3-Hydroxy palmitic acid methyl ester

Bacterial wilt pathogen Ralstonia solanacearum causes severe crop loss of eggplant, which is of economic importance in India. 3-hydroxy palmitic acid methyl ester (3OH-PAME) is the main quorum sensing molecule governing the expression of virulence factors in R. solanacearum. Ability of 164 bacterial isolates from the xylem of eggplant (Solanum melongena L.), chilli pepper (Capsicum annuum L.) and wild eggplant (Solanum torvum Sw.) to degrade 3OH-PAME was tested by disc diffusion assay. Enzymatic degradation of 3OH-PAME by five bacteria was confirmed by High-Performance Liquid Chromatography-Mass Spectrometry analysis. 3OH-PAME degrading bacteria were identified as Stenotrophomonas maltophilia, Pseudomonas aeruginosa and Rhodococcus corynebacterioides. 3OH-PAME degrading bacteria reduced the expression of virulence factors (exopolysaccharides and endoglucanase) of R. solanacearum in vitro and reduced wilt incidence in eggplant seedlings under greenhouse conditions. Isolates with quorum quenching activity successfully re-colonized eggplant seedlings. Quorum quenching bacteria produced antagonistic compounds, which may act synergistically with quorum quenching in reducing bacterial wilt in eggplant.Significance and impact of the study:This is the first report on endophytic bacteria of class Gammaproteobacteria and phylum Actinobacteria having 3OH-PAME degrading activity. This study demonstrates the potential use of endophytic bacteria as quorum quenching biocontrol agents for management of bacterial wilt in eggplant.

Autoinducers are indispensable for bacterial cell-cell communication. However, due to the reliance on culture-based techniques, few autoinducer-hydrolyzing enzymes are known. In this study, we characterized soil metagenome-derived unique enzymes capable of hydrolyzing 3-hydroxypalmitic acid methyl ester (3-OH PAME), an autoinducer of the plant pathogenic bacterium Ralstonia solanacearum. Among 146 candidate lipolytic clones from a soil metagenome library, 4 unique enzymes capable of hydrolyzing the autoinducer 3-OH PAME, termed ELP86, ELP96, ELP104, and EstDL33, were selected and characterized. Phylogenetic analysis revealed that metagenomic enzymes were novel esterase/lipase candidates as they clustered as novel subfamilies of family I, V, X, and family XI. The purified enzymes displayed various levels of hydrolytic activities towards 3-OH PAME with optimum activity at 40-50 °C and pH 7-10. Interestingly, ELP104 also displayed N-(3-oxohexanoyl)-L-homoserine lactone hydrolysis activity. Heterologous expression of the gene encoding 3-OH PAME hydrolase in R. solanacearum significantly decreased exopolysaccharide production without affecting bacterial growth. mRNA transcription analysis revealed that genes regulated by quorum-sensing, such as phcA and xpsR, were significantly down-regulated in the stationary growth phase of R. solanacearum. Therefore, metagenomic enzymes are capable of quorum-quenching by hydrolyzing the autoinducer 3-OH PAME, which could be used as a biocontrol strategy against bacterial wilt.

Bacterial wilt pathogen Ralstonia solanacearum causes severe crop loss of eggplant, which is of economic importance in India. 3-hydroxy palmitic acid methyl ester (3OH-PAME) is the main quorum sensing molecule governing the expression of virulence factors in R. solanacearum. Ability of 164 bacterial isolates from the xylem of eggplant (Solanum melongena L.), chilli pepper (Capsicum annuum L.) and wild eggplant (Solanum torvum Sw.) to degrade 3OH-PAME was tested by disc diffusion assay. Enzymatic degradation of 3OH-PAME by five bacteria was confirmed by High-Performance Liquid Chromatography-Mass Spectrometry analysis. 3OH-PAME degrading bacteria were identified as Stenotrophomonas maltophilia, Pseudomonas aeruginosa and Rhodococcus corynebacterioides. 3OH-PAME degrading bacteria reduced the expression of virulence factors (exopolysaccharides and endoglucanase) of R. solanacearum in vitro and reduced wilt incidence in eggplant seedlings under greenhouse conditions. Isolates with quorum quenching activity successfully re-colonized eggplant seedlings. Quorum quenching bacteria produced antagonistic compounds, which may act synergistically with quorum quenching in reducing bacterial wilt in eggplant.Significance and impact of the study:This is the first report on endophytic bacteria of class Gammaproteobacteria and phylum Actinobacteria having 3OH-PAME degrading activity. This study demonstrates the potential use of endophytic bacteria as quorum quenching biocontrol agents for management of bacterial wilt in eggplant.