STUDIES ON ELICITOR-SIGNAL TRANSDUCTION LEADING TO DIFFERENTIAL EXPRESSION OF DEFENSE GENES IN CULTURED TOBACCO CELLS

To elucidate the mechanism of elicitor-inducible gene expression, we investigated the induction of the accumulation of mRNAs transcribed from defense genes that encode class I basic chitinase (BCHN) and class II acidic chitinase (ACHN), class I basic beta-1,3-glucanase (GLN) and 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAHPS) in tobacco cells treated with a fungal elicitor. The elicitor treatment was shown to increase the steady-state levels of mRNAs for BCHN and GLN, and for ACHN and DAHPS. Expression of these pairs of genes was induced by elicitor with different kinetics of mRNA accumulation and differentially sensitive to pharmacological agents. Increases in the extracellular concentration of Ca2+ ions and the treatment with Ca2+ ionophore induced the accumulation Of ACHN and DAHPS mRNAs. Calcium channel blockers prevented elicitor-inducible increases in levels of BCHN and GLN mRNAs. Cycloheximide inhibited the elicitor-inducible accumulation of BCHN and GLN mRNAs. The elicitor-inducible accumulation of transcripts from these defense genes was inhibited in the presence of staurosporine. Expression of DAHPS was induced by Jasmonate-related compounds but that of BCHN, ACHN and GLN was not. Constructs of fusions of the 5'-upstream sequences of genes for BCHN and GLN with the reporter gene for beta-glucuronidase were found to be elicitor-responsive in transgenic cells. Thus, it appears that expression of the genes for BCHN and GLN is regulated primarily at the transcriptional level.