Agrobacterium tumefaciens: soil microbe and genetic engineer by profession. A THREAD on the spectacular biology of the 100% NATURAL genetic modification process, and how we can use it to make better crops #GMO
A. tumefaciens lives in the soil, and is attracted to plant roots by chemicals that are released from naturally occurring wounds. When the microbe makes contact with the plant root, it spits out a plant hormone called cytokinin, which helps it to stick to the plant cells.
High concentrations of plant wound compounds like acetosyringone induce a range of genes in Agrobacterium, changing it from nondescript soil microbe to genetic engineer!
The microbe has circular DNA called a plasmid, which contains everything it needs to re-engineer the plant cell. It contains all the tools it will need along with the DNA instructions that it wants to insert into the plant cell.
I'm going to pause for coffee - we'll resume shortly!
Back to Agrobacterium, the all-natural genetic engineer that you'll find in a handful of soil. When the microbe is sufficiently attached to a plant cell, it gathers up its engineering tool kit. Every tool needs to be made from scratch - FROM DNA - proper artisanal stuff this!
VirA / VirG proteins are the microbes eye's & ear's (in a manner of speaking) - they will let everything else know that it's time for action. VirD2 is assembled and cuts out the DNA instructions that the microbe wants to paste into the plant genome. A biochemical Ctrl+C, Ctrl+V.
GET THIS - the DNA instructions - a recipe for the microbes dinner. Not even kidding. It tells the plant how to make it's fav food.
VirD2 grabs onto the microbe's recipe (the DNA instructions) and functions like a little proteinaceous tugboat that will chug along to the plant nucleus, dragging the DNA with it. We call this 'T-DNA', as in Transfer DNA.

Biology is crackers. Properly 'LOL'ing as I write this.
Oh man - the Agrobacterium needs to get the T-DNA instructions into the plant cell...but what about that there plant cell wall? Not a problem, it builds a FRIGGING injection apparatus to get it across: VirB1-VirB11 & with VirD4 assemble like an organic transformer.
@MicrobioSoc @plantae_org @ASPB @kevinfolta @QUBIGFS @gatesfoundation @AgBioWorld @mem_somerville @ScienceAlly @GeneticLiteracy I should credit @PlantTeaching for heavily inspiring this figure - check out the FREE learning / teaching resources at… @ThePlantCell Amazing resources!
The plant under siege is nobodies fool, and won't take kindly to DNA swimming around in the cytoplasm without an escort. So, it has an armory of DNA nucleases that try to chop up the T-DNA. The microbe has planned ahead though, and clothed the T-DNA in a proetinaceous VirE2 coat!
VirE2 = biological armor for The Agrobacterium T-DNA.
Once the microbe's T-DNA / VirD2 / VirE2 entourage makes it to the nucleus, it looks for naturally occurring double-stranded DNA breaks, and jams the T-DNA in there while the plant attempts to stick it all back together again.


That sucker has been genetically modified!
We can use this 100% natural process, to piggyback in additional pieces of DNA that we think will help the plant, in any of a variety of ways. I'm going to take a break for lunch, but when I get back, we'll look at some of the ways this awesome technique has been used!
We've explored the incredible world of nature's genetic engineer, the soil microbe, A. tumefaciens, and established that we can use this process to add DNA instructions of our own into plants. This allows us to improve them - more yield / higher nutrition / pest resistance etc.
Before I talk about any specific examples - disclaimer time - I have no links to these companies, no financial interests or obligations of any kind. I'm highlighting innovative science that ticks my personal interest box :) Nothing more than that.
@ArcticApples, take a bow! This is one of my favs - it reduces food waste, and it uses one of my favourite biochemical pathways to do it - RNA interference (RNAi)! RNAi is like a volume dial for genes, it lets you 'turn them down' a bit, producing less of the protein!
If you have less of the protein, you have less of the function. In this case, RNAi was used to reduce the amount of a POLYPHENOL OXIDASE gene (PPO), which is responsible for tissue browning following damage / cutting…
When plant tissue is wounded (say for example when a pathogen invades), PPO gets exposed to polyphenols, making compounds called quinones. And, well, quinones - they just want to see the world burn. They spontaneously erupt into melanins, laying waste to everything in sight.
If the plant can't have those cells, nobody can hav'em! The pathogen certainly isn't keeping them. The great news is that there are several PPO genes that cooperate on this kind of thing, and some will have a big impact on browning, but won't have a big impact on pathogen defence
@ArcticApples found that PPO gene, and reduced the amount of protein coming out of it, using RNAi. Apples don't brown when cut, don't bruise when handled. Nice! Very, very nice.
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