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Firing a Gun at Seeds

The tools used by plant scientists will surprise you. It involves neutered bacteria and guns. Lots of guns.

Scary pictures circulating online would have you believe that scientists use a syringe to inject DNA into food. Don’t be silly. They use a gun.

But before we get to the gun (which, like Chekhov’s, will go off before this story has ended), we must talk about genes.

Banging on the hood of a car

We genetically change crops because we desire certain traits in them. For example, could we make a strawberry more flavourful? Can a tomato be made more resistant to frost? These traits are observable, but what creates these traits in the first place are things we can’t directly see: genes.

You can think of a gene as a functional unit of DNA. Boiled down simplistically, a gene is a blueprint for the making of a protein, and the protein is what results in the observable trait. Since the dawn of agriculture, the questions have become: how can we get a suite of favourable traits in one particular plant?

The oldest method is known as “selective breeding”. Mendel, the father of genetics, was doing just that with his pea plants. You select two plants with different traits. You emasculate one by removing its male flower parts. You then apply pollen from the second plant to the female part of the emasculated plant. Before you know it, you have a third plant, a baby plant, that has a different set of traits.

A hundred years ago, scientists began exposing plant seeds to mutagens like chemicals and gamma radiation. What a mutagen does is randomly create mutations in the DNA of the plant. Sometimes the mutation results in a desirable trait. The number of mutations and the location of these mutations is not known; rather, scientists would look once again at the observable traits of the plant that would grow from these mutant seeds. Once the right plant was found, it could be used to establish a cultivated variety. The next time you indulge in some fine organic Italian pasta, remind yourself that its wheat is most likely the product of atomic irradiation. 

It is important to point out that both selective breeding and mutagenesis on their own can be used without ever bothering to learn which underlying genes are affected or how they impact the plant.

It’s a little bit like hearing a strange noise coming out of the hood of your car and deciding to fix it by banging on the hood with a crowbar until the noise disappears. Firstly, some anger management classes would do wonders for your temper. Secondly, you never discovered what was causing the noise in the first place. Thirdly, you don’t know what other changes were created underneath that hood by your “rage against the machine”.

We have finally reached a point where we can begin to cast aside the crowbars of the past, open the hood, understand the way the engine works, and fix exactly what’s wrong with it. This is genetic engineering.

Turning a natural weapon into a force for good

Before we get to the heavy artillery of genetic engineering, a word or two about bacteria.

These little microorganisms have evolved to be quite the pain in the bark for a number of plants. For example, one such microorganism calledAgrobacterium can cause plants to develop tumours. How does it do this? Agrobacterium carries within its membrane a potent weapon in the shape of a ring of DNA. This DNA ring is known as a “plasmid”. When the bacterium comes close to a plant cell, it can erect a tether to it in preparation for a most insidious attack. Inside the bacterium, the ring of the plasmid is cut, so that its DNA turns into a long strand that can snake its way through the tether and into the unsuspecting plant cell.

Once inside the plant cell, the plant’s molecular machinery recognizes a particular signal in the invading DNA: this signal means “take me to the nucleus where the plant DNA is”. Having entered the control centre of the cell, this bacterial plasmid DNA is then integrated into the larger plant DNA. Some of this bacterial DNA contains genes that will act as an accelerator for plant growth, thus resulting in tumours.

This type of natural genetic Trojan horse has been repurposed by genetic engineers in order to enact positive transformations in the plant. First, the tumour-causing genes are removed from the Agrobacterium plasmid. Then, a gene of interest is inserted, along with a selection marker. The selection marker will have a visible effect on the plant cell and will act as confirmation that the Trojan horse successfully delivered its payload. The gene of interest is what we want to add to the plant genome, for example a gene from a different plant that confers resistance to a bacterial disease.

This way, when our plant is exposed to this genetically engineered bacterium, it will acquire a bacterial resistance gene and become a genetically engineered plant.

Reconditioning a bacterial Trojan horse is certainly a popular way of modifying plants, but there is a louder way.   

Gun-totin’ scientists

You may be imagining a lab-coat-wearing nerd holding a Magnum, but the handheld gene gun looks more like 1960s Star Trek phasers. In its mechanism of action, however, it resembles a semi-automatic weapon.

First, the DNA plasmid containing the gene of interest is coated onto tiny particles of either gold or tungsten. These metal particles are then loaded into a plastic cartridge, which is inserted into a cartridge holder, much like bullets fit into a revolver’s cylinder.

For this instrument to be a gun it requires a propellant. Something must pull these DNA-coated metal particles off of the cartridge and push them out the barrel and into the target at breakneck speed. It may make you sound like a Chipmunk, but helium can actually double as a good, inert propellant for this system.

When the gene gun trigger is pulled, a burst of compressed helium gas is released and it pulls the gold particles away from the cartridge and into the barrel of the gun. A particular model is known to create a 108-dB sound wave at a pressure of 400 psi. These accelerated DNA-coated beads leave the barrel and pierce through the plant cell wall, delivering their DNA plasmids which can be taken up by the cell, integrated into the chromosome, and replicated.

You may think the scientists who use these guns have gone ballistic, but the proper term is “biolistics”.

The next time you see an alarming picture of a scientist injecting an apple with a syringe, just remember it’s a scare tactic used by scientifically illiterate activists. Real genetic engineers don’t use needles; they strip bacteria of their tumour-causing properties and they shoot plant cells with guns.