Functional divergence of a trypsin protease underpins phosphate signaling in a diatom

Phosphate availability constrains growth across plant and algal lineages, yet the intracellular regulatory mechanisms linking external phosphate signals to acclimation responses remain incompletely understood. Protease families are highly expanded in photosynthetic eukaryotes, but whether individual protease paralogues have evolved regulatory roles in nutrient signaling is largely unexplored. Here, we identified a trypsin protease, PtTryp8, as a key regulatory component of phosphate acclimation in the diatom Phaeodactylum tricornutum. Loss of PtTryp8 impaired growth and phosphate uptake dynamics. Transcriptome profiling revealed coordinated dysregulation of phosphate-responsive genes, including transporters, signaling components, and membrane trafficking pathways in PtTryp8 mutants. Notably, PtTryp8 disruption selectively altered Ca²⁺-dependent protein kinase expression, consistent with a role in signal integration rather than bulk proteolysis. Comparative analyses indicated that PtTryp8 has diverged functionally from other trypsin paralogues, supporting isozyme specialization within diatom protease families. Together, these findings reveal a previously unrecognized role for proteolytic signaling in phosphate sensing and acclimation and suggest that regulatory diversification of proteases represents a general strategy for nutrient adaptation in photosynthetic eukaryotes.