We’ve been reading a lot about "plant biosensors," which are crops engineered to sense molecules in the environment (like pesticides or toxins) and then change color in response.
They could be an incredibly useful tool for farmers to stop plant pathogens before they spread. Here's how they work, briefly.
About two decades ago, a botanist named Sean Cutler helped discover protein receptors that bind to a plant hormone called abscisic acid, or ABA. This plant hormone regulates how plants respond to stress during droughts.
First, ABA binds to a protein called PYR1, forcing the protein to shift into a different shape and revealing a "binding pocket" that latches onto another protein, called HAB1. This causes “free” HAB1 levels to drop in the cell, which plants use as a signal to activate their stress response.
So those are the basics of ABA sensing. But how are these proteins actually engineered to sense new molecules?
Well, Cutler's group at UC Riverside is really good at engineering the ligand-binding pocket in the PYR1 protein. They've solved its crystal structure, and they know exactly which amino acids are responsible for grabbing onto the ABA molecules. For a recent paper (nature.com/articles/s41…) they randomly mutated these amino acids and then searched for PYR1 mutants that are able to grab onto new molecules; not just ABA. And it actually worked.
They found mutated PYR1 proteins that can grab onto lots of different things, including banned organophosphate pesticides, like azinphos-ethyl and diazinon, with nanomolar sensitivity.
So that’s the “sensing” part. But when the mutated version of PYR1 grabs onto a pesticide, how does that trigger the plant to physically change color?
This is where the synthetic biology comes in. Cutler’s group has engineered plants so that, when PYR1 binds to its target molecule, it instead grabs onto a slightly modified form of HAB1 (called HAB1*) that then activates genes of their choosing (not the drought response). For example, researchers can encode the betalain biosynthesis pathway into these plants (the same genes that give beets their dark red color) and have the plants make those pigments after sensing a desired molecule. They've basically built orthogonal, programmable biosensors inside of plants.
In the years ahead, there will likely be field trials for these plant biosensors. Farmers could plant just a few of them in their fields, amongst the tens of thousands of other plants, without selling them as food. Perhaps this will ease regulatory oversight. The plant biosensors would be discarded at the end of every growing season.