I really love the game Minecraft. I was thrilled, then, to realize that there is a discussable link (however tenuous) between Minecraft and an interesting topic in biology. First, however, remind me never to force my character to eat apples in-game ever again. To do so would be criminally inhumane and a serious breach of (virtual) human rights. Steves and Alex’s are people too.
For the uninitiated, you don’t get apples from apple trees, there is no such thing in Minecraft. Instead, you get apples from oak trees. Here’s the lie though, oak trees don’t produce apples. However, they do “produce” things with a passing resemblance to apples. These things are called “gall apples.” Any similarities though between apples and gall apples end at their names and the oddly bumpy, but unrough exterior of the gall apples. Here’s a picture.
My pimple-scarred mid-pubescent face comes to mind. I don’t miss those days at all.
It gets worse though.
As you might have guessed by now gall apples aren’t really apples at all. Instead, they are deformities that are prompted by some wasps and their larval progeny. The larva secretes certain chemicals that induce the formation of a gall, which feeds and houses the baby wasp until it is ready to leave. This is what the inside looks like apparently:
Galls like the one above are reported to be rather rubbery and squishy, not at all like a crisp apple. As it isn’t an actual fruit, it’s doubtful that it would be sweet either, though surprisingly I didn’t find a source describing the taste of these odd pseudofruit. I can’t imagine these would make for very pleasant eating, with or without the prospect of consuming a worm-like larva.
In the case of apple galls, it’s a wasp that manipulates a plant to form a gall to its benefit (but not much detriment to the plant, apple galls for the most part only cause superficial damage) [https://bygl.osu.edu/index.php/node/750]
Soo, about the bacteria...
There are other types of galls, caused by different types of organisms. Here we arrive at what I really wanted to talk about in this post.
Credit goes to John L. Ingraham's March of the Microbes for giving me the low-down. Pretty interesting read, discussing the ways that microbes leave their calling card in everyday activities and objects, from a bottle of wine to patches of lichen.
But I digress. There’s a particular type of gall, called crown galls, whose causative agent is a bacterium called Agrobacterium tumefaciens. If you’re also cursed with the knowledge of the Latin language, you probably can piece together an idea of what this bacteria does, even without the benefit of the previously provided context. A. tumefaciens makes “tumors” or galls. It can do this in a wide variety of plants, from grapevines to roses.
As you can imagine though, I’m not so interested in the plants that A. tumefaciens infects, or the galls it creates in its hosts, but the bacteria itself, and the process by which it makes these galls.
Viruses aren’t the only biological entities capable of transferring genetic material, bacteria actually can do it too, through a process called conjugation. In this process, genetic material is transferred from one bacteria to another. Small rings of genetic material, called plasmids, which are separate from a bacteria’s main circular chromosome are copied and transferred to the receiving bacteria.
A. tumefaciens is also capable of transferring genetic material, except as you now know, its recipient is the cells of the plant it chooses to infect. I say “choose” somewhat deliberately, since A. tumefaciens can sense compounds released by a “wounded” plant, and move in to stay. It requires a break in the plant tissue in order to successfully do so, so this act of “chemotaxis” (movement in response to chemical detection) is I suppose something of a necessity for plant infection to happen more than occasionally.
This little bacteria differs from its bacterial-conjugating peers in yet another outstanding way. In bacterial conjugation, whole single-stranded copies of plasmids are copied and sent over to the receiving cell. In A. tumefaciens case, only a small section of its plasmid (called a Ti or tumor-inducing plasmid) is sent over. Once the double-stranded DNA fragment is inside the plant cell, it is integrated into the plant cell’s genome. Although this DNA fragment does not contain instruction for more A. tumefaciens, in many respects, this behavior quite virus-esque.
However, this DNA does instruct the plant cell to act in a way that facilitates the survival and wellbeing of the bacteria and its progeny. Firstly, it encodes plant growth hormones, which result in the abnormal gall structure which houses the bacteria. Furthermore, it also encodes for amino acids and sugars that A. tumefaciens can use as food. Quite clever, isn’t it?
It doesn’t stop there though. Excess opines (type of amino acids) produced by the infected plants leak out of the roots into the area called the “rhizosphere,” the dirt around a plant’s roots. A. tumefaciens not only feed on this excess but, prompted by the presence of the opine, begin conjugating with each other. As a result, cells that previously didn't have the Ti plasmid, and the ability to manipulate plant cells, now do. It all works out quite nicely for A. tumefaciens. And it must be said, the plant itself, though now bearing unsightly plant warts, isn’t actually done much harm. While it’s a not a win-win for both parties, it’s not a total win-loss either.
Finally, because of A. tumefaciens unique abilities, it’s been employed for genetic engineering of plants, much like how the HIV I used at my first internship was used to genetically modify animal cells. I think it’s a very interesting organism, even though I mainly think viruses are cooler.
Thanks for reading!
- BJW
Comments