Abstract
Poor survival on plants can limit the efficacy of Biological Control Agents (BCAs) in the field. Yet bacteria survive in the atmosphere, despite their exposure to high solar radiation and extreme temperatures. If conditions in the atmosphere are similar to, or more extreme than, the environmental conditions on the plant surface, then precipitation may serve as a reservoir of robust BCAs. To test this hypothesis, two hundred and fifty-four rain-borne isolates were screened for in vitro inhibition of Erwinia amylovora, the causal agent of fire blight, as well as of other plant pathogenic bacteria, fungi and oomycetes. Two isolates showed strong activity against E. amylovora and other plant pathogenic bacteria, while other isolates showed activity against fungal and oomycete pathogens. Survival assays suggested that the two isolates that inhibited E. amylovora were able to survive on apple blossoms and branches similarly to E. amylovora. Pathogen population size and associated fire blight symptoms were significantly reduced when detached apple blossoms were treated with the two isolates before pathogen inoculation, however, disease reduction on attached blossoms within an orchard was inconsistent. Using whole genome sequencing, the isolates were identified as Pantoea agglomerans and P. ananatis, respectively. A UV-mutagenesis screen pointed to a phenazine antibiotic D-alanylgriseoluteic acid synthesis gene cluster as being at the base of the antimicrobial activity of the P. agglomerans isolate. Our work reveals the potential of precipitation as an under-explored source of BCAs, whole genome sequencing as an effective approach to precisely identify BCAs, and UV-mutagenesis as a technically simple screen to investigate the genetic basis of BCAs. More field trials are needed to determine the efficacy of the identified BCAs in fire blight control.
Highlights
There has been a growing effort in scouting for bacteria and fungi to be deployed as biological control agents (BCAs) against crop pests (Jaffuel et al, 2019) and diseases (Durán et al, 2018)
Precise genomebased classification and identification methods, such as the Life Identification Number (LIN) system implemented in the LINbase web server (Tian et al, 2019), could aid in regulation of BCAs, but those methods have not been thoroughly explored within this context
The assay was performed in Petri dishes containing yeast extract dextrose agar NYDA (Nutrient agar 23 g l−1, dextrose 10 g l−1, yeast extract 5 g l−1). 100 μl of E. amylovora suspensions were spread on each plate and up to five 10 μl-droplets of suspension of rain-isolated bacteria were placed at equal distance from each other
Summary
There has been a growing effort in scouting for bacteria and fungi to be deployed as biological control agents (BCAs) against crop pests (Jaffuel et al, 2019) and diseases (Durán et al, 2018). Precise genomebased classification and identification methods, such as the Life Identification Number (LIN) system implemented in the LINbase web server (Tian et al, 2019), could aid in regulation of BCAs, but those methods have not been thoroughly explored within this context Another challenge with BCAs is that their efficacy under field conditions is more variable than synthetic pesticides since BCAs contain living organisms, whose survival on plant surfaces and/or internal tissues depends on environmental conditions (Sundin et al, 2009; Bonaterra et al, 2012). The most promising BACs were identified to species using genome-based methods, including the LINbase Web server (Tian et al, 2019), and the genetic basis of the biocontrol activity for one of the bacteria was explored using a combination of UV-mutagenesis and genome sequencing
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