DETERMINATION OF THE DISTRIBUTION AND MULTIPLICATION SITES OF FLAVESCENCE-DOREE MYCOPLASMA-LIKE ORGANISMS IN THE HOST-PLANT VICIA-FABA BY ELISA AND IMMUNOCYTOCHEMISTRY

A mycoplasma-like organism (MLO) is known to be the aetiological agent of Flavescence Doree, an important grapevine yellows disease which oc curs in Europe, Indirect enzyme-linked immunosorbent assay (ELISA) was used to monitor the distribution of the pathogen in the experimental host plant Vicia faba, during the course of infection after inoculatio n by the leafhopper Euscelidius variegatus. Post-embedding colloidal g old indirect immunolabelling was developed to identify, without ambigu ity, the various forms of MLO cells in the different infected parts of the plant, by transmission electron microscopy. Silver enhancement of the gold probe gave accurate histological and cellular localization o f MLOs in tissue sections, by light microscopy. Both ELISA and immunol ocalization first detected MLO in roots 17 days after inoculation with infectious leafhoppers. Small vesicles and filamentous bodies were id entified as MLOs in the cytoplasm of phloem cells, and ELISA indicated the occurrence of a multiplication phase in the roots 17-24 days afte r inoculation. MLOs then reached the collar and systemically colonized the basal axillary shoot. They preferentially multiplied in the apica l area of this growing shoot. MLO bodies were located in mature sieve tubes and in non-functional phloem cells on the periphery of the vascu lar bundles, and necrotic cells were observed in the phloem tissue. El ectron dense and distorted MLOs were identified in these collapsed cel ls.