing amyloplasts. Amyloplasts on perceiving the
gravity stimulus, sediments on cell boundary. This abundance of
amyloplasts triggers changes in auxin transport which ultimately leads
to various growth responses including bending of roots towards stimulus
of gravity (Smith & De Smet, 2012; Swarup et al., 2005) (Figure
2).