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).

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.

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