The colonization of plants in the terrestrial land occurred around 450 to 490 Mya, which obligates several adaptations, including the modification and formation of specialized structures or plant organs. The roots are multicellular organs of plants that comprise of gravitropic response, root hairs, endogenous branching, and a protective root cap. There are two major types of root systems categorized as the taproot system and the fibrous root system. The plant’s root has been evolved in order to perform a variety of processes which include water and nutrient uptake, mechanical support and anchoring, and storage functions. The root system acts as a major connection between the plant-soil interface involving various biotic and abiotic factors. Thus, roots are an essential part of the plants for their survival and growth. Recently a variety of new traits has been identified that was developed as a result of adaptations of complex root systems. The root systems in plants contain a set of characteristics or distinguishable units of the plant phenotype, called phenes, or traits. The root growth, structure, and adaptation to the habitat influence the plant growth and yield. The adaptations of the root system depend on several factors that include soil type, availability of nutrients, water availability, amount of light, and climatic conditions of the environment. The root system of plants in the temperate climate region has adapted a long taproot system and smaller root hair structures known as rhizomes. Such that the plants that grow in tropical humid climates have adapted several root structures such as root and stem tubers, fusion tubers, and lomentaceous tubers. The root system of plants in savannas has adapted to digitate root tubers and caudex.
The uptake of food and water is the major function of the root system of the plants. There are two types of plants categorized on the basis of their mode of obtaining the food namely autotrophic and heterotrophic. Autotrophic organisms produce their food from sunlight through photosynthesis, while heterotrophic organisms were not able to produce their food and depend on onother plants for their food. They may be either totally parasitic or partially parasitic. The plants also depend on the microbial partners in the process of nutritional uptake. Some species of bacteria and fungi evolved that play a symbiotic relationship with the root system of plants where they are associated with the plants. The symbiotic relationship between the plants and the microbes improves the uptake of nutrients for both the organisms and hence it is mutualistic or symbiotic. The nutritional adaptations of the root system to symbiosis resulted in the formation of specialized structures including nodules in legume plants and mycorrhization in plants with fungal symbionts. Nitrogen is an essential macronutrient required for the growth and development of the plant. The atmospheric nitrogen is the easily available nitrogen pool for the plants in the terrestrial ecosystem. However, the plants cannot directly utilize the atmospheric nitrogen present in diatomic form, to the lack of enzymes required to convert the nitrogen to a biologically usable form. The biological conversion of diatomic nitrogen to ammonia, a useful form of nitrogen by the plants, is performed by the symbiotic bacteria present in the root nodules of the legume plants. The bacteria present in the soil-root interface is collectively known as rhizobia that interact with the plants to form a specialized structure called nodules where the process of nitrogen fixation occurs using the enzyme nitrogenase. Simultaneously, the process of nitrogen fixation underwrites the soil fertility as the root system of the plant leaves behind a certain amount of the available nitrogen. The plants obtain ammonia from the nitrogen fixed by the bacteria and in return, the bacteria get the carbon compounds synthesized by plants through photosynthesis, and the root system exist as a protected niche for the bacteria. Like in symbiosis with bacteria, most plants are symbiotic with some fungi that facilitate the uptake of minerals from the soil. The symbiotic association formed by the fungi is called mycorrhizae where the fungi are integrated into the physical structures of the root system during the active plant growth. The hyphae of mycorrhizae allow the plant in obtaining phosphate, zinc, and copper from the soil. In addition to the absorption of minerals, the mycorrhizae also aid in the host defense mechanism and function as a physical barrier to pathogens. Mycorrhizae are two types, namelyectomycorrhizae, and the endomycorrhizal.
