Grapevine flavescence dorée symptoms © Sandrine Eveillard

Grapevine flavescence dorée

Phytoplasma, the pathogenic agent

INRA researchers identified a flavescence dorée phytoplasma surface protein that allows the interaction between the bacteria pathogen and the insect vector of the disease. The researchers also developed a technique to visualise what happens inside plant vessels when a sucking insect feeds, thereby better understanding how the phytoplasma is acquired and transmitted from one plant to another during the feeding process.

By Pascale Mollier - Julien Chuche - Denis Thiéry - Daciana Papura - Sylvie Malembic - Alain Blanchard - Xavier Foissac, translated by Daniel McKinnon
Updated on 09/06/2016
Published on 03/26/2013

Phytoplasma, an unusual pathogen

Phytoplasma are bacteria in the Mollicutes class that lack cell walls. They are intracellular obligate parasites that reproduce in plant phloem and in sucking insects. As with most Mollicutes, phytoplasma have a small genome size of less than 1 Mpb.

The “Mollicutes” team of the joint research unit UMR 1332 BPF focuses on the flavescence dorée bacterium and its biological cycle interactions with its hosts.

Difficult to cultivate the organism

Attempts to cultivate phytoplasma in a laboratory have all failed. Phytoplasma can only be maintained in planta and may only be propagated by cuttings or by reproducing the natural cycle while keeping infected material contained. The rudimentary metabolism of the bacterium may likely explain its inability to multiply in vitro and its status as an obligate parasite.

The phytoplasma–insect vector relationship: surface proteins involved

Close-up of the head of an adult grapevine leafhopper, Scaphoideus titanus. This insect does not cause any direct damage to the vine but disseminates flavescence dorée phytoplasma. © INRA, WALKER Anne-Sophie
Close-up of the head of an adult grapevine leafhopper, Scaphoideus titanus. This insect does not cause any direct damage to the vine but disseminates flavescence dorée phytoplasma © INRA, WALKER Anne-Sophie

The S. titanus leafhopper acquires flavescence dorée phytoplasma when feeding on an infected vine. The phytoplasma then colonises the insect’s body and multiplies. There is a one-month latency period before the leafhopper becomes infectious and can transmit the phytoplasma to new plants, which it does each time it feeds. Insects remain infectious until death, but phytoplasma is not transmitted to offspring.

The phytoplasma cycle in the vector crosses several tissue barriers. This requires interaction between phytoplasma and insect proteins. Using the genome sequencing of the flavescence dorée phytoplasma, researchers in the BFP research unit are working to identify and characterise the phytoplasma surface proteins involved in vector transmission, with a view to being able to block this interaction. They were able to demonstrate that the surface protein, VmpA, binds to two proteins in the vector that are currently being identified. The research is being completed with genomic engineering techniques, in particular the use of synthetic biology tools. The aim is to better identify on a molecular level phytoplasma’s pathogenic determinants and to better understand the genetic deficiencies responsible for their inability to multiply in vitro.

The insect vector–grapevine relationship: electropenetrography to see the unseen

Electropenetrography device (EPG) © J. Chuche
Electropenetrography device (EPG) © J. Chuche

The leafhopper feeds by piercing leaf vessels. The feeding behaviour of piercing insects is very difficult to observe because it happens inside plant tissues. Electropenetrography (EPG) was developed as an indirect observation method. It uses electrodes to capture variations in electric resistance generated when the insect feeds. EPG can be used to locate stylets in the tissue and also to identify different behavioural steps, such as the injection of saliva and food intake, and measure their duration. Consequently, EPG is essential to the study of S. titanus feeding behaviour, in particular to understand the phytoplasma acquisition and transmission processes. It has already been used to identify different feeding behaviour between males and females, which may serve to explain the higher transmission capacity in males. Developed in Bordeaux, EPG is also beneficial when screening plant material that is resistant to the flavescence dorée vector. It can also be used to test the biological activity of molecules that stimulate or inhibit leafhopper feeding.

References

- Carle, P., Malembic-Maher, S., Arricau-Bouvery, N., Desque, D., Eveillard, S., Carrere, S., Foissac, X. 2011. Flavescence doree phytoplasma genome: a metabolism oriented towards glycolysis and protein degradation. Bulletin of Insectology 64, S13-S14.

- Salar, P., Charenton, C., Foissac, X. et Malembic-Maher, S. 2013. Multiplication kinetics of Flavescence doree phytoplasma in broad bean. Effect of phytoplasma strain and temperature. European Journal of Plant Pathology 135:371-381.