In Situ Detection of Grapevine Flavescence Doree Phytoplasmas and their Infection Cycle in Experimental and Natural Host Plants

Abstract

Phytoplasmas are unculturable wall-less prokaryotes classified in the class Mollieutes. They are characterized and differentiated by symptoms on plants, host plant specificity and insect vectors [1, 2, 3]. Based on electron microscope observations, especially in thin sections, these microorganisms were described as being morphologically similar to animal mycoplasmas and, consequently, they were referred to for a long time as mycoplasma-like organisms. They contain DNA, ribosomes, but have no vacuole or internal membranes. They are heterogeneous in shape, size (80 to 1000 nm diameter) and electron density. However molecular analysis based on the 16S rRNA genes [4] and 16S rDNAs [5] indicate that they are a homogeneous phylogenetic group close to acholeplasmas and different from mycoplasmas. This genetic homogeneity has led to the recent classification of these organisms in a single group named “phytoplasmas” (International Committee on the Taxonomy of Mollieutes, International Organization for Mycoplasmology, Bordeaux France, 1994). Phytoplasmas are transmitted by grafting diseased plants onto healthy ones, by dodder (Cuscuta) bridges and by phloem feeding vector insects, especially leafhoppers and psylla. Phytoplasma cells in host tissues are commonly detected in situ by classical transmission electron microscopy and fluorescent microscopy using DNA-speciflc staining.