Small Interfering RNA Pathway Modulates Initial Viral Infection in Midgut Epithelium of Insect after Ingestion of Virus

Abstract

IMPORTANCE Many viral pathogens that cause significant global health and agricultural problems are transmitted via insect vectors. The first bottleneck in viral infection, the midgut epithelium, is a principal determinant of the ability of an insect species to transmit a virus. Southern rice black streaked dwarf virus (SRBSDV) is restricted exclusively to the midgut epithelium of an incompetent vector, the small brown planthopper (SBPH).Here, we show that silencing of the core component Dicer-2 of the small interfering RNA (siRNA) pathway increases viral titers in the midgut epithelium past the threshold (1.99 ؋ 10 9 copies of the SRBSDV P10 gene/g of midgut RNA) for viral dissemination into the midgut muscles and then into the salivary glands, allowing the SBPH to become a competent vector of SRBSDV. This result is the first evidence that the siRNA antiviral pathway has a direct role in the control of viral dissemination from the midgut epithelium and that it affects the competence of the virus's vector. Many viral pathogens that cause significant global health and agricultural problems are arthropod borne and transmitted via an insect vector. Many plant viruses are transmitted in a persistent manner by sap-sucking insects, including planthoppers, leafhoppers, thrips, and aphids (1-3). Plant viruses, when ingested by their insect vectors, establish their initial infection in the midgut epithelium, from where they disseminate to the midgut visceral muscles (1-3). The viruses then move from the midgut muscles into the hemolymph and finally into the salivary glands, from where they can be introduced into plant hosts (1-3). Efficient viral infection of the midgut epithelium, where initial infection is established, is thus a determinative factor in the ability of an insect species to transmit a certain plant virus.This ability of an insect species to transmit a virus, which has been described as vector competence, is common in nature (4, 5). For many years, the midgut barrier has been believed to be the principal determinant of vector competence (1-9). For example, infection of the incompetent insect vectors of Rice dwarf virus and SRBSDV, two plant reoviruses, and Tomato spotted wilt virus (TSWV), a tospovirus, is restricted to the midgut epithelium (7-10). The inability of incompetent insect vectors to transmit these viruses may be caused by a failure of the virus to efficiently replicate in or disseminate from the midgut epithelium (1-5). It is possible that viral titers in the midgut epithelium of incompetent vectors are unable to reach the threshold needed for viral dissemination. The innate immune response in the midgut epithelium may effectively control viral accumulation, thus blocking viral transmission by incompetent vectors. However, whether the initial antiviral immune pathway(s) in the insect midgut can modulate the competence of plant virus vectors is not unknown.Several mosquito and Drosophila innate immune responses have been reported to have an antiviral effect. These responses include RNA interfere...

Understanding the immune system architecture and transcriptome responses to southern rice black-streaked dwarf virus in Sogatella furcifera

“…Most interestingly, 66 immune genes were identified as MIEGs, indicating these immune pathways accumulated in response to SRBSDV replication. The RNAi pathway is the typical response; 56% (15/27) of genes in the pathway were identified as MIEGs, which is consistent with the previous finding that the RNAi pathway plays a crucial role in the modulation of the persistent propagation of SRBSDV in its competent vectors48. Considering that many cellular and humoral immune response genes were suppressed in the DEG analysis, the immune interaction between WBPH and SRBSDV is very complicated.…”

“…Invasion and dissemination of the virus are considered the infection barrier and spread-limiting steps in the body of vector insects4849. The first line of defense is provided by physical barriers, such as the chitin exoskeleton and tracheal and intestinal epithelia that protect the insect against entry by invading pathogens5051.…”

Rice Reoviruses in Insect Vectors

“…Our recent report provides new experimental evidence to show that the infection of RGDV through vertical transmission, which eliminates the indirect effects from infected plant hosts, has a direct adverse effect on the fitness of R. dorsalis (22). Thus, infection by rice reoviruses causes a limited adverse, but not pathogenic, effect on their vectors (57,58). Infection by rice reoviruses causes cytopathological changes of insect tissues.…”

“…SRBSDV is exclusively restricted to the intestinal epithelium of the incompetent L. striatellus (45). Our recent report shows that silencing the core component Dicer-2 of the siRNA pathway increases viral titers in the intestinal epithelium above the threshold (1.99 × 10 9 copies of SRBSDV P10 gene/µg midgut RNA) required for viral dissemination into the visceral muscles, then into the salivary glands, allowing L. striatellus to become a competent vector of SRBSDV (57). Thus, the siRNA pathway can modulate initial viral infection in the intestinal epithelium of the insect after ingestion of the virus, eventually affecting vector competence for the virus.…”

“…Indeed, the small interfering RNA (siRNA) antiviral pathway, a major innate immune response, is triggered by the persistent infection of rice reoviruses in respective vectors (57,58,60,124). Quantification of SRBSDV accumulation in the competent vector S. furcifera and incompetent vector L. striatellus has identified a clear threshold titer for a certain insect to be able to transmit the virus (6,36,70).…”