1.2.40
RELATIONSHIP BETWEEN THE RESISTANCE REACTION INDUCED BY RICE BLAST FUNGUS AND THE CALCIUM SIGNALLING IN RICE LEAVES

A HIGA, N SUGA, Y MINAI, Y MATSUOKA and M HAGA

Department of Agricultural Chemistry, Faculty of Agriculture, Tamagawa University, Machida-shi, Tokyo 194, Japan

Background and objectives
It is not clear that calcium signalling involves signal transduction in the defence response induced by plant-pathogen interactions. In this study, in order to analyse signal transduction induced by infection in rice leaf cells, the dynamics of cytoplasmic calcium concentration [Ca2+]cyt was measured.

Results and conclusions
The transmembrane signalling system was operated by elicitor stimulation, and this system resulted in activation of phospholipase C (PLC) and calmodulin [1, 2]. Superoxide anion (O2-) generation and [Ca2+]cyt of rice leaf protoplasts stimulated by elicitor were measured under conditions of IAA application. O2- generation and [Ca2+]cyt were markedly enhanced by IAA pretreatment. It is shown that IAA functions as a priming effector. Decreasing O2- and [Ca2+]cyt following receptor downregulation were observed in elicitor pre-stimulated and IAA-treated protoplasts. These results suggest that IAA is related to recognize elicitor in rice plants. In order to establish involvement of inositol phospholipid turnover (PI turnover) in the signal transduction pathway, O2- and [Ca2+]cyt were measured in the presence of PLC and PI turnover inhibitors. O2- and [Ca2+]cyt were significantly inhibited by these reagents. These results show that PI turnover acting Ca2+ release exist in signal transduction pathway.

In order to demonstrate the Ca2+ transport mechanism, [Ca2+]cyt and cytoplasmic pH were analysed by the channel blockers. Increasing [Ca2+]cyt was suppressed by the channel blockers. It is indicated that Ca2+ influx is mediated to the Ca2+ channel. Cytoplasmic pH was changed from 5.3 to 6.1 by elicitor stimulation. Changed cytoplasmic pH by elicitor stimulation was followed to Ca2+ release, indicating the existence of Ca2+/H+ antiporter. ATPase activity involving Ca2+ pumping was found in the membrane fraction. Optimum temperature, pH and Km values for this enzyme are 35C, 6.0 and 1.6 mM, respectively. It is suggested that ATPase activity was enhanced in this pH range for pumping out cytoplasmic Ca2+. These results indicate the relation of calcium signalling to the rice plant signal transduction pathway.

References
1. Kanoh H, Haga M, lwata M, Sekizawa Y, 1993. Journal of Pesticide Science 18, 299-308.
2. Kanoh H, Haga M, Sekizawa Y, 1993. Journal of Pesticide Science 1 8, 325-332.