The
emergence and widespread occurrence of plant bacterial diseases
that cause global production constraints have become major challenges
to agriculture worldwide. To promote the discovery and development
of new bactericides, imidazole-labeled 1,3,4-oxadiazole thioethers
were first fabricated by integrating the crucially bioactive scaffolds
of the imidazole motif and 1,3,4-oxadiazole skeleton in a single molecular
architecture. Subsequently, a superior antibacterial compound A
6
was gradually discovered possessing
excellent competence against plant pathogens Xanthomonas oryzae pv oryzae and Xanthomonas axonopodis pv citri with EC50 values of 0.734 and
1.79 μg/mL, respectively. These values were better than those
of commercial agents bismerthiazol (92.6 μg/mL) and thiodiazole
copper (77.0 μg/mL). Further modifying the imidazole moiety
into the imidazolium scaffold led to the discovery of an array of
potent antibacterial compounds providing the corresponding minimum
EC50 values of 0.295 and 0.607 μg/mL against the
two strains. Moreover, a plausible action mechanism for attacking
pathogens was proposed based on the concentration dependence of scanning
electron microscopy, transmission electron microscopy, and fluorescence
microscopy images. Given the simple molecular structures, easy synthetic
procedure, and highly efficient bioactivity, imidazole (or imidazolium)-labeled
1,3,4-oxadiazole thioethers can be further explored and developed
as promising indicators for the development of commercial drugs.
In this study, a type of thiazolium-labeled 1,3,4-oxadiazole thioether bridged by diverse alkyl chain lengths was constructed. The antimicrobial activity of the fabricated thioether toward plant pathogenic bacteria and fungi was then screened. Antibacterial evaluation indicated that title compounds possess specific characteristics that enable them to severely attack three phytopathogens, namely, Xanthomonas oryzae pv. oryzae, Ralstonia solanacearum, and Xanthomonas axonopodis pv. citri with minimal EC 50 values of 0.10, 3.27, and 3.50 μg/mL, respectively. Three-dimensional quantitative structure−activity relationship models were established to direct the following excogitation for exploring higher active drugs. The in vivo study against plant bacterial diseases further identified the prospective application of title compounds as alternative antibacterial agents. The proteomic technique, scanning electron microscopy patterns, and fluorescence spectrometry were exploited to investigate the antibacterial mechanism. Additionally, some target compounds performed superior inhibitory actions against three tested fungal strains. In view of their simple molecular architecture and highly efficient bioactivity, these substrates could be further explored as promising surrogates for fighting against plant microbial infections.
In recent years, naturally occurring tetrahydro-β-carboline (THC) alkaloids and their derivatives have been of biological interest. However, few studies and developments have reported the use of such structures in managing plant bacterial diseases. Herein, an array of novel THC derivatives containing an attractive 1,3-diaminopropan-2-ol pattern were prepared to evaluate the antiphytopathogen activity in vitro and in vivo and explore innovative antibacterial frameworks. Notably, target compounds exhibited excellent activities against three rebellious phytopathogens, namely, Pseudomonas syringae pv. actinidiae (Psa), Xanthomonas axonopodis pv. citri, and Xanthomonas oryzae pv. oryzae, at related optimal EC 50 values of 2.39 (II 9 ), 2.06 (I 23 ), and 1.69 (II 9 ) μg/mL, respectively. These effects were superior to those of the parent structure 1,2,3,4-THC and positive controls. In vivo assays showed that II 9 exhibited excellent control efficiencies of 51.89 and 65.45% at 200 μg/mL against rice bacterial blight and kiwifruit bacterial canker, respectively, and I 23 substantially relieved the citrus canker on the leaves. Antibacterial mechanisms indicated that these THC compounds could induce the increment of reactive oxygen species and subsequently endow the tested bacteria with distinct apoptotic behavior. In addition, II 9 could alleviate the hypersensitive response and pathogenicity of Psa. Overall, these simple THC derivatives can be further developed as versatile antibacterial agents.
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