Abstract
Citrus canker is a very destructive disease of citrus species. The challenge is to find new compounds that show strong antibiotic activity and low toxicity to plants and the environment. The objectives of the present study were (1) to extract, purify and evaluate the secondary metabolites with antibiotic activity produced by Pseudomonas aeruginosa LV strain in vitro against Xanthomonas citri subsp. citri (strain 306), (2) to determine the potential of semi-purified secondary metabolites in foliar application to control citrus canker under greenhouse conditions, and (3) to identify antibiotic activity in orange leaf mesophyll infected with strain 306, by electron microscopy. Two pure bioactive compounds were isolated, an organocopper antibiotic compound (OAC) and phenazine-1-carboxamide. Phenazine-1-carboxamide did not show any antibiotic activity under the experimental conditions used in this study. The OAC showed a high level of antibiotic activity with a minimum inhibitory concentration of 0.12 μg mL-1. In greenhouse tests for control of citrus canker in orange trees, the semi-purified fraction F3d reduced lesion formation by about 97%. The concentration used was 500 times lower than that for the recommended commercial copper-based product. Electron microscopy showed that F3d altered the exopolysaccharide matrix and caused cell lysis of the pathogen inside the citrus canker lesions. These results suggest that secondary metabolites produced by inducing P. aeruginosa LV strain have a high potential to be used as a bioproduct to control citrus canker.
Highlights
Brazil is one of the largest producers of orange juice concentrate in the world (Neves et al, 2010), earning over US$ 2 billion a year
Previous studies have shown the antibiotic effect of Pseudomonas secondary metabolites against various bacterial species
We evaluated the effect of F3d, a semi-purified fraction, on plants infected by Xcc 306, evaluating antibiotic effectiveness in the initial infection process on the leaf and the antibiotic control of Xcc 306 infecting the mesophyll tissue inside the lesions, and we determined the active pure compound present in the F3d fraction (30% organocopper antibiotic compound (OAC))
Summary
Brazil is one of the largest producers of orange juice concentrate in the world (Neves et al, 2010), earning over US$ 2 billion a year. Citrus canker is one of the most destructive diseases. Frontiers in Microbiology | www.frontiersin.org de Oliveira et al. Organocopper Antibiotic Compound Against Xanthomonas citri of many commercial citrus species (Gottwald and Graham, 2000), and it is found in the biggest sweet orange producing areas, such as São Paulo State, Brazil, and Florida, USA. The etiological agent is Xanthomonas citri subsp. The infection begins by X. citri subsp. Growers use integrated management approaches such as cultivating lesssusceptible citrus genotypes produced in X. citri subsp. Citri-free nurseries, control of P. citrella, planting windbreaks as barriers to hinder the spread of the disease, and spraying synthetic copper bactericides (Schubert and Sun, 1996; Belasque and Behlau, 2011) Growers use integrated management approaches such as cultivating lesssusceptible citrus genotypes produced in X. citri subsp. citri-free nurseries, control of P. citrella, planting windbreaks as barriers to hinder the spread of the disease, and spraying synthetic copper bactericides (Schubert and Sun, 1996; Belasque and Behlau, 2011)
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