Antimicrobial compounds effective against Candidatus Liberibacter asiaticus discovered via graft-based assay in citrus

Authors: YANG, CHUANYU - University Of Florida; ZHONG, YUN - University Of Florida; POWELL, CHARLES - University Of Florida; Doud, Melissa; Duan, Ping; ZHANG, MUQING - University Of Florida

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2018
Publication Date: 11/23/2018
Citation: Yang, C., Zhong, Y., Powell, C., Doud, M.S., Duan, Y., Zhang, M. 2018. Antimicrobial compounds effective against Candidatus Liberibacter asiaticus discovered via graft-based assay in citrus. PLoS One. 8(1):17288. https://doi.org/10.1038/s41598-018-35461-w.
DOI: https://doi.org/10.1038/s41598-018-35461-w

Interpretive Summary: Citrus Huanglongbing (HLB) is one of the most devastating diseases for citrus plants and has caused enormous economic losses to many citrus growing regions of the world. Effective strategies against HLB bacterium in citrus production are still limited. To date, there are still no commercial genetically modified citrus varieties available due to lack of consumer acceptance of transgene technology. Chemical control is a critical short-term strategy against Candidatus Liberibacter asiaticus (Las). Currently, application of antibiotics in agricultural practices is limited due to public concerns regarding emergence of antibiotic-resistant bacteria and potential side effects in humans. Due to unprecedented epidemics of citrus HLB in Florida and other regions of the world, screening of non-antibiotic or other chemical compounds that have already been registered for fruit tree production and can reduce the emergence on antibiotic-resistant bacteria is urgently needed for the survival of the Florida citrus industry. In the present study, 39 antimicrobial (non-antibiotic) compounds were screened for effectiveness against HLB and phytotoxicity using an optimized graft-based screening system with Las-infected citrus scions. 39 antimicrobials were clustered into three groups: “effective” (Group I), “partly effective” (Group II), and “ineffective” (Group III). Despite different modes of action, 8 antimicrobials (Aluminum aluminum hydroxide, D,L-buthionine sulfoximine, nicotine, surfactin from Bacillus subtilis, SilverDYNE®, colloidal silver, EBI-601, and EBI-602), were all as effective as ampicillin at eliminating or suppressing Las, showing both the lowest Las infection rates and titers in treated scions and inoculated rootstock. These effective antimicrobials are potential candidates for HLB control either via rescuing infected citrus germplasms or restricted field application.

Technical Abstract: Huanglongbing (HLB), the most destructive citrus disease, is caused by three species of phloem-limited Candidatus Liberibacter. Chemical control is a critical short-term strategy against Candidatus Liberibacter asiaticus (Las). Currently, application of antibiotics in agricultural practices is limited due to public concerns regarding emergence of antibiotic-resistant bacteria and potential side effects in humans. The present study screened 39 antimicrobials (non-antibiotics) for effectiveness against citrus Huanglongbing and phytotoxicity using an optimized graft-based screening system with Las-infected citrus scions; the antibiotic ampicillin was used as a positive control, and water (CK-1), ethanol (CK-2), and dimethyl sulfoxide (DMSO) were used as negative controls. Results of principal component and hierarchical clustering analyses demonstrated that 39 antimicrobials were clustered into three groups: “effective” (Group I), “partly effective” (Group II), and “ineffective” (Group III). Despite different modes of action, 8 antimicrobials (Aluminum hydroxide, D,L-buthionine sulfoximine, nicotine, surfactin from Bacillus subtilis, SilverDYNE®, colloidal silver, EBI-601, and EBI-602), were all as highly effective as ampicillin at eliminating or suppressing Las, showing both the lowest Las infection rates and titers in treated scions and inoculated rootstock. The ineffective group, which included 21 antimicrobials (e.g., 2-amino-5-chlorobenzoxazole and 2-methyl-4-isothiazolin-3-one), did not eliminate or suppress Las at the tested concentrations, resulting in plants with increased titers of Candidatus Liberibacter, similar to CK-1, CK-2, and DMSO negative controls. The other 10 antimicrobials (e.g., Carvacrol and P-cymene) partly eliminated/suppressed Las in treated and graft-inoculated plants. These effective antimicrobials are potential candidates for HLB control either via rescuing infected citrus germplasms or restricted field application.