Galls of various sorts, as has been said, result from infections in the plant that result in unrestrained growth. Plants are also subject to various viral infections that can cause strange growth as well, such as Tobacco Mosaic Virus.
However, plant tumors are different from cancers per se because plant cells are like stem cells, they are totipotent. The bacterium may cause the plant to loose control of the growth rate of the cells, but the cells remain able to change into a functional type. In addition, because of a lack of a circulatory system the infection cannot spread to consume the entire plant body. Hence plants don’t get “cancer’” just localized tumors.
Here is how one expert puts it:
“Plants CAN get cancer of sorts. However, because of some of the differences between animals and plants, plant cancers behave differently to those in animals.
First a definition: What is cancer?
Well, in animals, cancer can be defined as a disease where the cells in part of the body divide out of control to produce extra, abnormal, cells (which can also divide out of control). This can happen because the cell’s DNA, which gives instructions to the cell, is damaged in some way. The uncontrolled production of extra cells can lead to a tumour (or clump of extra abnormal cells) developing. The tumour can interfere with the normal functioning of the body. Eventually, some of the abnormal (cancerous) cells may split off the tumour and circulate around the body’s blood and lymph systems and cause the cancer to spread, potentially distributing tumours to other parts of the body (a process called “metastasis”). It is this spreading of the cancer around the body that is particularly deadly because it allows the cancer to simultaneously mess up several areas of the body. If the areas affected are important, like the brain or lungs, this will have devastating effects.
Plants are fundamentally different to animals, and these differences mean that their cancers are also fundamentally different. So how do plants differ from animals in relation to cancer?
1) plant cells are special
Normal plant cells have the ability to reorganise when they divide in order to become different kind of cells. Such cells are known technically as “totipotent” (from “total potential” to differentiate into any other kind of cell). In animals this special ability is only held by special cells called stem cells. This difference explains how you can take a cutting from a plant shoot and grow a complete plant from it, but you cannot take a “cutting” from an animal and grow another animal!
This special ability defends the plant against cancer: a cell will only become cancerous if it loses control of both it’s division process AND it’s ability to be totipotent. If it can still change it’s “type” (root cell or shoot cell etc), then the extra cell growth is not such a problem, because the extra cells can function normally.
2) plants don’t have circulatory systems.
In animals, cancers can spread through the circulatory system (blood and lymph) and cause damage to many parts of the body at once (this is “metastasis” which I mentioned above). Plants don’t have these circulatory systems and, therefore, cancers in plants will remain in a fixed location and only cause problems to that small part of the plant. Even if a tumour (known as a “gall” in plants) develops, it will not spread to other parts of the plant.
These important differences in the way cancers work in plants compared to animals, mean that there is still a lot of debate in the area. For example some people argue about whether the plant cancers can even be classified as cancers as they are defined in animals.”
And then here is a bit more about the organism that causes galls
“In plants, there is an environmental organism, it’s called Agrobacterium tumefaciens, this is a soil dwelling bacterium and it has something called a transposon. This is a piece of genetic material which the bacterium injects into the plant’s own genetic material and that transposon carries genes which code for growth factors. And it causes the plant cells to begin to grow out of control. And the idea is to produce a big growth locally on the plant that then gives a home and provides protection to bacteria and that’s a Gall. And it’s very, very common, it’s called Crown Gall disease when the plants actually have it, but it doesn’t spread predictably to other bits of the plant. So there are some similarities between human cancers and animal cancers and plant tumours like Crown Gall disease, but it’s not the same disease.”
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