Different Mechanisms of Induced Systemic Resistance and Systemic Acquired Resistance Against Colletotrichum orbiculare on the Leaves of Cucumber Plants

Abstract

Defense mechanisms against anthracnose disease caused by Colletotrichum orbiculare on the leaf surface of cucumber plants after pre-treatment with plant growth promoting rhizobacteria (PGPR), amino salicylic acid (ASA) or C. orbiculare were compared using a fluorescence microscope. Induced systemic resistance was mediated by the pre-inoculation in the root system with PGPR strain Bacillus amywlquefaciens EXTN-1 that showed direct antifungal activity to C. gweosporioides and C. orbiculare. Also, systemic acquired resistance was triggered by the pre-treatments on the bottom leaves with amino salicylic acid or conidial suspension of C. orbiculare. The protection values on the leaves expressing SAR were higher compared to those expressing ISR. After pre-inoculation with PGPR strains no change of the plants was found in phenotype, while necrosis or hypersensitive reaction (HR) was observed on the leaves of plants pre-treated with ASA or the pathogen. After challenge inoculation, inhibition of fungal growth was observed on the leaves expressing both ISR and SAR. HR was frequently observed at the penetration sites of both resistance-expressing leaves. Appressorium formation was dramatically reduced on the leaves of plants pre-treated with ASA, whereas EXTN-1 did not suppress the appressorium formation. ASA also more strongly inhibited the conidial germination than EXTN-1. Conversely, EXTN-1 significantly increased the frequency of callose formation at the penetration sites, but ASA did not. The defense mechanisms induced by C. orbiculare were similar to those by ASA. Based on these results it is suggested that resistance mechanisms on the leaf surface was different between on the cucumber leaves expressing ISR and SAR, resulting in the different protection values.