Image credits: Imperial College London.
A lettuce plant can now make a protein normally found inside a pig’s muscles.
Researchers have engineered lettuce and tobacco chloroplasts to stably produce myoglobin, the iron-binding protein that helps give meat its red colour, distinctive flavour and nutritional iron. They used lettuce and tobacco becase these plants have some of the most well-studied genomes. But don’t expect meaty lettuce anytime soon.
Turning a leaf into a protein factory
Myoglobin is an iron- and oxygen-binding protein found in the heart and skeletal muscle tissues.
The small protein sits inside vertebrate muscle and binds heme, an iron-containing molecule. Oxygen attaching to that heme helps determine meat colour, while myoglobin also contributes to the metallic and umami notes associated with meat. Heme iron is generally more readily absorbed by the human body than the non-heme iron that dominates plant foods.
That combination has made heme proteins valuable targets for the alternative-meat industry. Some products already use leghemoglobin, a related protein naturally found in legume root nodules, made through engineered yeast.
But what if instead of asking microbes in fermentation tanks to manufacture the ingredient, you could use the plant itself as a production system? That was the idea behind researchers from Imperial College London, the Bezos Centre for Sustainable Protein, Kyomei and collaborating institutions.
The approach is called plant molecular farming, and basically, it works by installing genetic instructions in plants so that their cells manufacture useful proteins. It’s not a new approach, it’s been used for roughly four decades for medicines, vaccines and industrial products.
Chloroplasts and protein
In this instance, the team targeted chloroplasts — the structures inside plant cells best known for photosynthesis. Chloroplasts have their own genomes, because they evolved as free-living bacteria. A single plant cell can contain many chloroplasts, each carrying multiple copies of that DNA. That gives researchers many templates from which to manufacture a desired protein, which is a big advantage.
The chloroplasts did what the researchers hoped: they turned plant cells into better myoglobin factories. Tobacco produced the most, lettuce somewhat less, and both substantially outperformed engineered algae. In tobacco, putting the gene in the chloroplast also produced at least three times more myoglobin than putting it in the plant’s nucleus.
But “more” does not yet mean “a lot.” A kilogram of fresh engineered lettuce contained only about 48 milligrams of myoglobin. Tobacco contained about twice that amount. Once differences in water content were removed, the two plants performed similarly. Beef muscle, by comparison, contains roughly 10 times more myoglobin by dry weight.
There’s still plenty of room for improvement, but it’s still promising. Even though the quantities are small, they can be grown over large areas using sunlight as their main energy source, while livestock requires feed, land and other inputs before an animal produces muscle.
Not a meat replacement crop (for now)
Illustration generated with the aid of AI.
The significance of the study is straightforward: the researchers showed that a plant can be genetically programmed to make an animal muscle protein, pass that ability to its offspring and continue growing normally enough to flower and produce seeds.
The study didn’t calculate whether growing and extracting myoglobin from plants would ultimately be cheaper or more sustainable than making it in microbes or obtaining it from animals. The researchers say that question will require a dedicated economic analysis.
The next challenge is to make that protein fully functional. The plants produced myoglobin, but much of it lacked heme, the iron-containing molecule that gives myoglobin its useful properties. In other words, the researchers have shown that plants can build the protein. Now they need to improve the final assembly.
Co-author Dr Kyoko Morimoto, Chief Scientific Officer at Kyomei, said: ‘We hope that edible lettuce, modified to express myoglobin, could also one day serve as a heme-iron-enriched biofortified food, depending on legislative approval’.
Demonstrating stable myoglobin production in plant chloroplasts, including in an edible crop, raises the possibility of using plants as scalable, low-input production platforms alongside microbial fermentation” concluded Professor Rodrigo Ledesma-Amaro, Director of the Bezos Centre for Sustainable Protein.
The study was published in Frontiers.
