Asked by: Pete Jones, Flitwick
Plants can develop neoplasms or tumours, where the plant cells divide uncontrollably to form hard outgrowths. By far the most common example of this is crown gall disease, which is caused by Agrobacterium tumafacians, a bacterium that inserts part of its own DNA into the plant genome.
But these tumours do not aggressively invade surrounding tissues or move to other parts of the plant (known as metastasis) and so are not normally classified as cancers. Part of the reason for this is that plants don't have a circulatory system that would allow tumour cells to be transported around it. Also, in animals, cells become specialised during embryonic development so they can only divide to form new cells of a certain type. This means that as an animal tumour grows it invades surrounding tissues with cells of the wrong type for that location, which interferes with the function of the organs.
Plant cells are generally totipotent, meaning they retain their ability to divide to form cells of any type. So when a plant tumour grows, the cells can continue to form multicellular structures correctly and the plant just gets larger, possibly with a slightly distorted shape. Some plant tumours that deactivate the totipotency of the tumour cells have been found but they still don't metastasize so they aren't fatal to the plant.