Abstract
The flavescence dorée phytoplasma undergoes a propagative cycle in its insect vectors by first interacting with the insect cell surfaces, primarily in the midgut lumen and subsequently in the salivary glands. Adhesion of flavescence dorée phytoplasma to insect cells is mediated by the adhesin VmpA. We hypothesize that VmpA may have lectin-like activity, similar to several adhesins of bacteria that invade the insect gut. We first demonstrated that the luminal surface of the midgut and the basal surface of the salivary gland cells of the natural vector Scaphoideus titanus and those of the experimental vector Euscelidius variegatus were differentially glycosylated. Using ELISA, inhibition and competitive adhesion assays, and protein overlay assays in the Euva-6 insect cell line, we showed that the protein VmpA binds insect proteins in a lectin-like manner. In conclusion, the results of this study indicate that N-acetylglucosamine and mannose present on the surfaces of the midgut and salivary glands serve as recognition sites for the phytoplasma adhesin VmpA.
Highlights
Diseases caused by phytoplasmas affect more than 1000 plant species, resulting in severe symptoms and notable economic losses in agricultural c rops[1,2,3,4]
This was hypothesized for P38 by demonstrating its interaction with insect proteins in vitro[20], and for the immunodominant membrane proteins Imp and IdpA and the variable membrane proteins Vmps, partly because, as a measure for antigenic membrane protein (Amp), they are subjected to strong selective pressure[21,22,23,24,25]
To determine which carbohydrates are present at the surfaces of the two main barriers that phytoplasmas must cross, that is, the luminal surface of midgut epithelial cells and the basal surface of salivary gland cells, we determined the binding capacity to these organs exhibited by fluorescent lectins with different specificities
Summary
Diseases caused by phytoplasmas affect more than 1000 plant species, resulting in severe symptoms and notable economic losses in agricultural c rops[1,2,3,4]. The spatiotemporal dynamics of the ‘Ca. P. asteris’-related strain onion yellows (OY) phytoplasma in the insect vector Macrosteles striifrons were precisely described by Koinuma et al.[16] The study by these researchers showed that the first organ to be colonized was the anterior midgut between 7 and 14 days after acquisition began. Primary sugar specificity αGalNAc α,βGalNAc Galβ3GalNAc βGal α,βGlcNAc (GlcNAc)[2,3,4] α-1,3Man αMan, αGlc striifrons, Macrosteles quadripunctulatus and Euscelidius variegatus[17,18,19] These interactions contribute to insect transmissibility, probably enabling phytoplasma movement within insect cells. Several other surface-exposed proteins were found in the genome of phytoplasmas and are supposed to be implicated in cell invasion by interacting with insect proteins. ELISA, inhibition and competitive adhesion assays, and protein overlay assays were used to show that VmpA attaches to insect vector cells in a lectin-like manner
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