Study on The Mechanism of Suppression Mediated Rhizobacterial Strains against Citrus Canker Caused by Xanthomonas citri subsp. citri

Abstract

Citrus canker caused by Xanthomonas citri subsp. citri (Xcc) is one of the important diseases which have been strictly quarantined in many countries. By increasing side effect of the chemicals due to the mostly controlling by chemical applications, the alternative method of disease control has been extremely required. Four rhizobacterial strains such as Burkholderia gladioli MRL408-3 and TRH423-3 and Pseudomonas fluorescens THJ609-3 and TRH415-2 showing anti-bacterial activity against Xcc on the artificial medium, were selected in the previous study. When these rhizobacterial strains were pre-treated on the citrus leaves following inoculation with Xcc, disease severity was significantly suppressed. In this study, in order to illustrate the mechanism mediated by these selected rhizobacterial strains, the number of Xcc on the citrus leaves pre-treated with the selected strains was counted and the infected leaves were observed using a scanning electron microscope (SEM). The number of Xcc colonies from the rhizobacterial strains pre-treated leaves was effectively reduced compared to that of non-treated control one on the semiselective medium, indicating the direct suppression by the selected rhizobacterial strains against Xcc on the citrus leaf surface. The counted colonies were identified as Xcc and the rhizobacteria, respectively, by the gel electrophoresis of polymerase chain reaction (PCR) amplified and analysis of bacterial rDNA sequence using the specific primer sets. Based on the SEM image, it revealed that the number of total bacterial cells on the leaves pre-treated with the rhizobacterial strains after inoculating Xcc was reduced compared to those on the leaves inoculated with Xcc only. On the other hand, to determine whether a resistance induced by the treatment with the rhizobacterial strains, pathogenesis-related protein -1 (PR-1 protein) gene was extracted from the citrus leaves. As expected, PR-1 protein gene from the leaves pre-treated with THJ609-3 was amplified. Based on these results, it is suggested that these rhizobacteria could express any anti-bacterial activity on the citrus leaves against Xcc and reduce the population of Xcc resulting in suppression of disease severity. Therefore, it is expected that rhizobacterial strains could be used as the biological control agents in a green house and field where chemical application is prohibited or limited.