Conservation of an immune homeostasis module in land plants

Calcium-dependent protein kinases (CDPKs or CPKs) are a unique family of Ca2+-regulated kinases with diverse functions in plants. CPK28 regulates immune homeostasis, stress responses, and growth in multiple angiosperms including tomato, rice, cotton, and Arabidopsis thaliana (hereafter, Arabidopsis). In Arabidopsis, CPK28 phosphorylates and activates the E3 ubiquitin ligases PLANT U-BOX 25 (PUB25) and PUB26 that target the major immune signaling protein BOTRYTIS-INDUCED KINASE 1 (BIK1), resulting in its turnover. The CPK28-PUB25/26-BIK1 regulatory module maintains precise levels of BIK1 in the cell and is thought to optimize immune responses. Separated from angiosperms by 450M years of evolution, the liverwort Marchantia polymorpha (hereafter, Marchantia) has emerged as a model system to study the evolution of signaling modules across land plants. Here, we demonstrate that the function of CPK28 is conserved in Marchantia. MpCPK28 displays Ca2+-dependent protein kinase activity and is inhibited by calmodulin in vitro. Over-expression of MpCPK28 results in pronounced developmental phenotypes and decreased chitin-induced oxidative burst, and MpCPK28 can functionally complement the mutant phenotypes of the Arabidopsis cpk28-1 mutant. MpCPK28 associates with and phosphorylates multiple residues on Marchantia AVRPPHB SUSCEPTIBLE 1 (PBS1)-like (PBL)-a (MpPBLa), a functional ortholog of AtBIK1, as well as MpPUB20e (a putative ortholog of AtPUB25/26). MpPBLa undergoes proteasomal degradation in Marchantia and can be polyubiquitinated by MpPUB20e. The occurrence of the CPK28-PUB25/26-BIK1 module across land plants reveals that the regulation of immune amplitude has been conserved throughout plant evolution.