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

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, apparatus enabling Ca2+-signal decoding in diatoms remains poorly understood. Phaeodactylum tricornutum possesses several Ca2+-dependent protein kinases (CDPKs) that are upregulated 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 characterised PtCDPK2. We show that PtCDPK2 localises 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.