Association of endoplasmic reticulum associated degradation (ERAD) with the transmission of Liberibacter solanacearum by its psyllid vector

Immunity responses as checkpoints for efficient transmission of begomoviruses by whiteflies

Begomoviruses are a group of single stranded DNA plant viruses exclusively transmitted by the sweet potato whitefly Bemisia tabaci in a persistent, circulative manner. After acquisition from plant phloem, this group of viruses circulate and are retained within the whitefly, interacting with tissues, cells and molecular pathways for maintaining the safety of the infective intact virions, by exploiting cellular mechanisms and avoiding degradation by the insect immune responses. During retention, the virions are internalized in the midgut cells, exit and spend hours-days in the hemolymph and cross into salivary gland cells, before transmission. Destroying this group of viruses by the insect immune system seems inefficient for the most part, by examining their very efficient transmission. Thus, within the various sites along the transmission pathway especially in the midgut, it is thought that the immune system with its various layers is activated for avoiding the damage caused by the viruses on one hand, and for ensuring their safe circulation and transmission on the other hand. Begomoviruses have evolved mechanisms for counteracting and exploiting the activated immune system for their safe translocation within the whitefly. In this review, we discuss the various levels of immunity activated against begomoviruses in B. tabaci, taking other pathogen-vector systems as examples and reflecting relevant components on the interactions between B. tabaci and Begomoviruses.

An insect virus differentially alters gene expression among life stages of an insect vector and enhances bacterial phytopathogen transmission

Diaphorina citri transmits Candidatus Liberibacter asiaticus (CLas) between citrus plants which causes the expression of huanglongbing disease in citrus. D. citri flavi-like virus (DcFLV) co-occurs intracellularly with CLas in D. citri populations in the field. However, the impact(s) of DcFLV presence on the insect vector and its interaction with the CLas phytopathogen remain unclear. We compared CLas acquisition and transmission efficiencies as well as transcriptomic expression between viruliferous and non-viruliferous psyllids at multiple life stages. Viruliferous nymphs acquired higher titers of CLas than non-viruliferous nymphs, whereas viruliferous adults acquired less CLas than those without virus. The presence of DcFLV increased the transmission of CLas by both nymphs and adults. Furthermore, RNA-seq and functional gene expression analyses revealed that endoplasmic reticulum stress-, autophagy-, and defense-related genes were significantly upregulated in viruliferous adult psyllids, whereas most of these genes were downregulated in viruliferous nymphs. Our work demonstrates that DcFLV differentially modulates various cellular and physiological functions in D. citri in a life stage-dependent manner and promotes the acquisition of CLas at the nymphal stage and transmission of the pathogen at the adult stage of the vector. Collectively, our results suggest that D. citri vectors with DcFLV exhibit greater pathogen transmission efficiency than those without virus.

IMPORTANCE

Huanglongbing (HLB), caused by fastidious bacteria from three Candidatus Liberibacter species, is the most damaging disease impacting the citrus industry worldwide. Spread by the Asian citrus psyllid (Diaphorina citri) in Asia and the Americas, HLB causes substantial financial losses, and has reduced citrus production in Florida by more than 90%. Although there are ongoing efforts to limit spread of the disease, effective HLB management remains elusive. Suppressing vector populations and decreasing CLas transmission are the two strategies that need to be urgently improved. Recently, a D. citri flavi-like virus (DcFLV) was characterized within its D. citri host, and it co-occurs intracellularly with CLas in psyllid populations. Here, we show that viruliferous nymphs exhibit higher CLas acquisition than non-viruliferous nymphs. Furthermore, both viruliferous adults and nymphs exhibit increased CLas transmission efficiency. We suggest the possibility of manipulating DcFLV in D. citri populations to reduce CLas transmission for HLB disease management.

Accumulation and Transmission of 'Candidatus Liberibacter solanacearum' Haplotypes by the Nymphs of Two Psyllid Vectors

'Candidatus Liberibacter solanacearum' (Lso) is a plant pathogenic bacterium transmitted by psyllids that causes significant agricultural damage. Several Lso haplotypes have been reported. Among them, LsoA and LsoB are transmitted by the potato psyllid Bactericera cockerelli and infect solanaceous crops, and LsoD is transmitted by the carrot psyllid B. trigonica and infects apiaceous crops. Several studies evaluated the transmission of these haplotypes by adult psyllids. However, fewer data are available on the transmission of different Lso haplotypes by psyllid nymphs. In this study, we investigated the transmission of these three haplotypes by psyllid nymphs to expand our basic understanding of Lso transmission. Specifically, the objective was to determine if the haplotypes differed in their transmission rates by nymphs and if LsoA and LsoB accumulated at different rates in the guts of nymphs as it occurs in adults. First, we quantified LsoA and LsoB titers in the guts of third- and fifth-instar potato psyllid nymphs. We found similar LsoA titers in the two nymphal stages, while LsoB titer was lower in the gut of the third-instar nymphs compared to fifth-instar nymphs. Second, we assessed the transmission efficiency of LsoA and LsoB by third-instar nymphs to tomato plants, revealing that LsoA was transmitted earlier and with higher efficiency than LsoB. Finally, we examined the transmission of LsoD by carrot psyllid nymphs to celery plants and demonstrated an age-related difference in the transmission rate. These findings provide valuable insights into the transmission dynamics of different Lso haplotypes by nymphal vectors, shedding light on their epidemiology and interactions with their psyllid vectors.