A Phytotoxin Solanapyrone-A Downregulates Calcium-dependent Protein Kinase Activity in Potato

We previously demonstrated that alternaric acid, a host-specific toxin produced by the plant pathogenic fungus Alternaria solani, in the presence of Ca(2+) and Mg(2+), stimulated in vitro phosphorylation of His-tagged calcium-dependent protein kinase 2 from potato cultivar Rishiri (RiCDPK2). Herein, we report that Solanapyrone-A (SpA), a non-host-specific toxin produced by A. solani, inhibited the phosphorylation of RiCDPK2 in the presence of Ca(2+) and Mg(2+). However, SpA stimulated RiCDPK2 phosphorylation in the absence of these cations. Based on the current findings, we suggest that RiCDPK2 may mediate SpA-induced signaling independent of Ca(2+) and Mg(2+), leading to a compatible interaction between potato and A. solani.