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Induction of PR-1 Proteins and Potentiation of Pathogen Signals by Salicylic Acid Exhibit the Same Dose Response and Structural Specificity in Plant Cell Cultures

Based on the marked difference in both dose response and structural specificity, it has been recently proposed that the induction of acquired resistance and pathogenesis-related (PR) proteins in intact plant leaves and the potentiation of pathogen signals in plant cell cultures by salicylic acid (SA) and its analogues are mediated by different SA signaling pathways initiated from different SA receptor systems. We show here, however, that the induction of PR-1 proteins, the most widely used marker of acquired resistance, by SA and its analogues in tobacco cell cultures exhibits the same dose response and structural specificity as the potentiation of pathogen signals demonstrated in soybean cell cultures. The different dose response and structural specificity for the induction of PR-1 proteins by SA between intact tobacco leaves and cell cultures appear to be caused largely by the difference in uptake and/or metabolism of these chemical inducers by different types of plant cells. These results suggest that the potentiation of pathogen signals by SA, which may contribute greatly to SA function, does not involve a different, more potent signaling pathway from the one(s) responsible for the induction of acquired resistance and PR protein accumulation.