Quorum Sensing Inhibition Attenuates the Virulence of the Plant Pathogen Ralstonia solanacearum Species Complex.

Strains of Ralstonia solanacearum species complex (RSSC) cause "bacterial wilt" on a wide range of plant species and thus lead to marked economic losses in agriculture. Quorum sensing (QS), a bacterial cell-cell communication mechanism, controls the virulence of RSSC strains by regulating the production of extracellular polysaccharide (EPS) and secondary metabolites, biofilm formation, and cellular motility. R. solanacearum strain OE1-1 employs (R)-methyl 3-hydroxymyristate (3-OH MAME) as a QS signal, which is synthesized by the PhcB methyltransferase and sensed by the PhcS/PhcRQ two-component system. We describe the design, synthesis, and biological evaluation of inhibitors of the phc QS system. Initial screening of a small set of QS signal analogues revealed that methyl 3-hydroxy-8-phenyloctanoate, named, PQI-1 (phc quorum sensing inhibitor-1), inhibited biofilm formation by strain OE1-1. To improve its inhibitory activity, the derivatives of PQI-1 were synthesized, and their QS inhibition activities were evaluated. PQIs-2-5 evolved from PQI-1 more strongly inhibited not only biofilm formation but also the production of ralfuranone and EPS. Furthermore, RNA-Seq analysis revealed that the PQIs effectively inhibited QS-dependent gene expression and repression in strain OE1-1. On the other hand, the PQIs did not affect the canonical QS systems of the representative reporter bacteria. These antagonists, especially PQI-5, reduced wilting symptoms of the tomato plants infected with strain OE1-1. Taken together, we suggest that targeting the phc QS system has potential for the development of chemicals that protect agricultural crops from bacterial wilt disease.

Antibacterial activity of tannins isolated from Sapium baccatum extract and use for control of tomato bacterial wilt.

In the search for new antibacterial agents from natural sources, we revealed that a crude methanol extract of Sapium baccatum was highly active against Ralstonia solanacearum, a causal agent of a serious disease called bacterial wilt of tomato. The bioassay-guided fractionation of this extract resulted in the isolation of seven known active compounds, including gallic acid, methyl gallate, corilagin, tercatain, chebulagic acid, chebulinic acid, and quercetin 3-O-α-L-arabinopyranoside. Their chemical structures were determined by electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy. An in vitro antibacterial bioassay using a broth microdilution method revealed that, except for quercetin 3-O-α-L-arabinopyranoside (MIC = 250 µg/mL), the isolated compounds exhibited strong antibacterial activity against R. solanacearum (MIC = 26-52 µg/mL). Among the seven compounds, methyl gallate exhibited the strongest broad-spectrum activity against most of the plant pathogenic bacteria tested (MIC = 26-250 µg/mL). In the in vivo experiments, the crude extract of S. baccatum at 2000 and 1000 µg/mL reduced the development of tomato bacterial wilt by 83 and 63%, respectively, under greenhouse conditions after 14 days of infection. The results suggested that the extracts of S. baccatum or isolated tannins could be used as natural bactericides for the control of bacterial wilt of tomato.

Inhibition of tobacco bacterial wilt with sulfone derivatives containing an 1,3,4-oxadiazole moiety.

A series of new sulfone compounds containing the 1,3,4-oxadiazole moiety were designed and synthesized. Their structures were identified by (1)H and (13)C nuclear magnetic resonance and elemental analyses. Antibacterial bioassays indicated that most compounds exhibited promising in vitro antibacterial bioactivities against tobacco bacterial wilt at 200 µg/mL. The relationship between structure and antibacterial activity was also discussed. Among the title compounds, 5'c, 5'h, 5'i, and 5'j could inhibit mycelia growth of Ralstonia solanacearum in vitro by approximately 50% (EC(50)) at 39.8, 60.3, 47.9, and 32.1 µg/mL, respectively. Among them, compound 5'j was identified as the most promising candidate due to its stronger effect than that of Kocide 3000 [Cu(OH)(2)] within the same concentration range. Field trials demonstrated that the control effect of compound 5'j against tobacco bacterial wilt was better than that of the commercial bactericide Saisentong. For the first time, the present work demonstrated that sulfone derivatives containing 1,3,4-oxadiazole can be used to develop potential bactericides for plants.