Faustine Ryckebusch defended her thesis on 13 March 2020. Carried out within the framework of the E-Space flagship project, and financed by Agropolis Fondation, her subject was "Characterization of the transmission of geminiviruses belonging to the new genus Capulavirus typical for aphid transmission".
Faustine was supervised by Michel Peterschmitt and hosted by Montpellier, SupAgro, in the framework of Biodiversity, Agriculture, Food, Environment, Land, Water, in partnership with BGPI - Biology and Genetics of Plant-Parasite Interactions (laboratory).
Abstract
The family Geminiviridae is the first group of plant viruses in terms of number and economic importance. They have circular single-stranded (css) DNA genomes and are transmitted in a circulative non-propagative (cnp) manner by leafhoppers, whiteflies, treehoppers and aphids (Hemiptera). While diseases induced by geminiviruses were reported for more than a century, aphid transmitted geminiviruses were discovered only recently by metagenomics. This delayed discovery is puzzling because aphids are the hemipterans that transmit the most plant viruses including the viruses of families Nanoviridae and Luteoviridae, transmitted in a cnp manner. Aphid transmission was detected among a group of 4 diverging geminiviruses exhibiting a typical genome organization and for which the genus Capulavirus was created. Aphid transmission was shown only for 2 of them, the Alfalfa leaf curl virus (ALCV) transmitted by Aphis craccivora and the Plantago latent virus (PlLV) transmitted by Dysaphis plantaginae. By demonstrating aphid transmission of a third capulavirus, Euphorbia caput-medusae latent virus (EcmLV), aphid transmission was validated as a generic taxonomic criterion. The population of A. craccivora that transmits EcmLV does not transmit ALCV, which reflect high transmission specificity among capulaviruses. ALCV was selected to investigate further geminivirus transmission by aphids because it was isolated on cultivated plants in France and associated with symptoms. Its localization in plant tissues (FISH) and the accurate analysis of the feeding behavior of the vector by electropenetrography (EPG) revealed that ALCV is phloem restricted and that its distribution is heterogeneous. Transmission tests and estimations of virus contents (qPCR) show that 12% of aphids, at maximum, transmit ALCV and that transmission is dependent on a critical threshold of virus content. We also show that ALCV accumulates and persist efficiently in the digestive tract of the vector aphid, with amounts that are similar to those of geminiviruses in their leafhopper of whitefly vectors. On the contrary, the amount and persistence of ALCV in hemolymph and heads are low, consistently with the low detection of ALCV by FISH in the salivary glands unlike in the digestive tract. These results may explain the low individual transmission rate of ALCV by A. craccivora, much lower than the 90% rate observed with geminiviruses transmitted by whiteflies and leafhoppers. Thanks to a broad bean nanovirus transmitted by A. craccivora, Faba bean necrotic stunt virus (FbNSV), we showed that the low transmission of ALCV is not imputable to the aphid. Indeed the comparisons carried out with the same A. craccivora population show that the transmission rate of FbNSV is 2 to 5 times higher than that of ALCV and FbNSV was excreted much more efficiently. FISH observations show that ALCV and FbNSV do not colocalize in midgut cells, which is consistent with distinct pathways. While nanoviruses and capulaviruses have both a circular single-stranded DNA genome, only nanoviruses encode a helper component for transmission (NSP). In geminiviruses, only CP is determining vector transmission. A yeast two-hybrid analysis carried out against the CP of ALCV identified three proteins of the digestive tract of A. craccivora. Their role in ALCV transmission will have to be validated with transmission and complementary interaction tests. Together with a similar study carried out in parallel by another Ph.D. student with the CP and the NSP of FbNSV, we expect to validate the hypothesis of distinct pathways with molecular data. The generalization of the low transmission efficiency of ALCV to all capulaviruses may explain both their late discovery and their preferential adaptation to perennial hosts. This hypothesis is discussed according to some rare descriptions of capulaviruses in natural conditions and in comparison with viruses of families Nanoviridae and Luteoviridae.