Candidatus Liberibacter asiaticus reduces callose and reactive oxygen species production in the phloem

Huanglongbing (HLB) causes significant economic loss in citrus production worldwide. HLB is caused by Candidatus Liberibacter asiaticus ( C Las), a gram-negative bacterium which inhabits the phloem exclusively. C Las infection results in accumulation of callose and reactive oxygen species in the phloem of infected plants, but little is known about the specific processes that take place during infection because of the sparse distribution of bacteria and the inaccessibility of the phloem inside the tree. In this study, we used the seed vasculatures, which accumulate a high number of C Las, as a model tissue to study C Las-host cellular interactions. In vasculature where C Las is abundant, sieve pore callose and H 2 O 2 concentration were reduced compared to healthy seed vasculature. The expression of callose synthases ( CalS ) and respiratory burst oxidase homolog ( RBOH ) genes were downregulated in infected seeds compared to healthy ones. In leaves of HLB-infected plants, H 2 O 2 concentration and CalS expression increased compared to uninfected leaves, but cells with C Las had lower levels of sieve plate callose compared to cells without C Las. Our results provide evidence that the bacteria manipulate cell metabolism to disable plant defenses and suggests that HLB disease is the result of a constant arms-race between the pathogen and a defense response, which is ultimately harmful to the host plant.