HISTOPATHOLOGY AND WITHIN-PLANT DISTRIBUTION OF THE PHYTOPLASMA ASSOCIATED WITH AUSTRALIAN PAPAYA DIEBACK

Dieback-affected papaya plants were characterized by a discoloration o f the contents of laticifers, while the anatomy of sieve elements was healthy in appearance until the necrotic stages of the disorder were r eached. Laticifer discoloration was not always associated with the pre sence of phytoplasma in affected tissue, as judged by polymerase chain reaction (PCR) using primers based on the 16S rRNA gene and 16S-23S i ntergenic spacer region. Phytoplasma DNA was detected in a range of pl ant tissues, including roots, but not in mature leaves which would act as photoassimilate sources. As plants recovered from a dieback period , the extent of the distribution of both laticifer discoloration and p hytoplasma DNA decreased. Phytoplasma cells were not observed in trans mission electron microscopy studies of mature sieve elements of diebac k-affected leaf, stem, or fruit tissue from plants at various stages o f symptom expression, although PCR tests indicated the presence of phy toplasma DNA. Membrane-bound structures, similar in shape and size to phytoplasma cells but interpreted as autophagic vesicles or latex vesi cles in immature laticifers, were observed within vacuoles of cells in phloem tissue in leaves displaying tissue breakdown in the form of a water-soaked appearance to veins (''X-Y'' patterning). In contrast, ph ytoplasmas were readily observed in papaya leaves displaying symptoms of yellow crinkle. We conclude that phytoplasma cells are present in v ery low titer in dieback-affected tissues and that, while the plant ap pears to limit proliferation of the dieback-associated pathogen, this defense strategy is ultimately unsuccessful because it is associated w ith a rapid decline of the papaya plant.