A phosphate-sensing organelle regulates phosphate and tissue homeostasis.

Inorganic phosphate (P i ) is one of the essential molecules for life. However, little is known about intracellular P i metabolism and signalling in animal tissues 1 . Following the observation that chronic P i starvation causes hyperproliferation in the digestive epithelium of Drosophila melanogaster, we determined that P i starvation triggers the downregulation of the P i transporter PXo. In line with P i starvation, PXo deficiency caused midgut hyperproliferation. Interestingly, immunostaining and ultrastructural analyses showed that PXo specifically marks non-canonical multilamellar organelles (PXo bodies). Further, by P i imaging with a Förster resonance energy transfer (FRET)-based P i sensor 2 , we found that PXo restricts cytosolic P i levels. PXo bodies require PXo for biogenesis and undergo degradation following P i starvation. Proteomic and lipidomic characterization of PXo bodies unveiled their distinct feature as an intracellular P i reserve. Therefore, P i starvation triggers PXo downregulation and PXo body degradation as a compensatory mechanism to increase cytosolic P i . Finally, we identified connector of kinase to AP-1 (Cka), a component of the STRIPAK complex and JNK signalling 3 , as the mediator of PXo knockdown- or P i starvation-induced hyperproliferation. Altogether, our study uncovers PXo bodies as a critical regulator of cytosolic P i levels and identifies a P i -dependent PXo-Cka-JNK signalling cascade controlling tissue homeostasis.