Phospholipid-Activated Protein Kinase in Plants: Coupled to Phospholipase A₂?

Abstract

The search for second messenger systems in signal transduction mechanisms in cells of higher plants has always strongly been influenced by the concepts which had been developped for animal cells. To date, however, the only molecular signal transduction pathway providing a valid working hypothesis for signal transduction in plant cells is the phosphatidylinositol (PI) cycle which, in animals, produces two second messengers, myo-inosito1,1,4,5-trisphosphate (IP₃) and diaclyglycerol (DAG) (Berridge 1987; Morse et al. 1989). The function of one of it, IP₃, is likely to be the release of Ca²⁺ from the vacuole and the ER (Schuhmaker and Sze 1987) which would be similar to its function in animal cells. The function of DAG remains unclear because evidence for a DAG-activated protein kinase in plants is still scarce and controversial (Morré et al. 1984; Elliott et al. 1988; Martiny-Baron and Scherer 1989). Our own work originated from our interest in the mechanism of proton transport stimulation in zucchini microsolnes by the platelet-activating factor (PAF) (Scherer 1985) and by auxin (Scherer and Morré 1978; Scherer 1981). The at first bizarre observation of transport stimulation of plant H⁺-ATPase by an animal lipid