NATIVE BACTERIA IN THE RASPBERRY CROWN GALL REDUCE THE SEVERITY OF AGROBACTERIUM TUMEFACIENS

Abstract

Native bacterial populations in crown galls caused by Agrobacterium tumefaciens may harbor bacteria of interest for the biocontrol of this pathogen. In this study, the density of native bacterial populations in the crown gall of raspberry (Rubus ideaus) was explored and antagonism in vitro and in vivo against A. tumefaciens was evaluated. Bacteria morphologically similar to A. tumefa-ciens were isolated from six gall samples, identified by sequencing of the virD2 gene. Bacterial population density was calculated by direct plate count in nutrient agar and R2A media. The in vitro antagonism efficiency index against A. tumefaciens of the most frequent bacteria was evaluated by dual confrontation in nutrient agar medium and in vivo by inoculation of 1.5 x 108 CFU mL-1 in the root of tomato plants (Solanum lycopersiucm). under glass. Direct sequencing identified A. tumefaciens 70DZCEMU016 plasmid Ti virD2 biovar 3 in raspberry galls. There are different densities of native populations in the galls and their diversity is limited. Partial amplification of the 16S rRNA gene and sequence comparison identified 13 strains with higher frequency in the genera Pseudomonas (61.5%), Bacillus (15.3%), Alcaligenes (15.3%) and Delftia (7.6%). Among these, Alcaligenes faecalis showed the highest rate of antagonism in vitro (p ≤ 0.05) against A. tumefaciens followed by Delftia sp. and Pseudomonas citronellolis. In vivo inoculation of these antagonists with A. tumefaciens in greenhouse tomato plants does not prevent infection; however, with the inoculation of Alcaligenes faecalis, the severity of tumors in the stem of the plants was significantly reduced (p ≤ 0.05). A. faecalis is the most efficient antagonist in vitro and in vivo against A. tumefaciens.