Endophytes and plant extracts as potential antimicrobial agents against Candidatus Liberibacter asiaticus, causal agent of huanglongbing

Authors: Krystel, Joseph; Stover, Eddie; DOMINGUEZ, JESSICA - Florida International University; JAYACHANDRAN, KRISHNASWAMY - Florida International University; SHETTY, KATEEL - Florida International University

Submitted to: Microorganisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2023
Publication Date: 6/8/2023
Citation: Krystel, J.A., Stover, E.W., Dominguez, J., Jayachandran, K., Shetty, K. 2023. Endophytes and plant extracts as potential antimicrobial agents against Candidatus Liberibacter asiaticus, causal agent of huanglongbing. Microorganisms. https://doi.org/10.3390/microorganisms11061529.
DOI: https://doi.org/10.3390/microorganisms11061529

Interpretive Summary: Citrus production is threatened by citrus greening disease. There are no effective treatments. The control methods used also have potential negative side effects. This project studied natural plant extracts and bacteria present in healthy citrus trees to find potential therapies. The plant extracts showed strong antimicrobial activity but were toxic to trees at high concentrations. Compounds from the bacteria were shown to fight greening disease. These bacteria may provide a low-cost treatment for citrus greening without negative environmental effects.

Technical Abstract: Huanglongbing (HLB), also known as citrus greening is an insidious yet relentless disease of citrus. There is no known cure nor treatment to effectively control HLB. Current control methods are primarily based on the use of insecticides and antibiotics, but effectiveness is limited and has negative impacts on beneficial and non-target microorganisms and insects. Thus, there is an urgent need for the development of effective, yet sustainable treatment options to reduce or eliminate CLas from infected trees. We screened citrus endophytes from asymptomatic CLas infected trees, their cell-free culture supernatants (CFCS), and crude plant extracts for antimicrobial activity against two culturable surrogates of CLas, Sinorhizobium meliloti and Liberibacter crescens and for effectiveness directly against CLas, in vitro. In a leaf disk assay, the crude plant extracts of 5 medicinal plant species Christmas bush (Alchornea cordifolia), oregano (Origanum vulgare), thyme (Thymus vulgaris), cinnamon (Cinnamomum aromaticum), turmeric (Curcuma longa) demonstrated statistically significant antimicrobial activity against CLas. However, results also indicated possible phytotoxicity toward citrus. A psyllid homogenate assay incorporating a viability qPCR was used to measure antimicrobial activity. Statistically significant reductions of viable CLas cells were observed for each of 5 bacterial CFCS. Subsequent 16S rRNA gene sequencing revealed that each of the 5 bacterial isolates were most closely related to Bacillus amyloliquefaciens. These have the potential to be used as sources of novel antimicrobials for the sustainable management of HLB.