Tree tumor: Agrobacterium’s Genetic Manipulation explained

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Genetic Modification Unveiled: The Intricate Dance of Organisms

Tree tumor is a genetically modified organism. However, it wasn’t genetically modified by humans. So, who’s the culprit? I wrote an article a while ago about horizontal gene transfer, which is the way bacteria mix their genes amongst themselves.

If you reproduce sexually, you mix up your genes with another organism every time you reproduce. But bacteria don’t reproduce sexually, so they need to find another way of mixing up their genes. This process is crucial because gene mixing can speed up the adoption of novel beneficial genes.

Conjugation: Bacterial Intimacy

Horizontal gene transfer happens in a few different ways, but I’ll talk specifically about conjugation. That’s when one bacteria grows a little tube that attaches to a neighboring bacteria. Then, the first bacteria injects DNA through the tube into the second bacteria. There’s a lot more to it than that, so I recommend you read the original horizontal gene transfer article because it’s really interesting stuff. It covers a couple of other types of horizontal gene transfer as well.

For a long time, it was thought that conjugation only happens between bacteria. Incredibly, it can happen between kingdoms. You’ve got the animal kingdom, the plant kingdom, and so on. It turns out you can have conjugation between bacteria and plants, two entirely different kingdoms, entirely different domains even. When it happens, it’s bad news for the plant and is responsible for Tree tumor.

Agrobacterium’s Role in tree tumor growth

We’re talking specifically here about a genus of bacteria called Agrobacterium. You find Agrobacterium all over the surfaces of plants. The process begins with a wound. Plants give off compounds in response to a wound, like a cut to the outer surface of the plant. In the same way, humans do. But these compounds trigger various processes in the bacteria that cause those bacteria to multiply.

You’ve got these multiplying bacteria in the sap of the wound, which makes sense because bark is a bit like human skin. It’s a protective outer layer of dead cells. By using this chemical signal, the bacteria won’t multiply until it has access to these vulnerable five cells inside the wound. At some point, the bacterial cells start to grow their little tubes. They’re called sex pili, and these tubes attach to the plant cells.

What happens next is inside the bacteria; there’s a little ring of DNA that’s called a plasmid. A copy is taken of a section of that ring and pushed through the sex pili into the plant cell. This rogue bit of DNA has with it an enzyme that can insert the DNA into the plant cell’s genome. So now, this rogue DNA is part of the plant cell’s genome and will be read by the plant cell’s genetic machinery.

The Unraveling of DNA Secrets

What does this snippet of DNA code for? This is where it gets really interesting. There are genes on this DNA for basically two different tasks. The first is producing essentially bacterial food. In other words, when the cell reads this bit of DNA, it produces enzymes, and those enzymes manufacture nutritious molecules for the bacteria—molecules that the bacteria can easily metabolize. In other words, the bacteria has turned the plant cell into a food factory. But it doesn’t stop there. This rogue strip of DNA also codes for the production of certain plant hormones, specifically growth hormones.

So these cells start to divide and multiply in an uncontrolled way, leading to a tree tumor. And that’s the familiar shape of a crown gall. So not only have these bacteria turned these plant cells into food factories, they’ve turned them into self-replicating food factories. How amazing is that?

By the way, this might not actually be a crown gall; it might be a burr. They’re different things, but it’s really hard to tell the difference. The point is, the tree tumor might be a crown gall. Interestingly, the wood inside a crown gall or a burr is quite sought-after. You get this beautiful, kind of gnarly pattern to the wood. So you can add interesting decorative elements.

Since figuring out the inner workings of Agrobacterium, scientists have put the knowledge to really good use. If you want to genetically modify a plant, pretty much the best way to do it is with Agrobacterium. To see why, take a look at that DNA ring, that plasmid. You can basically split it into three sections: you’ve got the payload, you’ve got the virulence section, and then some other stuff.

Agrobacterium’s Utility in Genetic Modification/tree tumor

The payload is the bit that gets copied and sent through the sex pili, and it contains the genes for plant growth hormones. The OP is those are the nutritious food molecules for the bacteria. The virulence section holds genes for transmission of the payload. So it codes for enzymes that snip out and take a copy of the payload. It’s got genes for making the sex pili, and it’s got genes for pushing the payload through the sex pili.

At each end of the payload, there’s a little bit of DNA called left border and right border. These bits of DNA are snipped by the molecular machinery when a copy is taken. Scientists are able to hijack that bit of machinery to snip out the payload and insert their own new payload, which might be DNA that codes for resistance to a pest, for example.

Scientists have been using Agrobacterium to produce all kinds of genetically modified plants like GMO soybean, corn, cotton. But it’s worth remembering that Agrobacterium was doing it before it was cool.

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