The good, the bad, and the phosphate: regulation of beneficial and detrimental plant–microbe interactions by the plant phosphate status

Summary Phosphate (P i ) is indispensable for life on this planet. However, for sessile land plants it is poorly accessible. Therefore, plants have developed a variety of strategies for enhanced acquisition and recycling of P i . The mechanisms to cope with P i limitation as well as direct uptake of P i from the substrate via the root epidermis are regulated by a conserved P i starvation response (PSR) system based on a family of key transcription factors (TFs) and their inhibitors. Furthermore, plants obtain P i indirectly through symbiosis with mycorrhiza fungi, which employ their extensive hyphal network to drastically increase the soil volume that can be explored by plants for P i . Besides mycorrhizal symbiosis, there is also a variety of other interactions with epiphytic, endophytic, and rhizospheric microbes that can indirectly or directly influence plant P i uptake. It was recently discovered that the PSR pathway is involved in the regulation of genes that promote formation and maintenance of AM symbiosis. Furthermore, the PSR system influences plant immunity and can also be a target of microbial manipulation. It is known for decades that the nutritional status of plants influences the outcome of plant–microbe interactions. The first molecular explanations for these observations are now emerging.