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).