Aetiology of citrus greening disease

Abstract

Greening disease of citrus is characterized by the presence of procaryotic organisms in the sieve tubes of infected plants. These procaryotes have often been called mycoplasma-like. We have previously shown that the envelope of the organism was composed of two membranes, each with a triple-layered structure: an inner membrane (cytoplasmic membrane) and an outer membrane. Penicillin treatment of greening-affected plants results in remission of symptoms, suggesting the presence of a peptidoglycan (PG) layer in the envelope of the organism. However, when observed by conventional electron microscopy, no PG layer could be detected in the envelope of the greening organism (GO). Recently, we were able to transmit the GO from citrus to periwinkles by dodder. In periwinkles, GO multiply to high titres and, therefore, characterization studies can be carried out directly on the organisms in situ. Using papain treatment of GO in greening-infected periwinkles, we were able to visualize a PG-like layer in the envelope of the GO. This layer was removed by lysozyme treatment. In these respects, the structure of the GO envelope was nearly identical to that of E. coli, a Gram-negative bacterium, but was different from that of Staphylococcus aureus (a Gram-positive bacterium) treated in the same way. From the presence of a membranous PG-containing cell wall, the GO appears to be a true bacterium of the Gram-negative type, and not a mycoplasma.