Antibacterial FANA oligonucleotides as a novel approach for managing the Huanglongbing pathosystem

Sandoval-Mojica et al., Scientific Reports, 2021 tested whether an antisense oligonucleotide can work as an antibacterial in agriculture, against the organism behind citrus greening disease. Huanglongbing is caused by Candidatus Liberibacter asiaticus, carried between trees by the Asian citrus psyllid, and there is no cure; growers rely on insecticides and on removing infected trees. RNA interference cannot reach the bacterium at all, because bacteria have no such pathway, which is what makes an antisense approach worth testing here rather than the usual double stranded RNA. AUM BioTech designed and made the oligonucleotides, and AUM is a co-author on the paper.

The work moved outward from cells to insects to trees. A scrambled control oligonucleotide ran in the cell culture, viability, leaf feeding and artificial diet arms. The tree infusion was compared with streptomycin sulfate and water, and the transmission assay with psyllids fed a sucrose solution. In Drosophila S2 cells carrying the psyllid's own Wolbachia endosymbiont, a labelled oligonucleotide was inside the cells within 24 hours with no transfection agent and no delivery conjugate. An AUMsilence sdASO against the Wolbachia gyrase gene, at 5 μM for 7 days, cut that transcript by 30% and reduced the number of viable bacterial cells by 88% against the scrambled control and 87% against untreated cells, which shows the oligonucleotide crossed into the bacteria as well as into the insect cell. Adult psyllids that fed for 7 days on citrus leaves standing in a 5 μM solution showed a 75% fall in the bacterial ligase transcript, and the labelled oligonucleotide could be seen in the oesophagus, filter chamber and anterior midgut.

The last two steps were in whole organisms. Psyllids fed the oligonucleotide in an artificial diet carried significantly less of the bacterium than untreated or scrambled control insects, and they passed it on to fewer plants: seedlings exposed to treated insects held significantly fewer bacterial gene copies when sampled 6 months later. Infused into a single root of an infected citrus tree, 5 mL of a 5 μM solution suppressed bacterial titre for the whole 30 day trial, with the largest falls at two and 7 days, 81% and 51.6%. Streptomycin sulfate infused the same way did not significantly reduce the titre, and trees given water alone rose by 67% and 89% at 14 and 30 days.