The oxidative burst constitutes one of the most rapid defence responses characterized in the Plant Kingdom. We have observed that four distinct elicitors of the soya bean oxidative burst activate kinases of masses ≈44kDa and ≈47kDa. Evidence that these kinases regulate production of reactive oxygen species include: (i) their rapid activation by oxidative burst elicitors, (ii) their tight temporal correlation between activation/deactivation of the kinases and activation/deactivation of the oxidative burst, (iii) the identical pharmacological profile of kinase activation and oxidant production for 13 commonly used inhibitors, and (iv) the autologous activation of both kinases and oxidant production by calyculin A and cantharidin, two phosphatase inhibitors. Immunological and biochemical studies reveal that the activated 44kDa and 47kDa kinases are mitogen-activated protein (MAP) kinase family members. The kinases prefer myelin basic protein as a substrate, and they phosphorylate primarily on threonine residues. The kinases are themselves phosphorylated on tyrosine residues, and this phosphorylation is required for activity. Finally, both kinases are recognized by an antibody against activated MAP kinase immediately after (but not before) cell stimulation by elicitors. Based on these and other observations, a preliminary sequence of signalling steps linking elicitor stimulation, kinase activation and Ca2+ entry, to initiation of oxidant production, is proposed.
Present address: Chemistry Department, Wabash College, P.O. Box 352, Crawfordsville, IN 47933, U.S.A.
The first two authors contributed equally to this work.