Gene Activation by Cytoplasmic Acidification in Suspension-Cultured Rice Cells in Response to the Potent Elicitor, N-Acetylchitoheptaose

N-Acetylchitoheptaose strongly induces a set of defense reactions in suspension-cultured rice cells including cytoplasmic acidification (K. Kuchitsu, Y. Yazaki, K. Sakano, and N. Shibuya, Plant Cell Physiol. 38:1012-1018, 1997) and the accumulation of mRNAs for two rapidly activated genes, EL2 and EL3 (E. Minami, K. Kuchitsu, D.-Y. He, H. Kouchi, N. Midoh, Y. Ohtsuki, and N. Shibuya, Plant Cell Physiol. 37:563-567, 1996), as well as phenylalanine ammonia-lyase (PAL), chitinase and β-1,3-glucanase. Treatment of cells with propionic acid resulted in the accumulation of the mRNAs for EL2, EL3, and PAL in a manner similar to the accumulation induced by N-acetylchitoheptaose. Concomitantly, there was a rapid decrease in the cytoplasmic pH as detected with in vivo 31P-nuclear magnetic resonance (NMR) spectroscopy. Interestingly, K-252a, a potent inhibitor of Ser/Thr protein kinases, strongly inhibited gene induction by N-acetylchitoheptaose, but showed much less inhibition of gene induction caused by propionic acid. Calyculin A, a protein phosphatase inhibitor, induced mRNA accumulations for EL2, EL3 and PAL, with concomitant acidification of the cytoplasm. On the other hand, chitinase and β-glucanase mRNA levels did not change after addition of propionic acid or calyculin A. Treatment of the cells with propionic acid did not induce the production of reactive oxygen species. These results strongly suggest that cytoplasmic acidification at the early stage of elicitor action could be a key step in the signal transduction events leading to the expression of elicitor-responsive genes. A hypothetical model of elicitor signal pathway is proposed based on these results.