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How Do Cannabis Trichomes Form? Scientists Are Using Firefly Genes to Find Out

By Alexander Beadle

Published: Jul 05, 2022   

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How Do Cannabis Trichomes Form? Scientists Are Using Firefly Genes to Find Out

The majority of cannabinoids are produced in cannabis plant’s trichomes, the white, sticky spikes that protrude from the surface of cannabis flowers. But beyond this, very little is known about how cannabinoids are synthesized and what factors may limit or boost cannabinoid production in plants.

Now, a new paper from scientists at the University of Connecticut, published in the journal Plants, reports the discovery and identification of a transcription factor gene that may be responsible for trichome development and cannabinoid synthesis.

To investigate how transcription factors may play a role in trichome development, the UConn researchers have received funding from the National Research Institute of the US Department of Agriculture. This will be used to support a new research project that will use a modified version of fireflies’ bioluminescent gene to study the interactions between DNA and transcription factors in cannabis.


Transcription factor CsMIXTA linked to trichome development

Transcription factor genes regulate how genes are expressed by determining whether a piece of DNA will be transcribed into RNA.

The transcription factor MIXTA has previously been found to play a role in trichome formation in the sweet wormwood plant, and so the UConn researchers carried out a database search for any similar factors in the cannabis plant. Such a transcription factor was identified, which the researchers named CsMIXTA.

The researchers found that CsMIXTA is expressed in high levels in cannabis flower tissues and is upregulated during the peak states of flower maturation. Further analysis also showed that CsMIXTA expression was significantly correlated with the expression of two other genes that are essential to synthesizing the precursor compounds needed to generate the major cannabinoids.

Speaking to audiences at the Analytical Cannabis Expo West Online 2021 soon after this discovery, study author and UConn research assistant professor Yi Ma said, “We don't have direct evidence showing that these kinds of transcription factors could be involved in [cannabis] trichome development, only that the homologs of these in other plant species have function in trichome development.”

“We are actually doing some transformation experiments in tobacco plants [with the] overexpression of cannabis [transcription factors], and then we will look at the phenotype after we get the transgenic plants to see whether there's a change in the trichomes on the tobacco plants. Hopefully, we can get some results very soon.”

Since then, Ma and her colleagues have completed this tobacco experiment, which is also detailed in the new Plants paper.

Overexpressing CsMIXTA in tobacco plants – which are easier to genetically modify than cannabis, according to the researchers – was found to result in increases in trichome density and size, as well as more trichome branching. Notably, the overexpression of other MIXTA-like genes did not affect tobacco trichomes in this way, suggesting that CsMIXTA may have some unique functions in trichome development.

Due to the gene’s potential applications in tailoring trichome development and thus cannabinoid production, UConn has also filed a provisional patent application on this technology.


Studying cannabis genetics with firefly genes

A grant from the National Research Initiative of the US Department of Agriculture will also allow the UConn researchers to continue exploring the role that transcription factors play in trichome development.

In an upcoming study, the researchers aim to clone certain promoters (the section of DNA that transcription factors bind to) and place this into the cells of a model plant. A copy of the firefly luciferase gene – the gene responsible for fireflies’ hallmark bioluminescence – will also be fused to the promoters.

This way, if the promoter controlling luciferase expression comes into contact with the relevant transcription factors, the luciferase reporter will begin to produce light. This bioluminescence can be detected and quantified using a luminometer device, which should allow the researchers to tell if the promoter regions are indeed being controlled by transcription factors. They may also be able to assess whether these promoters respond to other plant hormonal signals.

“It’s a nifty way to evaluate signals that orchestrate cannabinoid synthesis and trichome development,” said study author and UConn professor Gerry Berkowitz in a statement.

Investigating the effects of other factors on THC production is also key. In prior work, Ma and Berkowitz along with graduate student Peter Apicella found that the enzyme that makes THC within the trichomes may not be the rate limiter that regulates THC production. Instead, the synthesis of the precursor compounds and the transporter-facilitated movement of this precursor around the plant may be equally, if not more important.

“From [our results], we could see that there is a certain correlation between the expression of the [THCA] synthase with THC production, but it seems that synthesis genes are not the rate limiting factor in this biosynthetic pathway,” Ma told Analytical Cannabis Expo West Online 2021 attendees.

“We think that the cannabinoid synthesis, the THC synthase or the CBD synthase, may not be the rate limiting factors. But we think that aromatic prenyltransferase could be the rate limiting factor for cannabinoid biosynthesis. So, it is the step that makes the CBGA that is the rate limiting step in this pathway.”

A better understanding of how cannabis produces THC – whether that be how the trichomes work or the rate limiting factors determining maximal THC production – could have significant impacts on hemp farming. Theoretically, scientists could selectively knock out enzymes responsible for THC synthesis and create new strains of hemp that produce only very low levels of THC, and so are far less likely to violate federal law. Similarly, far more potent cannabis strains could be made, which could be of interest to cannabis industry actors looking to maximize their profitability.

“We envision that the fundamental knowledge obtained can be translated into novel genetic tools and strategies to improve the cannabinoid profile, aid hemp farmers with the common problem of overproducing THC, and benefit human health,” the researchers said.

 

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