AbstractsBiology & Animal Science

In 1967, Doi et al. proposed the hypothesis that 'yellows type diseases' are caused by mycoplasma. Since then more than 60 yellows diseases have been associated with mycoplasma-like bodies (MLB) (Maramorosch, 1973). The majority of these associations are based upon electron microscopic observations of mycoplasma-like bodies in situ. However, little is known about the in situ development of MLB. Hull (1971) pointed out that information is lacking about the relative abundance of MLB in different cell types, at different stages of development, and in perennial plants at different times of the year. Therefore, a developmental cytopathological study of X-disease in Prunus persica Batsch.and P. avium L. and Albino disease of P. avium was initiated using techniques of light and electron microscopy. Three X-diseased P. persica trees were located in an orchard with a history of X-disease problems at The Dalles, Oregon. A symptomless control tree was located in the same orchard and a healthy control tree was established in a screen house at Corvallis, Oregon. Two albino infected P. avium trees were located in an orchard at Medford, Oregon, with control trees as stated above. Leaves and peduncles of healthy and diseased trees were sampled sequentially during the 1972 and 1973 growing seasons by dicing the distil 2-5 mm of midvein directly in 4% glutaraldehyde. The trees at The Da lles were sampled every other week while those at Medford were sampled once a month. The samples were subsequently fixed in osmium, dehydrated, embedded, sectioned, and observed. Comparisons were made between diseased and healthy phloem parenchyma cells and sieve tube elements. Preliminary indications suggest that there are structures present during the early ontogeny of sieve tube elements and phloem parenchyma cells that may be confused with MLB. Structures resembling MLB were found in healthy, virus-indexed controls as well as diseased trees. The structures are unit membrane bound, contain ribosomes and fibrils similar to nucleic acid strands. They are spherical to irregularly ellipsoidal in shape and 200-450 nm in diameter. In cross and longitudinal serial sections they are found to be in close association with endoplasmic reticulum and the nuclear envelope. Observations also show the consistent presence of true mycoplasma-like bodies in mature sieve tube elements of diseased tissue during June, July, and August and the consistent absence of such structures in immature sieve tube elements, phloem parenchyma cells, and in comparable cells of healthy controls. Criteria are set forth for distinguishing between true MLB and the structures in healthy, virus-indexed plant tissue. The presence in healthy, virus-indexed controls of structures similar to or the same as MLB points out the need for more research in this area. A more complete study of a MLB-associated disease is needed. The ontogeny of healthy, virus-free phloem parenchyma cells and sieve tube elements needs to be studied in detail so that valid…