Plasma Membrane Perturbation in Association with Calcium Ion Movement Followed by Fungal Elicitor-stimulated Oxidative Burst and Defense Gene Activation in Potato Tuber

Chlortetracycline (CTC) is known to fluoresce when interacting with Ca2+ in hydrophobic environments such as membranes. CTC fluorescence in potato tubers was rapidly reduced by hyphal wall component-elicitor (HWC-elicitor) from Phytophthora infestans in a dose-dependent manner, suggesting that Ca2+ in the plasma membrane was released by the elicitor. Pre-treatments of the tuber slices with verapamil and EGTA, a Ca2+ channel blocker and an extracellular Ca2+ chelator, respectively, suppressed the reduction of CTC fluorescence induced by the HWC-elicitor. The reduced fluorescence was preceded by an oxidative burst observed in the tuber tissues loaded with luminol after stimulation with HWC-elicitor. Application with HWC-elicitor also immediately reduced fluorescence of the CTC-loaded, Ca2+-bound plasma membrane fraction in vitro. Moreover, RNA gel blot analysis showed that verapamil and EGTA dramatically suppressed the accumulation of transcript for phenylalanine ammonia-lyase induced by the elicitor. These results demonstrate that extracellular Ca2+ is rapidly released from the plasma membrane by the treatment with HWC-elicitor and plays a key role in signal transduction leading to the oxidative burst and the activation of a defense gene. (Received February 5, 1999; Accepted May 13, 1999)