Phosphorous in the form of orthophosphate (Pi) is an essential macronutrient involved in many essential cellular processes and amongst others required by plants in sufficient amounts to keep photosynthesis functioning at an optimal rate1‐3. The concentration of available Pi in soil is extremely low, therefore all plants in their natural environment are always under some degree of Pi deprivation4,5. In periods of Pi limitation the Pi concentration in the cytoplasm drops into the low μM range6,7. The low affinity of these transporters for Pi would, however, not allow them to function in soil grown conditions where phosphate is limiting, which is mostly the case in natural ecosystems. This work was initiated with several key questions relating to Pi homeostasis in plants and in particular how this process is controlled in the various sub–cellular compartments that makes up a cell. The immense complexity of the reaction networks that are involved in maintaining the Pi balance throughout the plant is evident from the large number of publications that are available in this field. Despite the great scientific interest, it is also evident that several key reactions and proteins responsible for them are still a mystery to scientific investigators. It is for instance still unknown how Pi concentrations are sensed by the plant and how many reactions are involved in this sensing. Especially important is the mechanism of Pi sensing involved in the activation of miR399 and IPS/AT4 gene expression controlling phosphate uptake under Pi limiting conditions (1). Other questions still unanswered relate to the substrates of PHO2 and how these substrates mediate the over–expression of some of the high affinity Pi transporters (1). We also do not know the mechanism of transport of the miR399 complexes via the phloem. Despite the large number of Pi transport proteins identified to date, evidence exist to support several other transport activities in plants that yet needs to be elucidated. Unexplained activities identified include, export of Pi from amyloplasts not coupled to the transport of phosphorylated organic compounds (2), import of Pi into the vacuole (3) and the export of Pi from the Golgi, where it is generated as a by–product of glycosylation reactions (4). Several discoveries therefore still need to be made before Pi transport and the process of Pi homeostasis can be fully understood.