Abstract
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.
Highlights
Phytopathogenic bacteria cause serious economic losses by reducing the yield of marketable quality crops [1]
The compounds were identified as gallic acid [24,25,26], methyl gallate [24,25,26], corilagin [25,26,27], tercatain [28, 29], chebulagic acid [25, 26, 30, 31], chebulinic acid [26, 30, 31], and quercetin 3-O-α-L-arabinopyranoside or guaijaverin [32,33,34] (Fig 1)
These compounds have previously been isolated from plant extracts, most of them belong to tannins group which possess various bioactivities including antibacterial activity [35]: gallic acid and methyl gallate from extracts of Sedum takesimense aerial parts [4]; Euphorbia helioscopia whole plants [24]; Dimocarpus longan seeds [25] and Terminalia spp. fruits, including T. bellerica, T. chebula, and T. horrida [26]; corilagin from extracts of the D. longan seeds [25] and Terminalia spp. fruits [26], Punica granatum leaves [27], and E. fischeriana roots [29]; tercatain from extracts of leaves of T. catappa [28] and E. fischeriana roots [29]; chebulagic and chebulinic acids from extracts of the D. longan seeds and the Terminalia spp. fruits [25, 26, 30, 31]; and quercetin 3-O-α-L-arabinopyranoside from extracts of Acidovorax avenae subsp. cattleyae
Summary
Phytopathogenic bacteria cause serious economic losses by reducing the yield of marketable quality crops [1]. Ralstonia solanacearum, which causes vascular wilt disease, is one of the most destructive pathogens [2]. This soil-born pathogen is found worldwide and has a large range of hosts, comprising more than 200 plant species, including tomato, potato, pepper, peanut, tobacco, and banana [2,3,4]. The direct yield losses caused by R. solanacearum differ widely. S. baccatum against tomato bacterial wilt (315007-03). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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