The morphological, symbiotic and molecular physiological below-ground adaptations for nutrient scarce environments: A South African perspective

Nutrient-poor ecosystems are classified as those where available nutrients have been degraded, leached or removed from the system. The most important contributor to soil nutrient profiles is soil age, which is determined by glacial events, tectonic activity and natural disasters. In ancient and weathered soils, these events result in the limitation of both nitrogen and phosphorus, coincidentally the two primary nutrients that limit plant growth and function. Using data collected from primary literature sources, the main focus of this review was to contextualise the physiological function and mechanistic workings of three major below-ground adaptations in low-nutrient environments. This is further explored in relation to the adaptations used by species found in the nutrient-poor fynbos biome. Consequently, plants growing on such soils have had to evolve adaptive mechanisms that allow for survival in these nutrient-poor environments. These adaptations have allowed fynbos species to thrive in these nutrient-poor ecosystems and are divided into two broad terms, both of which are specialised for the acquisition of either P or N or both. Symbiotic root-structures including mycorrhizal fungi (MF) and the formation of nodules in the family Legumaceae. Nodules are an infected area of the root system where N-fixing rhizobia are housed. The other symbiotic root-structure, MF, allows for extensive development of plant-fungal root associations that significantly increase the absorptive area for plant roots. The non-symbiotic root-structures include the proteiod roots or cluster roots of several plant families, including the Proteaceae, that are adaptive ecological traits to assist with mineral acquisition in nutrient-poor environments. Cluster roots are an evolutionary adaptation that combines metabolic alterations with morphological changes to increase the plants acquisition of P. Our understanding of these systems in relation to fynbos species has mainly been from a physiological context but the application of multi-omics approaches in the future will provide new insights into the regulatory mechanisms that control adaptations to nutrient scarce environments.