Root phenotypic plasticity

Root architecture refers to the spatial configuration of a plant's root system. The root architecture plays an important role in acquiring a secure supply of water and nutrients, as the acquisition of these resources drives plant growth. In addition to nutrient absorption, the root architecture provides a plant with anchorage and support. Root systems are considered to be very diverse, showing variation among species, genotypes of a given species and even within a single root system. We can classify the architecture of a root system using the following metrics: Branch magnitude (number of links), Topology (branching pattern), Link length (distance between branches), Root angle (radial angle between a branch and its parent root ) and the Link radius (root diameter). == Root phenes ==

Phenes are the fundamental units of the phenotype that are both unique and elementary to a given level of biological organization. A phene state is the outcome of complex synergistic developmental systems, influenced by genes and gene products, as well as the environment. Root architectural and anatomical phenes determine the temporal and spatial distribution of root foraging in specific soil domains, and therefore affect resource capture. Mobile resources, such as nitrate and water are generally found in deeper soil domains over time due to crop uptake, evaporation, and leaching throughout the growth season. Whereas, immobile soil nutrients, including phosphorus and potassium, are more available in the topsoil. In a heterogenous matrix of soil, plants that are able to acquire edaphic resources at reduced metabolic cost will be favored, as these plants can allocate more resources towards growth, continued soil resource and reproduction. == Root plasticity ==

Root phenotypic plasticity enables plants to adapt to an array of biotic and abiotic constraints that limit plant productivity. According to Lynch 2018, Phosphorus (P), nitrogen (N), and water are the three principal resources most often limiting plant growth. Given that soil resources may be unevenly distributed, or subject to local depletion, a plant's ability to adapt to spatiotemporal changes in their environment can provide a fitness advantage over others. In particular, plants can adjust root phenotype by 1) changing their investment of biomass in shoots and roots on an individual level, 2) change their architecture on an organ level, or 3) modify their root anatomy on a module-level. Even though the exploitation of soil resources through root activity is energetically costly, natural selection favors plants that can direct root activity to exploit efficiently the heterogeneous distribution of soil resources. Auxin's role in root plasticity The hormone that is found in multiple steps of lateral root development is called auxin. Auxin appears in the initiation, emergence and elongation of the roots. Auxin signals regulate the direction auxin efflux and auxin flow throughout the cell. This regulation is what directs and aids in lateral root development. Lateral root initiation requires auxin signaling and protein degradation in order to activate through a series of enzymes and protein interactions. With many well characterized cell divisions it gives rise to lateral root emergence. Auxin signaling and the activity of the PUCHI gene interact by PUCHI gene encoding a transcription factor that is unregulated by auxin. Finally, to quantify deep rooting capacity of the seedlings, they assessed the effects of root mass fraction (RMF), taproot mass fraction (TRMF), and specific taproot length (STRL) on relative root depth (RRD). The researchers' findings suggest that drought conditions caused a dramatic decrease in the biomass of different plant parts, total root length, the average length of lateral roots and distal roots, and taproot length. The results showed that RMF and TRMF increased markedly with increasing drought severity, while STRL did not significantly increase under drought condition. Which signified that drought stress had a positive effect on RRD. Under moderate drought conditions, root architectural changes exerted a predominant effect on increased RRD, but under severe drought, root-shoot allocation and root architecture played equally important roles. These results tell us that the root architecture adjustments being made and root-shoot allocation caused deep rooting in P. euphratica seedlings under high drought conditions. Morphological changes seemed to only play a small role. They concluded that P. euphratica seedlings rely mostly on the adjustment of root architecture to maintain root depth under moderate drought conditions. However, they found that root-shoot allocation responds more under severe drought conditions. == References ==