The pathogenesis related protein, chitinase, and its role in the systemic acquired resistance phenotype in cucumber plants (Cucumis sativus L.)

Systemic Acquired Resistance (SAR) refers to a distinct plant response that systemically confers broad-spectrum disease resistance after an induction of biotic or abiotic nature. Accumulation of Pathogenesis-Related (PR) proteins such as chitinase (PR-8) takes place during the induction of SAR.; In order to study chitinase activity in cucumbers, a new method for chitinase detection and quantification was developed. In addition to determine if SAR requires active metabolism after challenge, leaf disks of cucumber plants expressing SAR were treated with the protein synthesis inhibitor cycloheximide and then challenged with the fungus Colletotrichum orbiculare. Cyclohexamide treatment in SAR expressing plants caused an increase in penetration of the fungus and symptom development was comparable to non-induced plants. This result suggests that the induced state of cucumber plants is dependant of protein synthesis.; In field and greenhouse experiments, no correlation between the amount of chitinase induction and the degree of resistance acquired through SAR was found. In addition, a differential cultigen response to two pathogens with different infections strategy was found. Cucumber cultivars expressing SAR are generally resistant to Colletotrichum orbiculare but are not always resistant to Didimella bryoniae. Several cultivars expressing SAR developed more severe symptoms to D. bryoniae despite the high chitinase activity detected. This suggests a differential resistance pathway and questions the ability of chitinase to decrease pathogen growth within the tissue.; Transgenic cucumber plants bearing the cDNA sequence of chitinase type III in the sense and antisense directions were generated. Treatment of the transformed plants with the resistance activator acybenzolar-S-Methyl (ASM) demonstrated no significant differences in their ability to withstand pathogen attack or express induced resistance. Results obtained suggest that the acidic chitinase is important for the basal levels of resistance prior to induction.; The present study indicates that chitinase does not play a significant role in the SAR phenotype. It is possible that the timely activation of the entire set of SAR genes is more important than the effect of a single gene even if it is over expressed. (Abstract shortened by UMI.)...