Microbial elicitors induce activation and dual phosphorylation of the Arabidopsis thaliana MAPK 6

Protein kinases related to the family of mitogen-activated kinases (MAPKs) have been established as signal transduction components in a variety of pro cesses in plants. For Arabidopsis thaliana, however, although one of the ge netically best studied plant species, biochemical data on activation of mit ogen-activated protein kinases are lacking. A. thaliana MAPK 6 (AtMPK6) is the Arabidopsis orthologue of a tobacco MAPK termed salicylate-induced prot ein kinase, which is activated by general and race-specific elicitors as we ll as by physical stress. Using a C terminus-specific antibody, we show tha t AtMPK6 is activated in elicitor-treated cell cultures of A. thaliana. Fou r different elicitors from bacteria, fungi, and plants lead to a rapid and transient activation of AtMPK6, indicating a conserved signaling pathway. T he induction was equally rapid as medium alkalinization, one of the earlies t elicitor response observed in cell cultures. A similarly rapid activation of AtMPK6 was observed in elicitor-treated leaf strips, demonstrating that recognition of the elicitors and activation of the MAPK pathway occurs als o in intact plants. We demonstrate by in vivo labeling that AtMPK6 is phosp horylated on threonine and tyrosine residues in elicited cells.