[Endophytic bacterial community in the symptoms and symptomless tissues of HLB-affected citrus plant]

To analyze the endophytic bacteria in symptoms and symptomless tissues, reveal dominant bacterial population which may interact with Ca. Las.

The population structure and diversity of endophytic bacteria in symptoms and symptomless tissues of single HLB-affected citrus plant were studied based on 16S rDNA sequences analyzed by PCR-DGGE (PCR-denaturing gradient gel electrophoresis). Quantitative analysis of Ca. Las, dominant bacteria and total amount of bacterial population in symptoms and symptomless tissues of branches, leaves and fruits was done by quantitative PCR.

The amount of Ca. Las is significantly different in symptoms and symptomless tissues; bacterial population in symptoms citrus tissues was significantly higher than in symptomless tissues. PCR-DGGE analysis result shows that endophytic bacterial structures are basically the same in symptoms and symptomless tissues. Extraction, cloning and sequencing of 17 obvious bands in DGGE electrophoretogram reveal that 8 of 17 bands belonged to the genus Serratia, which accounted for 47.06%. Sequence alignment shows that they belonged to different strains of Serratia marcescens with the 99.63% similarity. Quantitative analysis of the total bacteria and Serratia marcescens shows that there is no significant difference in total bacterial count between symptoms and symptomless tissues whereas the amount of Serratia marcescens was negatively correlated with the amount of Las.

The Ca. Las is heterogeneously distributed among various tissues of HLB-affected citrus plant. The amount of Ca. Las was negatively correlated with Serratia marcescens and total amount of endophytic bacteria have no relevance with symptom of HLB disease. How the interaction of Serratia marcescens and Ca. Las in tissue of citrus needs further study.