Pi uptake and distribution
The limited provision of P has led to the evolution of various adaptations in plants to satisfy requirements: Over 90% of plant species form mycorrhizal symbioses that improve acquisition of scarce minerals (Figure 1b). The fungal hyphae can deliver up to 80% of plant P albeit at a considerable cost to the plant in the form of reduced carbon [11• and 12]. Several plant species form socalled protoid roots in the case of P deficiency. These dense clusters of fine lateral roots not only increase the invaded soil volume but also extrude large amounts of chelators in the form of organic acids to dissolve the sparingly soluble calcium phosphates.
Expression of high and low affinity Pi transporters greatly depends on P supply [13 and 2]. The presence of mycorrhizal associations also impacts on expression of specific transporters [14]. Vacuolar sequestration of P is particularly important during seed development when large amounts of P and other minerals are stored in complex myo-inositol salts such as phytate. These are contained in globoid inclusion bodies within seed protein storage vacuoles and hence require two transmembrane transport steps.