![]() Nevertheless, we can imagine that developmental plasticity also poses certain risks. It also allows plants to adjust their form to conditions as they change, as plants are able to selectively self-prune, and to initiate shoot and root growth from many locations within the plant body. This allows otherwise immobile plants to place structures in the optimal position with respect to water, nutrients, and sunlight. What causes the high degree of variability in plant architectures, and how is it tolerated without risking the integrity of the organism? Developmental plasticity provides a wide range of advantages to the plant, allowing it to collect signals and information from its environment and incorporate them into decisions about growth and development. The structure of the root organ itself is very consistent, but the number, placement, and direction of growth of each root in the system is highly variable, even among genetically identical plants. Growth and development of the root system, although poorly understood, has even fewer clear constraints. However, the growth and development of stem and leaf is highly variable, and the axial bud may or may not be triggered to produce phytomers of its own ( Sussex & Kerk 2001). The shoot system develops through production of phytomers (stem, leaf and axial bud). Within certain constraints, the rules during subsequent development seem to be quite loose. During embryogenesis, the apical and basal poles are established and stem cell populations are set aside to direct further growth of the root and shoot systems. The embryonic form provides only a basic blueprint for the ensuing plant. The plant kingdom differs from this model at a fundamental level. Most of the animal body plan is established by the time that embryogenesis is completed. The result is the production of organisms that closely resemble each other – in a larger sense, all mice, and all people for that matter, look very much alike, with similar numbers, placements and shapes of all organs and limbs. Growth and development in the animal kingdom tends to follow predefined, tightly regulated programmes. Finally, we discuss emerging approaches for understanding the regulation of root system architecture. All the pathways affect lateral root formation, but some specifically target initiation of the lateral root, while others target the development and activation of the lateral root primordium, or the elongation of the lateral root. Regulatory pathways are also organized based on their specific developmental effect in the root system. The current literature describing the regulation of root system development is summarized here within this framework. Response pathways co-ordinate environmental cues with development by modulating intrinsic pathways. This review proposes a framework for describing the pathways regulating the development of complex structures such as root systems: intrinsic pathways determine the characteristic architecture of the root system in a given plant species, and define the limits for plasticity in that species. Root systems also provide an optimal system for studying developmental plasticity, a characteristic feature of plant growth. The right architecture in a given environment allows plants to survive periods of water of nutrient deficit, and compete effectively for resources. Root system development is an important agronomic trait.
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