[Erh-Min Lai/Chih-Horng Kuo] Agrobacterial T6SS influences disease occurrence and microbiota of crown galls

The type VI secretion system (T6SS) is a widespread antibacterial weapon used by proteobacteria for interbacterial competition. However, knowledge regarding the effects of the T6SS in shaping microbiota in complex microbial community is limited. Previous studies showed that agrobacteria, a group of soil-borne phytopathogens causing crown gall disease on various plant species, deploys the T6SS to attack closely- and distantly-related bacterial species in vitro and in planta. Although the T6SS is not required for gall formation when agrobacteria are inoculated directly on plant wounding sites, it is assumed that agrobacteria may need to compete with other bacteria in soil to gain access to plant wounds and to thrive inside galls.

To test the aforementioned hypothesis, the research team led by Erh-Min Lai  and Chih-Horng Kuo designed experiments to determine whether the T6SS influences natural disease incidence and the microbial community within crown galls (i.e., gallobiome). By development of a Soil Inoculation method coupled with Blocker-mediated enrichment of Bacterial 16S rRNA gene Amplicon Sequencing, named as SI-BBacSeq, it was found that the T6SS influences both disease occurrence and gallobiome composition. By comparing the Agrobacterium wild-type strain C58 with two T6SS mutants based on multiple inoculation trials across seasons, the team showed that T6SS mutants had significantly lower disease incidences. While the season played a more important role than the T6SS in shaping the gallobiome, the influence of T6SS was evident in summer, in which two Sphingomonadaceae species and the family Burkhoderiaceae were enriched in the gallobiome induced by the mutants. Further in vitro competition and colonization assay demonstrated the T6SS-mediated antagonism to a Sphingomonas sp. R1 strain isolated from tomato rhizosphere in this study. In conclusion, this work provided evidence that the agrobacterial T6SS promotes disease occurrence and influences crown gall microbiota composition.

This work is mainly contributed by the first author Mr. Si-Chong Wang, a former Master Student at Department of Life Sciences, National Central University and currently a Research Assistant in the Lai lab.  Other authors include Ms. Shu-Jen Chou and Ai-Ping Chen from Genomic Technology Core in IPMB  for assistance in optimizing 16S rRNA gene amplicon sequencing. This work is published online in the premier ASM journal mBio (Wang et al., 2023) .