Spend all day in the sun and you run the risk of developing skin cancer, yet plants seem to bask in UV rays without a care.
Truth is, plants can develop tumours. Unregulated cell division occurs in plants just like animals, so why don’t we ever hear of a plant dying of cancer?
Types of plant cancer
Infections can cause tumours in plants, in a similar way to the HPV virus causing cervical cancer in humans.
The bacterium Agrobacterium tumefaciens causes tumours called crown galls in many species of plant. The bacterium inserts its own DNA into the plant and messes up its growth hormones (auxin and cytokinin), creating a Agrobacterium-friendly tumour where it can live happily ever after. It’s rarely fatal but can cause some yield loss in perennial crops like fruit trees because gall production steals energy that could have been used to make more delicious apples or cherries.
Fungi like Ustilago maydis can produce tumours in a similar way. U. maydis causes corn smut, turning ears of corn into the strangely grey and deformed Mexican delicacy known as huitlacoche.
Geminiviruses cause tumours by directly interfering with cell replication in plants.
Although it’s uncommon, certain types of plants are quite prone to spontaneous tumours too. In a fun twist of fate, tobacco (Nicotiana) plants are particularly prone to developing cancer; when two species cross-breed with each other, the resulting offspring often develop tumours because of hormone regulation problems.
How to grow a tumour
Plant tumours share some similarities with human cancers. Plant tumours are disorganised lumps of cells, similar to human cancers. They are often caused by problems with levels of the hormones auxin and cytokinin. Fluctuations in hormones like oestrogen can lead to cancer in humans too.
Cell replication is strictly regulated in animals and plants by genes that are amazingly similar in both groups. Auxin and cytokinins, as well as human hormones like oestrogen, can interact with these cell cycle genes. When the hormones are out of balance, cells can start to multiply out of control.
These cell cycle genes can mutate and stop functioning properly, causing cancers in animals. Plants are less likely to fall victim to these random mutations because they have many copies of most cell cycle genes, so another version can take over if one is put out of action.
Plants also have a few other fail safes to protect themselves from potential cancerous cells:
- Regeneration. Brain tumours are one of the deadliest human cancers, with an 85% mortality rate after five years. The brain is such a vital organ that we cannot survive without it, but plants can regrow any damaged organs reducing the impact of a tumour.
- Totipotency. Plant cells are totipotent, which means they can develop into any cell type. If too many cells are produced in a leaf they can be incorporated into a normal structure. Each cell will be smaller than normal to maintain roughly the correct leaf shape overall.
- Containment. Plant cells are contained within a cell wall. Cancer cells can’t squeeze into neighbouring tissues, so the tumour is restricted to one area. Plant veins are different to humans too; only water and things dissolved within it can move through the vascular system. Tumour cells can’t cause new tumours elsewhere.
There are really two main stages to developing cancer; the cells go wrong then spread throughout the body. Infections, unstable hormones and plain old mutations can cause plant cells to override regulation and begin to divide, but cell walls and a dynamic body plan means plants are able to stop tumours from spreading uncontrollably or doing too much damage.