An Improved Axenic System for Studying Pre-infection Development of the Parasitic Plant Orobanche ramosa

Parasite�s self-stimulation following host�s signal

One of the problems in studying early stages of infection of hosts by parasitic plants is that it is often difficult to separate host and parasite tissues. This, in turn, gives rise to problems in distinguishing between populations of molecules such as mRNA in the two organisms. However, if the parasite can be stimulated to initiate infection in the absence of the host plant, then the processes occurring specifically within the parasite may be studied. This is the approach adopted by Gonz�lez-Verdejo et al. (C�rdoba and Madrid, Spain, pp. 1121�1127), in their work on a broomrape, Orobanche ramosa. The first requirement was to develop a method for sterilization of seeds that did not significantly reduce their germinability. The sterilized seeds were then conditioned under moist conditions for 8 days, making them receptive to signals from the host plant. Indeed, in order to germinate, the conditioned seeds actually require a signal from the host that in these experiments was supplied either by a tobacco root exudate or by �G24�, an analogue of strigol (a compound involved in the germination of another parasitic plant, Striga). These were equally effective in inducing germination, but because G24 was more easily obtained it was used for most of the experiments. The effect of G24 was very rapid, requiring only a 5-minute immersion of seeds in the inducer. This suggests that once the inducer has been perceived by the plant tissues it sets off a signal transduction pathway that operates independently to induce germination. A few days after germination, attachment organs developed that were indistinguishable from those that develop in the presence of the host. It is not yet clear whether this developmental phase is part of the sequence of events triggered by G24 or whether a separate endogenous signal is involved. Preliminary evidence consistent with the latter was obtained by the authors, providing a clear stimulus for further research.

Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk