Hi,
I have been recently asked by friend in Germany if I know more about root or leaf galls that appear now more often on plants bought in supermarkets or big plant centers or even in specialized nurseries.
Here is an example of such an infestation (Agrobacterium) - the photo was taken in a plant center:
First of all such plant galls are not uncommon and widely available in nature. They are caused by different microorganisms. The most familiar ones are Agrobacterium rhizogenes, Agrobacterium tumefasciens and Rhodococcus facians. Agrobacteriums are widely spread in Nature and are used in modern gentechnology as transfer vehicles for small gene sequences. (you have heart right - nature uses besides gene transfer via pollen also other vehicles for genetic evolution of plants - especially with dicotyledons)
The mechanism behind this transfer is that these microorganisms have extrachromosomal plasmids specific to the host (material gen material for special enzymes) and act somehow like viruses. Plants are in most cases infestated by sores in the root, basis stem or stem area. Depending on the location the final image might vary a little bit.
What do this plasmids if transfered? - these vector plasmids have been heavily investigated by scientists in the past. It appears that phenolic substances that are produced by plants with sores activate the virulence of the microorganisms and the transfer of plasmids to the plant. They produce a substance class called nopalines that reprogramm plant tissues in their natural growth. So if you want a description of the above picture - you mainly see a clump of undifferentiated cells (callus - like you see it in tissue culture) that finally stop at a certain point. The in vivo callus effects the plant growth as all assimilation material and nutrient ressoureces are used to keep this callus in active growing state.
Depening on the side where you find these clumps it also happens that the material redifferenciates - that means that small leafes or even clusters of buds are produced. ri-plasmids from A. rhizogenes cause mainly formation of roots on place you do not expect them.
How are these microoragnisms transfered? - you might have an idea of it that such diseases are mainly transfered by unclean cutting tools- especially the rhodococcus which is an actinomycete that does not form spores. Agrobacterium is widely spread and the transfer is easier.
What to do with plants that show symptoms of it? - well some people say just cut of the gall and act as nothing has happened or treat it with disinfectant. This is acting in a shortsighted way - as you remove the symptom but not the organism which caused the symptoms. The best way is to remove a plant from a collection as especially the Agrobacterium has a tendency of spreading (e.g. during hibernation in the cellar).
I also know that thermo deactivation and the application of abscisic acid is used to stop the abnormal growth. But I have not tested it so far. I do not think that activating the heat shock cascade at temps above 45°C is enough to eliminate the microorganisms.
HJ