Submitted to: Biotecnological Approaches for the Integrated Management of Crop Diseases
Publication Type: Book / chapter
Publication Acceptance Date: 3/20/2004
Publication Date: 6/20/2004
Citation: Velusamy, P., Defago, G., Thomashow, L.S., Gnanamanickam, S.S. 2004. Role of 2,4-diacetylphloroglucinol for plant disease control: its importance to rice bacterial blight suppression in india. pp.182-191 in Biotechnological Approaches for the Integrated Management of Crop Diseases. Interpretive Summary: Bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most important and devastating diseases of rice worldwide. In this study, a collection of more than 637 plant-associated bacteria isolated from the roots of crops grown throughout India were tested for the ability to inhibit the blast pathogen in laboratory assays and on rice grown in nethouse and field studies. Biochemical and genetic analyses of the mechanisms involved in inhibition of the pathogen by one isolate, PTB 9, revealed that the strain produced the well-known antibiotic 2,4-diacetylphloroglucinol (DAPG). To determine whether DAPG was responsible for the inhibitory activity of the strain, mutants that no longer produced the antibiotic were generated and compared to the parental strain for the ability to suppress blast on rice. The mutants were significantly less effective, implicating DAPG as a key factor in the activity of PTB 9. The results are significant because this is the first time that a DAPG-producing strain has been identified in India, and the first report of DAPG controlling a bacterial pathogen.
Technical Abstract: We have used established methods for assembling and characterizing plant-associated bacteria from parts of Andhra Pradesh, Karnataka, Kerala, Maharashtra and Tamil Nadu states of India. This present investigation is on Pseudomonas strains as biocontrol agents of bacterial blight of rice but the emphasis is really on those fluorescent pseudomonads that produce a valuable antibiotic called 2,4-diacetylphloroglucinol (DAPG). There has been no previous report on the production of DAPG by plant-associated bacteria from India and therefore, this study assumes special importance as a major study on DAPG production by Indian strains of Pseudomonas which also act as biocontrol agents for rice bacterial blight. In the present study DAPG production has been implicated as an antibacterial compound involved in the suppression of one of the most important and devastating bacterial crop diseases, the bacterial blight of rice. To provide positive confirmation to the role of DAPG as the mechanism that mediates the biological suppression of rice bacterial blight, a genetic analysis of P.fluorescens strain PTB 9 was carried out. Transposon (Tn5) insertion mutants of this strain which were altered for either total or partial loss of its in vitro antibiosis towards X. oryzae pv. oryzae were generated and were evaluated in a greenhouse experiment to compare their biocontrol efficacy with that of the wild type strain. In the greenhouse experiment, the wild type strain afforded 59.5 per cent bacterial blight suppression while the mutants afforded only less than 50 per cent (range: 17.1 to 23.8 per cent) bacterial blight suppression. Therefore, this observation with the Phl- mutants on bacterial blight suppression strongly supports a positive role for DAPG production by P. fluorescens strain PTB 9.