Cannabis Tissue Culture: Present and Future

Photos in this article are courtesy of Dark Heart Nursery (by Jennifer Cole and Mike Rosati).

In case you had missed it, Strainly allows you to offer genetics under the form of tissue cultures. While seeds and clones have been quite common starting material among growers, micro propagation is usually less familiar. Fortunately, William Roberts, Researcher / Agronomist at Dark Heart Nursery, is sharing his knowledge with us in this article. William has been with Dark Heart Nursery for 7 years and tissue culturing cannabis since 2014! Happy reading…

William Roberts, Researcher / Agronomist at Dark Heart Nursery.

As the cannabis industry continues to climb out of the shadows of prohibition there are many areas where the industry is playing catch-up with the larger world of mainstream modern agriculture. Most of the new cannabis businesses that have sprung up within the last few years are, to at least some extent, trying to emulate successful agricultural companies using much of the same technology and methods that made them successful. One proven agricultural  technology that is generating an increasing amount of interest from cannabis growers is plant tissue culture.

Among certain types of cannabis growers, enthusiasts, and entrepreneurs plant tissue culture has always been a tempting area for exploration. Many people familiar with plant tissue culture think it will help to drive growth and innovation in new and unexpected ways throughout the cannabis industry for years to come. But what is plant tissue culture and can its use with cannabis ever live up to the hype?

A Reminder of What Plant Tissue Culture is

Photo by Mike Rosati from Dark Heart Nursery lab.

Plant tissue culture is the process of growing plants in an isolated, sterile laboratory environment on artificial media in sealed sterilized containers. Instead of soil, plants are grown on a moist gelled medium full of nutrients and sugar. The tissue cultured plantlets grow out in their containers and are periodically harvested and multiplied to provide a continuous source of vigorous young plants.

Tissue culture is widely used in the production of many types of plants including orchids, fruit and nut trees and many other crops. However the potential for propagating new plants through tissue culture is limited by high production costs, the specialized facilities and skills required to effectively use the technology, and the amount of human labor needed to make it work on a large scale. Because of these limitations tissue culture is really only well suited for high value plants such as orchard trees and, perhaps, cannabis.

Production costs of tissue cultured cannabis will likely remain high for quite some time and the technology is likely to remain imperfect and limited in scale. Many cannabis growers who try using tissue cultured plants will not see enough of an improvement to justify the costs, time and effort needed, particularly with seeds and clones readily available from increasingly reputable nurseries.

So why bother? When used properly tissue culture can be a tremendously useful method of producing high quality, genetically identical, uniform plants that are vigorously growing and largely free of pests and disease. For example, farmers in California’s central valley are able to quickly plant entire almond and walnut orchards with the highest quality, genetically superior tissue-cultured stock. The use of tissue cultured plants can make harvests much more consistent, productive and profitable. That at least is the potential.

The Quest For Truly ‘Clean’ Plants

Photo by Jennifer Cole from Dark Heart Nursery.

One of the main advantages of plant tissue culture is the ability to produce pest and disease-free plants. Almost every company producing cannabis tissue cultures promotes the pest free aspect of tissue cultured plants as a main selling point. This ‘clean’ aspect of tissue cultured plants comes from the first stage of tissue culture, where the starting plant material is carefully prepared and surface-sterilized, typically through bathing it in concentrated bleach, to remove every microbe on the plant surface. Contaminants like bacteria, fungi, and even insects quickly proliferate in tissue cultures if present and are easily weeded out over time.

This surface sterilization process can only guarantee that plants will be free of common and easily observable pests and diseases like powdery mildew, botrytis and aphids. Viruses, which are tiny, often invisible and surprisingly easy to spread, cannot be reliably eradicated by typical tissue culture methods. There is nothing about the initial surface sterilization process or tissue culture in general that eliminates viruses from the cultures. And once the cultures have been established any viruses present will not become visible in the way that bacteria and fungi will. This lack of visibility makes virus transmission a real problem for tissue culture labs aspiring to produce truly ‘clean’ plants.

The Fight Against ‘PCIA’ and the ‘Dud Virus’

Viruses and similar pathogens have been plaguing agricultural crops for thousands of years, and there is no reason to think that cannabis will be any different. There are many reasons to suspect that cannabis may have an unseen virus or viruses negatively impacting yields. During prohibition cannabis genetics spread mostly through anonymous seeds, cuttings and clones traded between growers, creating the perfect environment for unseen viruses to spread. Within the past few years many growers have noticed unexplained issues with their plants resulting in significant losses in both yield and plant quality. And the problems, to the extent that growers perceive and talk about them, are only getting worse.

One name for this mysterious cannabis pathogen is PCIA (Putative Cannabis Infectious Agent), also called the ‘dud virus.’ General symptoms can include weak drooping branches, small spiky leaves, twisted and stunted growth and ultimately very low flower yield and THC content. Currently the only treatment for PCIA is to cull out and replace infected plants as soon as they are identified. This problem creates an opportunity for tissue culture to become more about producing ‘clean’ plants that have gone through a rigorous process for eradicating viruses and similar pathogens.

Photo by Mike Rosati from Dark Heart Nursery lab.

Dark Heart Nursery, based in Oakland, CA, has been a pioneer in cannabis tissue culture since 2014. They have developed a patent-pending process for eradicating cannabis viruses through intensive treatments and advanced tissue culture techniques. Dark Heart’s other recent breakthrough is the identification of a pathogen that is likely responsible for PCIA and the development of a test that can reliably detect it, even in plants that show no symptoms. With these advances Dark Heart Nursery has been able to ‘clean’ their mother plant population, and thus the clones they sell to growers, since early 2018 with promising results. Similar processes and growing methods have been used to cure many other crops from debilitating viral diseases and are now industry standards.

This combination of identification, testing and virus eradication through tissue culture has the potential to produce truly clean, elite tissue cultured cannabis plants for the first time. In the next few years nurseries like Dark Heart will be able to provide increasingly clean plants to growers through systematic testing and eradication of infected stock plants. As legalization continues to spread across the globe more labs, universities and other organizations will identify viruses and similar pathogens and develop improved methods of testing for and controlling them. In time the losses caused by viruses will begin to fade into the past with the other problems caused by prohibition. This ability to identify, eradicate and control viruses will ultimately be how tissue culture makes its mark on the cannabis industry.

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