Tomato Mold

Tomato Mold Photo Credit: Rudi Riet. All rights reserved.

The battles fought in the invisible war between pathogens and the cells they attack can be as complex as wars between nations. Armies of microbes produce powerful and insidious chemical weapons, and in response, plant and animal cells come up with ingenious methods of defense. In this Science Update, you’ll hear about a researcher who’s helping tomato plants fight back against a particularly diabolical foe.


Why good tomatoes turn bad. I'm Bob Hirshon and this is science update.

Anyone who’s raised tomatoes knows the depressing symptoms of black mold. Plant pathologist David Gilchrist at the University of California at Davis describes them.

It’s a very sad moment, when the tomato that is ripening, is just about to be picked and all of a sudden the fruit begins to show little black spots, and pretty soon, the fruit is something that you wouldn’t want to take into your kitchen.

Gilchrist says the mold does its dirty work by tricking the tomato’s cells into committing suicide. He explains that inside every living cell are proteins that let the cell kill itself when it’s no longer useful. The black mold fungus produces chemicals that trick the tomato cells into activating these suicide proteins.

And it’s that death, then, that provides a mechanism for acquiring a food source, which the pathogen can then use to spread in the tissue.

There are genes found in many plants and animals that stop this programmed cell death. Gilchrist and his colleagues have been able to insert these genes into tomato plants and make them resistant to the black mold.

He says the research holds promise for the treatment of many other diseases as well, since many pathogens use the same trick to attack living cells.

For the American Association for the Advancement of Science, I'm Bob Hirshon.


Making Sense of the Research

It may seem strange, but the ability to die on command is one of the most important features of living cells. In fact, no plant or animal could grow, develop, or even survive without it. Every single cell in your body has a genetic program built into it that will cause the cell to self-destruct if the program gets switched on. This pre-programmed self-destruction – a kind of cellular suicide – is called apoptosis.

Apoptosis plays a key role in several processes. One of them is general housekeeping: when cells get old or damaged, they need to be cleared out to make way for new ones. Sometimes it’s a part of development: for example, the reason we don’t have webbed fingers and toes is because the cells in between them are programmed to die off while we’re still in the womb. And other times, it’s a form of self-defense: if a toxic chemical or a virus gets into a cell, the cell will destroy itself rather than create a safe haven for the invader to grow and spread.

When the attack is small in scale, this can save the organism’s life. But if too many cells are affected, the organism can suffer severe damage. That’s what happens when tomatoes get black mold: a chemical from the mold triggers apoptosis in the tomato cells, killing them off in large numbers. By the time you pick the fruit, the entire tomato just collapses. Gilchrist and his colleagues have genetically engineered the tomatoes so that they don’t respond to that particular chemical. As a result, the apoptosis never happens, and the tomato stays healthy and edible.

Scientists would like to apply this kind of knowledge to human diseases, including Alzheimer’s disease and AIDS, which trick cells into self-destructing prematurely. You might think they would try to stop apoptosis in all our cells all the time, but that would be deadly. In fact, the exact reason why cancer cells are so dangerous is because they won’t die; instead, they grow and multiply like weeds and take over the body. Some researchers are actually trying to treat cancer by triggering apoptosis in the renegade tumor cells.

Now try and answer these questions:

  1. What is apoptosis? What are some reasons why it’s important?
  2. How does apoptosis figure into the black mold disease in tomatoes?
  3. What would happen if all of your cells lost the ability to self-destruct?
  4. Can you think of other examples of things in which certain parts might self-destruct, shut off, or disappear for the good of the whole? Think broadly: it could be a group, an organization, a machine, a society, or any other system.
  5. Apoptosis is important in development because it helps shape certain parts of your body, like your fingers. Can you think of other examples of things that are created by taking parts of it away, rather than just building it up? Again, be as broad as you like.

For Educators

Read more about Dr. Gilchrist's work with tomatoes and apoptosis at Dateline UC Davis.

See pictures of apoptosis at Cells Alive.

The article, Researcher instigates cancer cell suicide, discusses how a Wayne State University School of Medicine physician-researcher has developed a personalized therapy to treat a wide range of cancers. The treatment is based on a naturally occurring human enzyme that has been genetically modified to fool cancer cells into killing themselves.

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