Identification and characterization of five new OP2-related Myoviridae bacteriophages infecting different strains of Xanthomonas oryzae pv. oryzae

Bacterial leaf blight caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most severe bacterial diseases of rice. This study identified and characterized five Xoo bacteriophages X1, X2, X3, X4, and X5, isolated from diseased rice leaves in China as potential biocontrol agents. Electron microscopy showed that the five phages have contractile tails thus all belong to the family Myoviridae possessing icosahedral heads, necks and base plates with tail fibres. The head diameters ranged from 57.49 to 85.27nm, while the tail lengths ranged from 75.88 to 112.47nm. It was confirmed that the phages had the expected double-stranded DNA genomes. Phylogenetic analysis of DNAP gene indicated that the five phages could be clustered into one group with phage OP2, but were well separated from other Xoo phages. Significant physiological changes were observed in Xoo after infection by these phages, which differed in their host range and infection ability. Phage X3 had the broadest host range, lysing 22 out of the 23 Xoo strains tested, and had the longest latent period of 40 min with a burst size of 50-plaque forming units per infected cell. It caused 53% degradation of exopolysaccharide production, 43% degradation of biofilm, while the highest rate of bacteriophage-resistant bacterial colonies emerged at log 4 PFU/ml of strain Gz 0011. Application of phage X3 decreased disease severity in vivo: it was more effective (83.1% decrease) if applied by spraying before pathogen inoculation rather than after (28.9-73.9%), and seed treatment was also very effective (95.4% decrease).