This miss folded protein known 'attack' brain or other neural tissue. Since no brain in the plant, prion have no capability to make disease in plant and other 'non brain'ed organism. May be, it was related to attachment site of prion into specific 'place' that only found in brain or other neural related tissue

I am more wondering, if any molecules in plants which are totally unrelated to prion proteins could -after uptake in human and animals- change the conformation of cellular prion proteins. Due to the long latency period of such diseases, I would expect no easy support or exclusion of such a hypothesis. But the lack of any evidence for such proteins could be considered as evidence for the lack of such molecules.

Perhaps it is possible that it is the transmittable agent is a species from the group of actinomycetes? After all, depended the outbreak of made ​​cow disease together with the consumption of meat waste.

I think that there are priion like disease in the long-life living things.