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Plant Physiol. 1997 Oct;115(2):853-861.

Rapid and Transient Activation of a Myelin Basic Protein Kinase in Tobacco Leaves Treated with Harpin from Erwinia amylovora.

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Department of Plant Pathology, 108 Waters Hall (A.L.A., S.P., M.E.H., A.N.), and Division of Biological Sciences, 308 Tucker Hall (J.M.S., J.C.W.), University of Missouri, Columbia, Missouri 65211.


Harpins are bacterial protein elicitors that induce hypersensitive response-like necrosis when infiltrated into nonhost plants such as tobacco (Nicotiana tabacum L.) (Z.-M. Wei, R.J. Laby, C.H. Zumoff, D.W. Bauer, S.Y. He, A. Collmer, S.V. Beer [1992] Science 257: 85-88). Activity of a 49-kD Mg2+-dependent and Ca2+-independent kinase in tobacco leaves increased 50-fold 15 min after infiltration of harpin from Erwinia amylovora (harpinEa). Much less pronounced and more transient activation was detected in water-infiltrated leaves. Biochemical characteristics of the harpinEa-activated protein kinase (HAPK) activity are consistent with those of the mitogen-activated protein kinase family. HAPK is cytosolic and phosphorylates myelin basic protein on serine/threonine residues. Treatment with a protein tyrosine phosphatase completely eliminated HAPK activity, suggesting that tyrosine phosphorylation is required for posttranslational activation. Sustained HAPK activation after cycloheximide treatment implies that HAPK may be negatively regulated by a translation-dependent mechanism. The extracellular Ca2+ chelator EGTA or the protein kinase inhibitor K252a, infiltrated in planta together with harpinEa, partially blocked HAPK activation. The Ca2+-channel blocker La3+ had no effect on HAPK activation, suggesting that phosphorylation events precede and/or do not depend on the entry of extracellular Ca2+ into the cell. These results suggest that early signal transduction events during harpinEa- induced hypersensitive response elicitation depend in part on the activation of HAPK.

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