Calcium-dependent protein kinases in plants: evolution, expression and function.

Calcium-dependent protein kinases (CPKs) are plant proteins that directly bind calcium ions before phosphorylating substrates involved in metabolism, osmosis, hormone response and stress signaling pathways. CPKs are a large multigene family of proteins that are present in all plants studied to date, as well as in protists, oomycetes and green algae, but are not found in animals and fungi. Despite the increasing evidence of the importance of CPKs in developmental and stress responses from various plants, a comprehensive genome-wide analysis of CPKs from algae to higher plants has not been undertaken. This paper describes the evolution of CPKs from green algae to plants using a broadly sampled phylogenetic analysis and demonstrates the functional diversification of CPKs based on expression and functional studies in different plant species. Our findings reveal that CPK sequence diversification into four major groups occurred in parallel with the terrestrial transition of plants. Despite significant expansion of the CPK gene family during evolution from green algae to higher plants, there is a high level of sequence conservation among CPKs in all plant species. This sequence conservation results in very little correlation between CPK evolutionary groupings and functional diversity, making the search for CPK functional orthologs a challenge.