Tissue Culture

Imagine looking into a room the size of a bedroom and seeing shelf after shelf of miniature plants growing in test tubes, thousands of plants growing in their own little glass houses. It isn’t science fiction—it’s science.

More specifically, it’s the science of tissue culture micro-propagation, and it’s being used by professional growers and hobbyists around the world. It’s all made possible by a unique characteristic of plants called totipotency.

Each plant cell contains all of the genetic information necessary to grow a complete new plant that is genetically identical to the host plant, so a small snip from a leaf, stem or bud can be divided and re-divided multiple times, producing literally thousands of clones in a relatively short time.

Many gardeners are familiar with taking a cutting and rooting it to produce a new plant, but tissue culture takes it to a whole new level. A small living tissue, usually from a growing tip, is sterilized and placed into a nutrient-rich gel of multiplication media.

Soon, little shoots start to pop up, without roots. Those shoots can then be transferred to other jars filled with multiplication media, and more shoots will appear. The multiplication process continues exponentially, and what started as a small clump of plant material soon turns into hundreds of un-rooted plantlets.

The un-rooted plantlets can be transferred into culture tubes containing special rooting hormones and tiny, fine roots soon begin to form. Presto! You now have a supply of miniature plants in test tubes that can be uncorked and grown on at will.

Keeping a room of full-grown mother plants can be time-consuming and expensive. Mother plants can take up a lot of space, they require daily watering and care, and they are always in danger of getting spider mites, powdery mildew and other potential threats.

Plants in test tubes are much easier to handle. They are easy to store, and require minimal heat and light. Plus, they are kept in their own sealed containers, safe from pests and diseases. Although it takes some time to make occasional transfers into fresh media, it’s a lot easier than maintaining huge mother plants.

Plant Tissue Culture Care

The trick is to start in a clean, uncontaminated environment with special attention to aseptic processing techniques. Rooting gels are rich in vitamins, nutrients and plant growth hormones, so a single speck of dust or mold can quickly grow and contaminate the culture jar.

Transferring tissue samples should be done in a draft-free room to ensure air is as sterile as possible. The best way to ensure success is by using HEPA filters, which filter out particles as small as .3 microns, removing mold spores, bacteria and even large viruses from the air.

Ideally, HEPA filters should clean the air in the entire transfer room, with additional HEPA filtration on the transfer bench. Even then it takes practice to get good at transfers.

Just reaching your hand over an open jar of tissue culture media can cause problems. A single skin cell can contaminate the jar with bacteria and fungi and eventually overrun and kill the plant, but once a transfer is done correctly and the container is properly sealed, it is easy to handle and transport.

If you just want to play with tissue culture and you don’t want to invest in expensive facilities and a lab, tissue culture kits are available that can be done in a home kitchen. Premixed rooting media can be mixed in hot water, poured into baby food jars and sterilized in a standard pressure cooker.

The tissue transfers can then be done in a homemade glove box with reasonable success. If you want to take the hobby to a professional level, it’s better to set up dedicated rooms for media preparation, transfers and culture growing.

A modest tissue culture lab with just three full-time employees can easily produce more than 200,000 clones per year. It then becomes a personal choice whether to grow the plants yourself or sell the tissue-cultured plants to growers who are willing to pay extra for guaranteed clean stock.

Why is Plant Tissue Culture Beneficial?

One of the biggest problems with conventional propagation from mother plants is the spread of disease. Long-term plants are prone to infection by many common pests and diseases, and moving infected clones from place to place can quickly spread diseases across the country. Tissue culture eliminates the spread of disease.

Since tissue-cultured plants start in a sterile environment and are kept in closed containers, there is almost no chance for the seedlings to have pest, mold or mildew issues. If the plant tissue is contaminated with a fungus, it will quickly show up on the growing medium and the contaminated culture can be easily identified and destroyed.

Therefore, tissue-cultured plants can be safely transferred from state to state or country to country without fear of spreading diseases.

Tissue culture can even be used to rescue mother plants infected by viruses. Viruses normally spread from the older plant tissues upwards to new growth. If a valuable mother plant contracted a virus, it is possible to take a tissue sample from the uninfected new growth at the top of the plant.

The healthy tissue culture can be used to produce a new, virus-free mother plant, or multiplied to produce thousands of clean, virus-free plants ready to be grown in the field.

I once had the privilege of visiting a nursery that provided “certified” virus-free stock to farmers. Plants were started from tissue culture in a sealed, HEPA-filtered clean room, and the floors, walls and ceilings were washed once a week. No visitors were allowed in the tissue culture area, but I was allowed in the greenhouse, after following strict protocols.

In the entry room, I was required to put on sanitized coveralls and rubber boots and step in a tray of sterilizing solution. Next, I entered a spotlessly clean service area where I was required to wash my hands with antibacterial soap. After stepping in sterilizing solution again, I entered a special positive-pressure entrance to the greenhouse. When I opened the door, pressurized air came out of the room instead of outside air being pulled in.

Finally, I got to step into the greenhouse for a peek. The plants were grown hydroponically with a sophisticated water-cooling and filtration system, and there wasn’t a single bug to be seen. Since insects are vectors for viral contaminants, extreme measures had to be taken to guarantee virus-free plants.

So what were they growing in such a pristine environment? Potatoes! The entire facility was dedicated to growing certified, virus-free seed potatoes for contract farmers in the region.

Aside from providing virus-free plant stock, tissue culture is also important for propagating rare and unusual plants. For example, if a breeder develops an award-winning plant with superior characteristics, tissue culture is a fast way to bring it to market.

Since every plant grown from tissue culture is genetically identical to the mother plant, all of the tissue-cultured plants will display the same superior characteristics, as long as they are given the same nutrients and grown in the same environment.

Tissue culture micro-propagation may open the door for many new opportunities in hydroponics and computer-controlled agriculture, especially in the field of plant biotechnology. New vaccines and anti-cancer drugs are already being produced by special transgenic plants.

If a new medicinal compound can be produced in a single plant, hundreds of thousands of identical plants can be produced in test tubes, then grown out in a controlled environment.

In the future, tissue culture micro-propagation and computer-controlled environments may allow for the efficient downstream processing of many new drugs, nutraceuticals and life-saving vaccines from plants. Science or science fiction? You decide!

By Harley Smith