Reduced cold tolerance of viral-infected leafhoppers attenuates viral persistent epidemics

Most arthropod-borne viruses produce intermittent epidemics in infected plants. However, the underlying mechanisms of these epidemics are unclear. Here, we demonstrated that rice stripe mosaic virus (RSMV), a viral pathogen, significantly increases the mortality of its overwintering vector, the leafhopper species Recilia dorsalis. Cold-stress assays indicated that RSMV reduces the cold tolerance of leafhoppers, a process associated with the downregulation of leafhopper cuticular protein genes. An RSMV-derived small RNA (vsiR-t00355379) was found to facilitate the downregulation of a leafhopper endocuticle gene that is mainly expressed in the abdomen (named RdABD-5) and is conserved across dipteran species. The downregulation of RdABD-5 expression in R. dorsalis resulted in fewer and thinner endocuticle lamellae, leading to decreased cold tolerance. This effect was correlated with a reduced incidence rate of RSMV in early-planted rice plants. These findings contribute to our understanding of the mechanism by which viral pathogens reduce cold tolerance in arthropod vectors and suggest an approach to managing the fluctuating prevalence of arboviruses.

IMPORTANCE

Increasing arthropod vector dispersal rates have increased the susceptibility of crop to epidemic viral diseases. However, the incidence of some viral diseases fluctuates annually. In this study, we demonstrated that a rice virus reduces the cold tolerance of its leafhopper vector, Recilia dorsalis. This effect is linked to the virus-derived small RNA-mediated downregulation of a gene encoding a leafhopper abdominal endocuticle protein. Consequently, the altered structural composition of the abdominal endocuticle reduces the overwinter survival of leafhoppers, resulting in a lower incidence of RSMV infection in early-planted rice plants. Our findings illustrate the important roles of RNA interference in virus-vector insect-environment interactions and help explain the annual fluctuations of viral disease epidemics in rice fields.

Diversity and Dynamics of Bacterial Communities in the Digestive and Excretory Systems across the Life Cycle of Leafhopper, Recilia dorsalis

Recilia dorsalis is a notorious rice pest that harbors numerous symbiotic microorganisms. However, the structure and dynamics of bacterial communities in various tissues of R. dorsalis throughout its life cycle remain unclear. In this study, we used high-throughput sequencing technology to analyze the bacterial communities in the digestive, excretory, and reproductive systems of R. dorsalis at different developmental stages. The results showed that the initial microbiota in R. dorsalis mostly originated from vertical transmission via the ovaries. After the second-instar nymphs, the diversity of bacterial communities in the salivary gland and Malpighian tubules gradually decreased, while the midgut remained stable. Principal coordinate analysis revealed that the structure of bacterial communities in R. dorsalis was primarily influenced by the developmental stage, with minimal variation in bacterial species among different tissues but significant variation in bacterial abundance. Tistrella was the most abundant bacterial genus in most developmental stages, followed by Pantoea. The core bacterial community in R. dorsalis continuously enriched throughout development and contributed primarily to food digestion and nutrient supply. Overall, our study enriches our knowledge of the bacterial community associated with R. dorsalis and provides clues for developing potential biological control technologies against this rice pest.

Rice Stripe Mosaic Disease: Characteristics and Control Strategies

Rice stripe mosaic disease (RSMD) is caused by the rice stripe mosaic virus (RSMV; genus Cytorhabdovirus, family Rhabdoviridae). In recent years, significant progress has been made in understanding several aspects of the disease, especially its geographical distribution, symptoms, vectors, gene functions, and control measures. Since RSMD was first detected in southern China in 2015, it has been found in more and more rice growing areas and has become one of the most important rice diseases in southern China. RSMV is transmitted by the leafhopper Recilia dorsalis in a persistent-propagative manner, inducing yellow stripes, a slight distortion of leaves, increased tillers, and empty grains in rice plants. The virus has a negative-sense single-strand RNA genome of about 12.7 kb that encodes seven proteins: N, P, P3, M, G, P6, and L. Several molecular and serological tests have been developed to detect RSMV in plants and insects. The disease cycle can be described as follows: RSMV and its vector overwinter in infected plants; viruliferous R. dorsalis adults transmit the virus to spring rice and lay eggs on the infected seedlings; the next generation of R. dorsalis propagate on infected seedlings, become viruliferous, disperse, and cause new disease outbreaks. Control measures include monitoring and accurate forecasting, selecting disease-resistant varieties, improving cultivation systems, covering rice seedling nurseries with insect-proof nets, and using pesticides rationally. Inappropriate cultivation systems, pesticide overuse, and climatic conditions contribute to epidemics by affecting the development of vector insects and their population dynamics.

Development of a Specific Polymerase Chain Reaction System for the Detection of Rice Orange Leaf Phytoplasma Detection

Rice orange leaf disease (ROLD), caused by rice orange leaf phytoplasma (ROLP), is transmitted by leafhopper vectors Recilia dorsalis and Nephotettix cinticeps. ROLD severely devastates rice production in Asia. Accurate detection of the pathogen is important for disease management. Current nested polymerase chain reaction (nested PCR) method using phytoplasma universal primers is widely used to detect phytoplasmas; however, it has shortcoming of inconvenience and inaccuracy, for it needs two round of PCR reactions and could produce false positive results due to nontarget amplification. In this study, we developed a PCR assay using a set of primers designed based on the ROLP genome sequence to amplify house-keeping gene FtsH-1 in rice and leafhopper vector samples. This method is simple and rapid, and its sensitivity up to 10 pg/μl of total ROLP DNA. It also minimizes the false positive problem produced by nested PCR. This method was used to survey the geographic distribution of ROLD in southern China from 2016 to 2018. The results showed that the distribution areas and vector carrying rate of ROLD had gradually increased.

Transmission Biology of Rice Stripe Mosaic Virus by an Efficient Insect Vector Recilia dorsalis (Hemiptera: Cicadellidae)

Rice stripe mosaic virus (RSMV) is a newly discovered species of cytorhabdovirus infecting rice plants that is transmitted by the leafhopper Recilia dorsalis. In this study, the transmission characteristics of RSMV by R. dorsalis were investigated. Under suitable growth conditions for R. dorsalis, the RSMV acquisition rate reached 71.9% in the second-generation population raised on RSMV-infected rice plants. The minimum acquisition and inoculation access periods of R. dorsalis were 3 and 30 min, respectively. The minimum and maximum latent transmission periods of RSMV in R. dorsalis were 6 and 18 d, respectively, and some R. dorsalis intermittently transmitted RSMV at 2-6 d intervals. Our findings revealed that the virus can replicate in the leafhopper body, but is likely not transovarially transmitted to offspring. These transmission characteristics will help guide the formulation of RSMV prevention and control strategies.

Rice Stripe Mosaic Virus, a Novel Cytorhabdovirus Infecting Rice via Leafhopper Transmission

A new rice viral disease exhibiting distinct symptoms-yellow stripes, mosaic and twisted tips on leaves-was found in China. Electron microscopy of infected leaf cells revealed the presence of bacilliform virions and electron-translucent granular-fibrillar viroplasm in the cytoplasm. The enveloped viral particles were 300 to 375 nm long and 45 to 55 nm wide. The leafhopper Recilia dorsalis was able to transmit the virus to rice seedlings, which subsequently exhibited symptoms similar to those observed in fields. The complete genome of the virus was obtained by small-RNA deep sequencing and reverse transcription-PCR product sequencing. The anti-genome contains seven open reading frames (ORFs). The deduced amino acids of ORF1, ORF5, and ORF7 are, respectively, homologous to the nucleocapsid protein (N), glycoprotein (G), and large polymerase protein (L) of known rhabdoviruses. The predicted product of ORF2 is identified as a phosphoprotein (P) based on its multiple potential phosphorylation sites and 12.6 to 21.0% amino acid (aa) identities with the P proteins of plant rhabdoviruses. The product of ORF4 is presumed to be the viral matrix (M) protein for it shares 10.3 to 14.3% aa identities with those of other rhabdoviruses. The above five products were confirmed as the viral structural proteins by SDS-PAGE and aa sequencing analyses of purified virus preparation. ORF3 and ORF6 are considered to encode two nonstructural proteins with unknown functions. Phylogenetic analysis based on protein N, G, and L amino acid sequences indicated that the isolated virus, which we have tentatively named Rice stripe mosaic virus (RSMV), is a new species in the genus Cytorhabdovirus. To our knowledge, RSMV is the only cytorhabdovirus naturally infecting rice and the first reported leafhopper-transmitted cytorhabdovirus. Our surveys of rice fields indicate that RSMV occurs frequently in Guangdong Province, China. Although the disease incidence is low at present, it might become serious with the vector insect population increasing.