|Abstract or Summary
- 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 comparisons can be made between healthy and diseased material
not only at chronological stages but at comparable physiological
stages of growth and development.