Abstract
Candidatus Liberibacter asiaticus (CLas) is a phloem-limited, gram-negative, fastidious bacterium that is associated with the development of citrus greening disease, also known as Huanglongbing (HLB). CLas is transmitted by the Asian citrus psyllid (ACP) Diaphorina citri, in a circulative manner. Two major barriers to transmission within the insect are the midgut and the salivary glands. We performed a thorough microscopic analysis within the insect midgut following exposure to CLas-infected citrus trees. We observed changes in nuclear architecture, including pyknosis and karyorrhexis as well as changes to the actin cytoskeleton in CLas-exposed midgut cells. Further analyses showed that the changes are likely due to the activation of programmed cell death as assessed by Annexin V staining and DNA fragmentation assays. These results suggest that exposure to CLas-infected trees induces apoptotic responses in the psyllid midgut that should be further investigated. Understanding the adaptive significance of the apoptotic response has the potential to create new approaches for controlling HLB.
Highlights
Candidatus Liberibacter asiaticus (CLas) is a phloem-limited, Gram-negative, fastidious bacterium that is implicated in causing the most serious disease of citrus, citrus greening disease, referred to as Huanglongbing
These results indicated that Asian citrus psyllid (ACP) which acquire the CLas as adults are poor vectors of the pathogen compared with adults that acquired the pathogen as nymphs, suggesting that bacterial multiplication in the psyllid may be essential for efficient transmission[13], replication of the bacterium in insects has not been conclusively shown
The CLas OmpA signal appeared as a punctate pattern of dots with a diameter smaller than 1 μm, which localized inside the gut cells
Summary
Candidatus Liberibacter asiaticus (CLas) is a phloem-limited, Gram-negative, fastidious bacterium that is implicated in causing the most serious disease of citrus, citrus greening disease, referred to as Huanglongbing. Due to the low rate of pathogen acquisition and long time period required for successful inoculation by adult ACP, it was not possible to determine the latent period[12] These results indicated that ACPs which acquire the CLas as adults are poor vectors of the pathogen compared with adults that acquired the pathogen as nymphs, suggesting that bacterial multiplication in the psyllid may be essential for efficient transmission[13], replication of the bacterium in insects has not been conclusively shown. The interactions and parameters for acquisition, transmission and retention of CLso by B. cockerelli have been investigated, but with fewer details as compared to CLas. One study showed that increasing the latency period increased the chance the bacteria would reach the salivary glands and be transmitted, while a two week latent period was a prerequisite for transmission by adults[14]. It is possible that this immune response is related to the inability of adults to efficiently acquire and transmit CLas[12,13]
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