NATIVE BACTERIA IN RASPBERRY CROWN GALL REDUCE THE SEVERITY OF Agrobacterium tumefaciens

Abstract

Native bacterial populations in crown galls caused by Agrobacterium tumefaciens may harbour bacteria of interest for biocontrol of this pathogen. In this study, we explored the density of native bacterial populations in crown galls of raspberry (Rubus ideaus) and evaluated their in vitro and in vivo antagonism against A. tumefaciens. Bacteria morphologically similar to A. tumefaciens were isolated from six gall samples and identified by virD2 gene sequencing. Bacterial population density was calculated by direct plate count on nutrient agar and R2A media. The in vitro antagonism efficiency index against A. tumefaciens of the most frequent bacteria was evaluated by dual confrontation on nutrient agar medium, and in vivo by inoculation of 1.5 x 108 UFC mL-1 in the root of tomato (Solanum lycopersiucm) plants under greenhouse conditions. By direct sequencing and biovar characterization, it was identified as A. tumefaciens biovar 1 in raspberry galls. Native bacterial populations in galls have variable density and their diversity is limited. By partial amplification of the 16S rRNA gene, 13 strains were identified with the highest frequency in the genera Pseudomonas (61.5 %), Bacillus (15.3 %), Alcaligenes (15.3 %) and Delftia (7.6 %). Among these, Alcaligenes faecalis showed the highest in vitro antagonism index (p ≤ 0.05) against A. tumefaciens, followed by Delftia sp. and Pseudomonas citronellolis. In vivo inoculation of tomato plants with these antagonists against Agrobacterium tumefaciens did not prevent infection; however, Alcaligenes faecalis significantly reduced (p ≤ 0.05) the severity of plant stem tumours. A. faecalis is the most efficient antagonist in vitro and in vivo against A. tumefaciens.