Résumé
Geminiviruses (family Geminiviridae) are plant viruses with circular ssDNA genomes encapsidated in geminate virions and transmitted by insect vectors in a persistent circulative manner, causing severe crop diseases globally. Gemiviruses of the genus Begomovirus (begomoviruses) are often associated with circular ssDNA betasatellites encoding a protein that suppresses plant RNA interference (RNAi)-based defenses and innate immunity. Geminiviruses replicate in plant cells through rolling circle replication (RCR) and recombination-dependent replication (RDR), generating circular ssDNA for encapsidation and linear concatemeric dsDNA, respectively. RDR products may serve for resurrection of cirlular ssDNA via RCR and/or for evasion of the antiviral RNAi generating virus-derived small RNAs. Recent studies showed that Tomato yellow leaf curl virus (TYLCV) can replicate in the salivary glands of its insect vector Bemisia tabaci (whitefly), which ensures persistent infection and efficient transmission to new plants. This discovery raises questions about the underlying replication mechanisms and how the whitefly antiviral RNAi and innate immunity respond. To validate the function of host plant and insect vector genes implicated in begomovirus replication and transmission, we applied a virus-induced gene silencing (VIGS) approach. We designed a disarmed betasatellite vector lacking an RNAi suppressor, for carrying inserts of candidate plant and insect genes. Initial tests confirmed efficient replication of empty VIGS constructs in host plants and successful transmission by whiteflies. Using VIGS to knock down expression of the plant PCNA gene essential for DNA replication led to severe developmental abnormalities, unlike controls. Current experiments involve silencing of the whitefly PCNA gene implicated in begomoviral DNA replication and two other whitefly genes implicated in begomovirus circulation and transmission. The results of these experiments will be presented and discussed. As an ultimate goal, we aim to develop new strategies for the control of viral disease spread at the transmission step without affecting insect viability.