Apr 11 2023

Using Plants as Biofactories

When you think about it, plants are self-reproducing solar-powered biological factories. They are powered by the sun, extract raw material from the air and soil, and make all sorts of useful molecules. Mostly we use them to make edible molecules (food), but also to make textiles, fuel, and drugs. Their use to make biofuel is still controversial, because of cost and scaling issues. Can we set aside enough land to grow enough feedstock for biofuels to make a difference? Otherwise plants are an indispensable method of manufacturing.

This may become even more true as our bioengineering technology advances. A recent study demonstrates how powerful this technology is becoming – Tunable control of insect pheromone biosynthesis in Nicotiana benthamiana. The specific application here, making pheromones, is actually not the real story. It is just one potential application. But let’s review what the researchers did.

The goal of the study was to bioengineer a species of tobacco plant to make female moth pheromones. Tobacco plants are often used in this research because it is a “model” organism, about which we know a great deal, including a full genome of many varieties.¬† (There are many other reasons as well – a short growth cycle of 3 months, good for tissue cultures, produce many seeds per cross, etc.) The interest in moth pheromones is in their potential use for pest control. The idea is to replicate the pheromones that the female moths of a specific species release to attract males. This will attract all of the local males and distract them from the females, therefore significantly reducing reproduction and the pest population. This can potentially be a form of pest control that does not involve applying a pesticide to the plants themselves.

Pheromones can be tricky to produce through normal chemical processes. The molecules are complex, and often they are released as a combination of molecules in specific ratios that are unique to each species. Bioengineering a plant like tobacco to mass produce pheromones in the proper ratios could make their use in agriculture more plausible and cost effective. The agricultural industry is huge, with razor thin profit margins, and therefore any even modest improvement in cost has a large impact.

What the researchers demonstrated was that they could introduce moth pheromone transgenes into this tobacco variety and the plants would start producing the desired pheromones. But that’s old news. The somewhat newer part is that they were able to demonstrate a high level of tunable control of the expression of these genes. With bioengineering its not enough to just insert a foreign gene to start making its protein. You also have to insert the regulatory components – the bits of DNA that turn on and turn off the gene. Controlling gene expression is critical, for a few reasons.

First, the researchers wanted to be able to insert several pheromone genes in the same plant and control the amount of each pheromone that the plant makes. This is to get the ratio correct for the target moth species. This requires a high level of understanding of how the regulatory DNA works and how to precisely control it.

Second, they needed to tweak the overall amount of pheromones produced compared to all the other genes in the plant. They found that when they cranked up the pheromone gene expression too high, this stunted the growth of the plant because it was not making enough of the proteins it needs to grow and thrive. Too much of its energy and machinery was being diverted to making pheromones. This reduced the net output of pheromones, by shrinking the factory. Therefore they had to find the sweetspot – the maximal amount of pheromones the plants could produce without stunting their growth, in order to maximize net output. This also requires the ability to precisely control gene expression.

This is also what they found:

We demonstrate that copper can be used as a low-cost molecule for tightly regulated inducible expression. Further, we show how construct architecture influences relative gene expression and, consequently, product yields in multigene constructs. We compare a number of synthetic orthogonal regulatory elements and demonstrate maximal yields from constructs in which expression is mediated by dCas9-based synthetic transcriptional activators.

Two things of note here. First, they were able to use copper (which is already used as a pesticide and is safe and cost-effective for agricultural use) as an external trigger of gene expression Рso-called inducible expression. Therefore they are not only controlling expression through the genetic changes themselves, but are able to externally induce expression and control the timing of production. Second, they found that the architecture of how multiple genes are combined together affects their expression.

As a result of these control mechanisms, they were able to maximize yields of pheromones. This specific application has promise, but it has to be tested in the field. They need to actually make and purify the pheromones and deploy them in a field to test their efficacy.

But more important than this one application is the underlying technology. While incremental, this illustrates an important advance in our ability to understand and control gene expression for complex molecules in plants. This can add to the bioengineering options we already have for making things like insulin. We can engineer bacteria or yeast, grow them in large vats, and have them pump out drugs that are otherwise difficult to synthesize or have to be harvested from animals. We can also do this in plants, but adding tunable control adds to the power and utility of this technology.

I don’t think any one form of this technology is inherently superior. Each will be used for the purposes for which they are best suited. It’s good to have more options, however. Plants may be better at producing certain kinds of molecules than yeast or bacteria.

Imagine if the vast tobacco farms in the world were converted (at least partially) from making a harmful product that kills people to make drugs and products useful for agriculture. This is not far-fetched. What matters is the value of these plants as a cash crop. If tobacco plants making pheromones are more valuable on the market than tobacco used for tobacco products, then farmers will have an incentive to switch over. Perhaps we need to find bioengineered commercial uses for coca plants.



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