Genetic approaches for manipulating crop plants to enhance root exudation and access low nutrients in the rhizosphere

Opportunities exist for promoting plant nutrient uptake through genetic manipulation of rhizosphere processes. An assessment of the genotypic differences in plants such as in the excretion of nod-gene inducers by symbiotic legumes, the release of protons and phytosiderophores by crop varieties suffering from iron (Fe) and zinc (Zn) deficiencies, the extrusion of organic acid anions under aluminium (Al) and phosphorus (P) stress, and the release of root border cells and/or enzymes (phosphatases and phytases) concerned with P nutrition, is essential for identifying crop cultivars adapted to low-nutrient conditions. Approaches for creating nutrient-efficient crops suitable for low-nutrient environments include: (1) the screening of adapted landraces for a better understanding of the genetic basis of root exudation, (2) the selection of plants with enhanced symbiotic performance when infected with stem/root nodule bacteria (rhizobia) and/or vesicular arbuscular (VA) mycorrhizae, (3) increased experimentation on ion-channels, transporter genes and the promoters controlling their expression, and (4) the use of molecular markers to select useful plant traits. Additionally, we need to understand the operational relationship of these various genes to other processes involved with mineral nutrition, and to integrate the latter with such whole-plant processes as photosynthesis in order to obtain higher crop yields in low-nutrient environments. But the real challenge with these low-nutrient environments lies in the production of food crops with clinically acceptable levels of nutrients and vitamins that, when consumed by humans, do not cause malnutrition. Breeding for micro-nutrient enriched crops could be one way to produce foods that meet the dietary requirements of human nutrition. This summary presents the tools currently available for manipulating rhizosphere processes and plant genes for increased nutrient uptake and assimilation in low-nutrient environments.