Insect Science

Abstract Huanglongbing, a highly destructive disease of citrus species, is associated with a fastidious, gram-negative, phloem-limited bacteria (Candidatus Liberibacter spp.). In Florida, the causative agent of Huanglongbing (HLB) is C. Liberibacter asiaticus (CLas) and it is transmitted by the insect vector, Asian citrus psyllid (Diaphorina citri Kuwayama). Previous investigations have revealed systemic infection of CLas with an erratic and uneven distribution of pathogen in tree phloem. However, previous investigations did not consider the potential impact of plant vegetative growth on presence/absence of CLas in planta. Our objectives were to determine: 1) the effect of vegetative growth of Citrus sinensis (L.) Osbeck cv Valencia on detection of CLas in mature leaves, and 2) the impact of CLas inoculation frequency on progression of CLas titer in citrus leaves through the first year of infection. Temporal dynamics of CLas detection were associated with vegetative flush growth. Surprisingly, there was no difference in CLas titer detected between plants exposed to infected vectors for a one-time 7 d inoculation access period, as compared with plants exposed to continuously breeding CLas-infected insects over the course of an entire year of plant infection. Our results suggest that the CLas bacterium is transported through phloem during annual movement of carbon compounds needed for vegetative plant growth, including transportation from roots to mature leaves. These results highlight the importance of vegetative growth on temporal dynamics of CLas in citrus, and suggest a critical role of the sink-source interaction on presence/absence of CLas in leaves.

AbstractRickettsia typhi and Rickettsia felis (Rickettsiales: Rickettsiaceae) are flea-transmitted pathogens. They are important causes of acute febrile illness throughout the world. We, therefore, sought to identify the rickettsial species present in the fleas of dogs and cats in the department of Cauca, Colombia. In this study, we collected 1,242 fleas from 132 dogs and 43 fleas from 11 cats. All fleas were morphologically identified as Ctenocephalides felis (Bouché) adults and organized in pools for DNA extraction (234 pools from dogs and 11 from cats). The gltA gene from rickettsiae was targeted for screening amplification using conventional PCR. In total, 144 of the 245 pools (58.7%) were positive. The positive samples were then processed for the amplification of the 17kDa antigen gene (144/144; 100% positive) and sca5 gene (140/144; 97.2% positive). In addition, restriction enzyme length polymorphism analysis using NlaIV on the amplified product of the sca5 gene demonstrated several organisms: 21/140 (15%) were R. felis, 118/140 (84.3%) were Rickettsia asemboensis, and 1/140 (0.7%) were Candidatus Rickettsia senegalensis. Subsequent sequencing confirmed Candidatus Rickettsia senegalensis in C. felis collected from dogs the first reported from Colombia.

Candidatus Liberibacter solanacerum (CLso), transmitted by Bactericera trigonica in a persistent and propagative mode causes carrot yellows disease, inflicting hefty economic losses. Understanding the process of transmission of CLso by psyllids is fundamental to devise sustainable management strategies. Persistent transmission involves critical steps of adhesion, cell invasion, and replication before passage through the midgut barrier. This study uses a transcriptomic approach for the identification of differentially expressed genes with CLso infection in the midguts, adults, and nymphs of B. trigonica and their putative involvement in CLso transmission. Several genes related to focal adhesion and cellular invasion were upregulated after CLso infection. Interestingly, genes involved with proper functionality of the endoplasmic reticulum (ER) were upregulated in CLso infected samples. Notably, genes from the endoplasmic reticulum associated degradation (ERAD) and the unfolded protein response (UPR) pathway were overexpressed after CLso infection. Marker genes of the ERAD and UPR pathways were also upregulated in Diaphorina citri when infected with Candidatus Liberibacter asiaticus (CLas). Upregulation of the ERAD and UPR pathways indicate induction of ER stress by CLso/CLas in their psyllid vector. The role of ER in bacteria–host interactions is well-documented; however, the ER role following pathogenesis of CLso/CLas is unknown and requires further functional validation.

Abstract Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), transmits Candidatus Liberibacter asiaticus (Las), the bacterial pathogen putatively responsible for citrus huanglongbing. Multiple studies have shown psyllids acquire Las more frequently, and are more likely to inoculate susceptible plants, when they acquire Las as nymphs. Understanding the transmission of Las to nymphs is critical to the Las lifecycle. The objective of this study was to determine the transmission Las by female D. citri to their offspring. Two transmission pathways were quantified: horizontal transmission (acquisition of Las via feeding at the oviposition site) and vertical transmission (transovarial). Eggs of individual, infected females were transferred to an uninfected seedling to assess vertical transmission. In a second experiment, horizontal transmission was evaluated by replacing eggs laid by infected females with uninfected nymphs. Nymphs exposed to Las via horizontal transmission of the oviposition site were more likely to acquire Las than from vertical transmission. Las deposited in flush by an infected adult female feeding during oviposition was sufficient for infecting nymphs. Combined results of both experiments suggest that vertical transmission allows Las to spread in low amounts even when infected plant hosts are not available and that inoculation of the oviposition site provides a source of Las to developing nymphs via the plant phloem. These data support the hypothesis that transmission through infected plant material via maternal inoculation is a primary pathway of Las transmission between vector and host.