Phenotypic plasticity is the ability of an individual organism to alter its behavior, morphology and physiology in response to changes in environmental conditions. Root phenotypic plasticity enables plants to adapt to an array of biotic and abiotic constraints that limit plant productivity. 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. Studying how plants adapt their root architecture to abiotic and biotic stressors can give us insights on how to increase food production. This is extremely important since we project our population to gain 2.3 billion people by the year 2050, which will require an increase in food production by 25–70%. Malnutrition afflicts approximately 795 million people, particularly in sub-Saharan Africa where one in four people are malnourished. Research on root architecture plasticity of staple crops may help us develop cultivars with increased capacity for soil resource acquisition. Hence, help us improve crop yields in marginal environments.
Root architecture 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. Root elongation requires polarized auxin to transport and create an auxin maximum at the very tip of the LRP. In order for this activation to occur, auxin influx and efflux activity must be regulated within the plant cell. Lateral root developmental strategies indicated adaptations of the root system to different environmental niches.
Root phenotypic plasticity of Phaseolus vulgaris L.
The Phaseolus vulgaris L, also known as the common bean, is native to the Americas. It is a principal food crop in tropical and subtropical regions, where its production is often limited by low P availability. Phosphorus is generally more available near the surface strata due to deposition of plant residues at the soil surface and low mobility in the soil. Therefore, root phenes that increase the exploration of surface strata have higher probability of capturing P. Studies on the common bean have observed longer and denser lateral branching in beans under low P conditions. Genotypic markers responsible for P efficiency are highly correlated to plants with highly branched root systems and a large number of apices. Recent bean studies support that architectural differences could widely influence a plant's efficiency of Phosphorus acquisition.
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