Abstract
Background: Calcium-dependent protein kinases (CDPKs) are crucial sensors and serine/threonine protein kinases in plants, involved in growth, development, and stress responses. However, research on CDPKs in a distinct eukaryotic lineage, such as brown algae, is scarce. Given its independent taxonomic position, brown algae CDPKs might exhibit unique structural and functional characteristics in ecological adaptation. Results: The CDPK family in brown algae was examined on a genome-wide scale in this study. A total of 16 SjCDPK genes were identified in the Saccharina japonica genome, and their phylogenetic relationships, conserved domains, gene structures, chromosomal locations, and cis-regulatory elements were analyzed. Phylogenetic analysis showed that brown algae CDPKs evolved independently from plants. Some SjCDPKs possessed unique structural features and clustered into a specific group. Gene duplication and synteny analysis revealed CDPKs in brown algae are evolutionarily conserved and their evolution exhibits a relatively stable pattern. Moreover, RNA-seq analysis revealed that the transcriptional levels of the majority of SjCDPK genes exhibited notable changes across different life cycle stages, blade tissues, and in response to Cu2+ exposure and pathogen infection. Under high-light stress, SjCDPK genes exhibited a similar expression pattern, reaching the highest expression levels at 6 h after exposure. The subcellular localization result showed that SjCDPK4 was primarily localized in the cytoplasm, while SjCDPK5 was mainly localized in the nucleus. Conclusion: The identification and characterization of SjCDPKs, along with comparative analyses across brown algae, diatoms, and plants, reveal a unique evolutionary trajectory tailored to the ecological niche of brown algae. Expression analysis demonstrates their involvement in gametophyte and sporophyte development, as well as responses to light, copper stress, and pathogen challenges. The different subcellular compartmentalization of SjCDPK4 and SjCDPK5 suggests their distinct functional specializations. These systematic analyses provide insights into the evolution of brown algal CDPKs and are valuable for functional identification of SjCDPKs.