Replication of Tomato Yellow Leaf Curl Virus in Its Whitefly Vector, Bemisia tabaci

The Whitefly Bemisia tabaci Knottin-1 Gene Is Implicated in Regulating the Quantity of Tomato Yellow Leaf Curl Virus Ingested and Transmitted by the Insect

The whitefly Bemisia tabaci is a major pest to agricultural crops. It transmits begomoviruses, such as Tomato yellow leaf curl virus (TYLCV), in a circular, persistent fashion. Transcriptome analyses revealed that B. tabaci knottin genes were responsive to various stresses. Upon ingestion of tomato begomoviruses, two of the four knottin genes were upregulated, knot-1 (with the highest expression) and knot-3. In this study, we examined the involvement of B. tabaci knottin genes in relation to TYLCV circulative transmission. Knottins were silenced by feeding whiteflies with knottin dsRNA via detached tomato leaves. Large amounts of knot-1 transcripts were present in the abdomen of whiteflies, an obligatory transit site of begomoviruses in their circulative transmission pathway; knot-1 silencing significantly depleted the abdomen from knot-1 transcripts. Knot-1 silencing led to an increase in the amounts of TYLCV ingested by the insects and transmitted to tomato test plants by several orders of magnitude. This effect was not observed following knot-3 silencing. Hence, knot-1 plays a role in restricting the quantity of virions an insect may acquire and transmit. We suggest that knot-1 protects B. tabaci against deleterious effects caused by TYLCV by limiting the amount of virus associated with the whitefly vector.

Acquisition, Replication and Inoculation of Candidatus Liberibacter asiaticus following Various Acquisition Periods on Huanglongbing-Infected Citrus by Nymphs and Adults of the Asian Citrus Psyllid

The Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), is the primary vector of Candidatus Liberibacter asiaticus (Las) implicated as causative agent of citrus huanglongbing (citrus greening), currently the most serious citrus disease worldwide. Las is transmitted by D. citri in a persistent-circulative manner, but the question of replication of this bacterium in its psyllid vector has not been resolved. Thus, we studied the effects of the acquisition access period (AAP) by nymphs and adults of D. citri on Las acquisition, multiplication and inoculation/transmission. D. citri nymphs or adults (previously non-exposed to Las) were caged on Las-infected citrus plants for an AAP of 1, 7 or 14 days. These 'Las-exposed' psyllids were then transferred weekly to healthy citrus or orange jasmine plants, and sampled via quantitative polymerase chain reaction (qPCR) analysis 1-42 days post-first access to diseased plants (padp); all tested nymphs became adults 7-14 days padp. Our results indicate that following 1 or 7 day AAP as nymphs 49-59% of Las-exposed psyllids became Las-infected (qPCR-positive), whereas only 8-29% of the psyllids were infected following 1-14 day AAP as adults. Q-PCR analysis also indicated that Las titer in the Las-exposed psyllids (relative to that of the psyllid S20 ribosomal protein gene) was: 1) significantly higher, and increasing at a faster rate, following Las acquisition as nymphs compared to that following Las acquisition as adults; 2) higher as post-acquisition time of psyllids on healthy plants increased reaching a peak at 14-28 days padp for nymphs and 21-35 days padp for adults, with Las titer decreasing or fluctuating after that; 3) higher with longer AAP on infected plants, especially with acquisition as adults. Our results strongly suggest that Las multiplies in both nymphs and adults of D. citri but attains much higher levels in a shorter period of time post-acquisition when acquired by nymphs than when acquired by adults, and that adults may require longer access to infected plants compared to nymphs for Las to reach higher levels in the vector. However, under the conditions of our experiments, only D. citri that had access to infected plants as nymphs were able to inoculate Las into healthy citrus seedlings or excised leaves. The higher probability of Las inoculation into citrus by psyllids when they have acquired this bacterium from infected plants during the nymphal rather than the adult stage, as reported by us and others, has significant implications in the epidemiology and control of this economically important citrus disease.

The autophagy pathway participates in resistance to tomato yellow leaf curl virus infection in whiteflies

Macroautophagy/autophagy plays an important role against pathogen infection in mammals and plants. However, little has been known about the role of autophagy in the interactions of insect vectors with the plant viruses, which they transmit. Begomoviruses are a group of single-stranded DNA viruses and are exclusively transmitted by the whitefly Bemisia tabaci in a circulative manner. In this study, we found that the infection of a begomovirus, tomato yellow leaf curl virus (TYLCV) could activate the autophagy pathway in the Middle East Asia Minor 1 (MEAM1) species of the B. tabaci complex as evidenced by the formation of autophagosomes and ATG8-II. Interestingly, the activation of autophagy led to the subsequent degradation of TYLCV coat protein (CP) and genomic DNA. While feeding the whitefly with 2 autophagy inhibitors (3-methyladenine and bafilomycin A1) and silencing the expression of Atg3 and Atg9 increased the viral load; autophagy activation via feeding of rapamycin notably decreased the amount of viral CP and DNA in the whitefly. Furthermore, we found that activation of whitefly autophagy could inhibit the efficiency of virus transmission; whereas inhibiting autophagy facilitated virus transmission. Taken together, these results indicate that TYLCV infection can activate the whitefly autophagy pathway, which leads to the subsequent degradation of virus. Furthermore, our report proves that an insect vector uses autophagy as an intrinsic antiviral program to repress the infection of a circulative-transmitted plant virus. Our data also demonstrate that TYLCV may replicate and trigger complex interactions with the insect vector.

Fluorescent in situ hybridization for the localization of viruses, bacteria and other microorganisms in insect and plant tissues

Methods for the localization of cellular components such as nucleic acids, proteins, cellular vesicles and more, and the localization of microorganisms including viruses, bacteria and fungi have become an important part of any research program in biological sciences that enable the visualization of these components in fixed and live tissues without the need for complex processing steps. The rapid development of microscopy tools and technologies as well as related fluorescent markers and fluorophores for many cellular components, and the ability to design DNA and RNA sequence-based molecular probes and antibodies which can be visualized fluorescently, have rapidly advanced this field. This review will focus on some of the localizations methods which have been used in plants and insect pests in agriculture, and other microorganisms, which are rapidly advancing the research in agriculture-related fields.

Temporal Effects of a Begomovirus Infection and Host Plant Resistance on the Preference and Development of an Insect Vector, Bemisia tabaci, and Implications for Epidemics

Persistent plant viruses, by altering phenotypic and physiological traits of their hosts, could modulate the host preference and fitness of hemipteran vectors. A majority of such modulations increase vector preference for virus-infected plants and improve vector fitness, ultimately favouring virus spread. Nevertheless, it remains unclear how these virus-induced modulations on vectors vary temporally, and whether host resistance to the pathogen influences such effects. This study addressed the two questions using a Begomovirus-whitefly-tomato model pathosystem. Tomato yellow leaf curl virus (TYLCV) -susceptible and TYLCV-resistant tomato genotypes were evaluated by whitefly-mediated transmission assays. Quantitative PCR revealed that virus accumulation decreased after an initial spike in all genotypes. TYLCV accumulation was less in resistant than in susceptible genotypes at 3, 6, and 12 weeks post inoculation (WPI). TYLCV acquisition by whiteflies over time from resistant and susceptible genotypes was also consistent with virus accumulation in the host plant. Furthermore, preference assays indicated that non-viruliferous whiteflies preferred virus-infected plants, whereas viruliferous whiteflies preferred non-infected plants. However, this effect was prominent only with the susceptible genotype at 6 WPI. The development of whiteflies on non-infected susceptible and resistant genotypes was not significantly different. However, developmental time was reduced when a susceptible genotype was infected with TYLCV. Together, these results suggest that vector preference and development could be affected by the timing of infection and by host resistance. These effects could play a crucial role in TYLCV epidemics.