Can plants get cancer, and how do they defend themselves against disease?

Cancer is one of the most common causes of death in people, and case numbers are rising.

At current rates, about one in two Australians can expect a cancer diagnosis by the age of 85.

Vets, livestock farmers, pet owners and anyone who spends time around animals will also know that cancer can strike a whole range of creatures.

But did you know it’s not just a disease in the animal kingdom?

Plants can get cancer too.

While cancer doesn’t affect plants like it does us, it can cause costly problems for horticultural and agricultural growers.

Cancer in plants

All cancers begin when one cell or a few cells in an organism start growing uncontrollably.

In humans and other animals, new healthy cells grow in an orderly and regulated way to replace damaged or dead cells. This keeps our organs and tissues functioning the way they should.

But sometimes cells with mistakes in their genetic blueprint (or genome) don’t stop multiplying, and if they clump together, they form a tumour.

Bits of that tumour can break away and spread via the bloodstream or lymphatic system to other parts of the body and form new tumours. This process is known as metastasis.

Cancer-causing mistakes in an animal cell’s genome can arise in a multitude of ways, such as exposure to ultraviolet light, a virus, or random errors that occur when a cell divides.

But the story’s a little different in plants.

Cancers in plants are usually the result of a bacterial, viral or fungal infection, which tinkers with the plant’s genome.

Similar to animals, if those changes are in parts of a plant’s genome that control cell growth, cells can multiply uncontrollably to form tumours.

Despite plants being at risk of developing tumours, they often aren’t as dangerous as most cancers in animals.

Nor can they spread, because each plant cell is enclosed by and held in place by cell walls.

That said, tumours are not entirely problem-free for plants.

One of the most common plant tumours is crown gall, a disease caused by the bacterium Agrobacterium tumefaciens.

The tumour is the “gall”, a swollen knot-like structure, that typically forms on the base of the trunk and roots of woody plants.

To create a gall, a bacterium first infiltrates a plant cell. Once inside, it slips a strand of genetic material, in the form of DNA, into the the plant’s genome, plant scientist John Rathjen from the Australian National University, says.

This new strand of DNA contains instructions that force the plant cell to divide uncontrollably and produce lots of sugary molecules.

“The only thing that can eat that [molecule] is the bacterium, so it has harnessed the plant as a little farm for itself,” Professor Rathjen says.

Crown gall disease is endemic to Australia, and is a significant global problem for horticultural and agricultural crops.

The galls caused by the disease don’t necessarily kill the plant, but they can affect the plant’s health and hinder its functions.

For instance, the knobbly galls can impede water and nutrient flow between roots and leaves.

Professor Rathjen says the disease is an ongoing problem for grapevine growers in Australia.

“You’d have decreased plant productivity — fewer grapes, or the tree wouldn’t be able to grow as many leaves as it wanted to.”

Plant defences

So a plant’s been infected by a tumour-causing bacterium. Is the plant defenceless against an overwhelming tide of galls?

Or what about neighbouring plants, which — unlike humans — can’t simply get up and move away from an infected individual?

Professor Rathjen says people are often surprised to learn that plants have an in-built “biochemical toolbox” for protection.

And it includes an immune system.

But a plant’s immune system is different from animal immune systems in several ways, he says.

While we largely depend on white blood cells to fight off and remember pathogens, plants don’t.

Instead, plant cells can recognise parts of a pathogen — perhaps a protein on the surface of, and unique to, a particular bacterium.

Once a plant cell senses that protein, it can activate defence systems, ramping up production of antimicrobial chemicals, for instance.

Or they can sacrifice themselves for the greater good.

Plants will kill parts of themselves, such as an infected leaf, to stop an invading pathogen from spreading.

Plants rely on this “altruism” of individual plant cells to curb infections, plant biologist Amy Mackenzie of University of Adelaide, says.

“Basically, that means it’s every cell for itself in the plant,” she says.

This response is called programmed cell death.

“Cancer is a problem where [a cell] is not killing [itself] when it should,” Dr Mackenzie says.

Professor Rathjen adds that the evolution of a plant’s defence against pathogens is key to its survival.

“If you walk around, you’ll see lots of healthy plants that are happily fighting off attacks from microbes, like bacteria, fungi, viruses and little worms called nematodes,” he says.

“That attack is constant, and yet somehow plants have evolved a beautiful immune system that’s able to cope with that, and make sure that most of them grow to the right age where they can have little babies or seeds of their own.”

SOURCE: ABCNEWS