Characterization of tomato-associated rhizobacteria recovered from various tomato-growing sites in Tunisia

In the present study, a total of 200 rhizobacterial isolates were obtained from rhizosphere of healthy tomato plants grown in fields with a history of severe soilborne diseases and mainly crown and root rots. Screened their capacity to suppress in vitro growth of Sclerotinia sclerotiorum and Rhizoctonia solani, 69 and 57 isolates out of the 200 tested were shown able to inhibit significantly the mycelial growth of target pathogens by 11-62% relative to control. The 25 most effective isolates, leading to suppression of both fungi by more than 45% over control, were selected and subjected to morphological, biochemical, molecular, and metabolic characterizations. This collection of tomato-associated rhizobacteria exhibited a great morphological and biochemical diversity. Sequencing of 16S rRNA and rpoB genes led to the identification of four genera namely Bacillus, Chryseobacterium, Enterobacter, and Klebsiella. The most frequent species were B. amyloliquefaciens, B. thuringiensis, B. megaterium, B. subtilis, E. cloacae, C. jejuense, and K. pneumoniae. Screening for their plant growth-promoting properties, 20 isolates were shown able to produce siderophore, 18 had solubilized phosphate, and 19 were capable to synthesize indole-3- acetic acid (IAA). PCR amplification of lipopeptide biosynthetic genes revealed the presence of genes encoding fengycin A and bacillomycin D biosynthesis in 18 and 16 isolates, respectively. Metabolic characterization performed using Biolog™ Ecoplates indicated that tomato-associated rhizobacteria displayed a large metabolic activity and they were able to use a wide range of carbon sources with the increase of the incubation duration. Based on their metabolic profiles, these rhizobacterial isolates were grouped into eight major clusters generated at the different sampling times (24, 48 and 120 h of incubation). Average well-color development (AWCD) values were found to be positively correlated with the Shannon diversity index.