A plasma membrane Ca2+-dependent protein kinase PtCDPK2 promotes phosphorus starvation resilience in Phaeodactylum tricornutum

Phosphorus (P) is an essential element limiting algal growth in aquatic ecosystems. Diatoms are abundant microalgae that thrive in nutrient-variable environments. Determining how diatoms regulate responses to P availability is thus crucial for understanding their ecological success. P-limited diatoms use Ca2+-dependent signalling to sense and coordinate responses to phosphate resupply. However, the apparatus enabling Ca2+ signal decoding in diatoms remains poorly understood. Phaeodactylum tricornutum possesses several Ca2+-dependent protein kinases (CDPKs) that are up-regulated by P starvation, although it is unknown whether they act to coordinate P starvation responses and/or transduce Ca2+ signals stimulated by P resupply. Here, we functionally characterized PtCDPK2. We show that PtCDPK2 localizes to the cell periphery, suggesting a role regulating plasma membrane processes. PtCDPK2 is co-expressed with the P starvation response regulator, PtPSR1. Increases in PtCDPK2 are also coordinated with the capacity for P-Ca2+ signalling. Quantification of extracellular phosphate indicates that the activation of P-Ca2+ signalling ability under P starvation is not controlled by external phosphate exhaustion, but decreased cellular P quota. Finally, Ptcdpk2 mutants have significantly reduced Fv/Fm and alkaline phosphatase activity under P starvation, indicating that they are impaired in their ability to cope with P depletion. Together, our findings demonstrate that Ca2+ signalling processes contribute to diatom tolerance to P limitation, alongside their established role mediating P resupply responses.