Abstract
Mitogen-activated protein kinase (MAPK) cascades play an important role in plant immunity. To investigate downstream signaling of StMPK1 (potato ortholog of SIPK), we isolated several PPSs (protein phosphorylated by StMPK1) using in vitro expression cloning method. To dissect the biological function of PPSs in plant immunity, we employed virus-induced gene silencing (VIGS) in Nicotiana benthamiana. VIGS of NbWRKY8 (NbPPS8) enhanced disease susceptibility to a virulent strain of Phytophthora infestans. MAPK-mediated phosphorylation of NbWRKY8 promoted the binding activity to W-box sequence. By using alanine scanning mutagenesis, we identified 5 serine residues in N-terminal region of NbWRKY8 as potential phosphorylation sites. Expression of NbWRKY8DDDDD, a gain-of-function NbWRKY8 mutant that mimics the phosphorylated form of NbWRKY8, highly induced the expression of NADP-ME, the target gene of NbWRKY8, compared with wild-type NbWRKY8. These results suggest NbWRKY8 is phosphorylated and activated by MAPK and induces expression of downstream genes involved in the defense responses.
