Viroids are a unique class of plant pathogens. They solely consist of a small,
single-stranded, circular RNA, which is not encapsidated in a protein coat (Fig. 1).
Fig. 1: Electron microscopic picture of potato spindle tuber viroid (PSTVd) (Courtesy Dr. G. Klotz)
These naked RNA molecules cause serious diseases among many crop plants, fruit trees
and ornamentals, including potato, tomato, cucumber, chrysanthemum, avocado, and
coconut palms, to name only a few (Fig. 2). The viroid induced diseases lead
to dramatic economic losses in agriculture and horticulture worldwide.
Fig. 2: Symptoms of viroid diseases: A: potato spindle tuber on tomato,
B: avocado sun blotch, C: chrysanthemum stunt,
D chrysanthemum chlorotic mottle
Our studies are focussed on
1) Viroid detection:
The rapid and reliable detection of viroids is the only way to control the spread of
viroids by eradication of all infested material at a very early stage of infection.
Molecular hybridisation techniques and PCR are most frequently used in practice.
A simple and rapid technique for viroid detection is the tissue print hybridisation (Fig. 3),
which is preferably applied under conditions, where no highly equipped laboratories
are available (Mühlbach et al. 2003). Since the detection is easily performed with
non radioactive materials, it is particularly useful field studies in tropical areas
The technique also allows a quick visualisation of the tissue specific distribution of
the pathogen (Stark-Lorenzen et al. 1995).
Fig. 3: Tissue print hybridisation (whole mount of chemiluminescence detection)
showing the distribution of potato spindle tuber viroid (PSTVd) in an
infected tomato plant (picture made by U. Weber)
2) Viroid host interaction:
Viroid RNA does not carry any protein specific genetic information. This means that
its biological function is completely dependent on its interaction with target
compounds of the host cell. In order to understand viroid pathogenesis, we are
analysing the presence and putative function of viroid RNA binding proteins in plants.
We use the best investigated combination of potato spindle tuber viroid (PSTVd) and
the tomato (Lycopersicon esculentum Mill.) as host plant. By screening of a cDNA
expression library (Werner et al. 1997) we found a CDC5 homologous protein that binds
to PSTVd. The deduced protein structure of viroid host plants is different from the
structure of related proteins of non-host organisms (Fig. 4). Present studies are
concentrated on the characterisation of the RNA binding capacity and the biological
function of the tomato CDC5 protein.
Fig. 4: Structural organisation of the cdc5 homologous gene of tomato