Isolation of novel leaf-inhabiting endophytic bacteria in Arabidopsis thaliana and their antagonistic effects on phytophathogens

Bacterial endophytes benefit the host with protection against abiotic and biotic stresses and through increased plant growth. In this study, we screened novel endophytes for the ability to endophytically inhabit leaf tissues. Four endophytic bacteria were screened from the apoplastic fluids extracted from the uninfected upper leaves of Arabidopsis thaliana 3 days postinoculation (dpi) with Pseudomonas syringae pv. tomato DC3000 (Pst). Under sterile plant growth conditions, we showed that the four isolates proliferated intercellularly in the leaf tissues of A. thaliana. Based on 16S rRNA sequencing analysis, the identities of the inoculated endophytes and the re-isolates from the leaf tissues were confirmed. Among the isolates, the two species of Rhodococcus were the first members of the genus to be identified as leaf-inhabiting endophytes. Additionally, three of the isolates showed antagonistic activities, with different levels of activity, against Fusarium oxysporum pv. conglutinans (F.o.) and/or Pst. Furthermore, the application of one isolate, Bacillus cereus KB1, on tomato plants significantly increased the disease resistance to Botrytis cinerea and Pst. In combination, these results indicate that these endophytic isolates can be used to develop potential biocontrol agents against a variety of pathogenic fungi and bacteria.