Trichomes

Development, Types, Functions and Microscopy


Introduction


Trichomes is the term used to refer to tiny outgrowths from the plant epidermis. Although the term "trichomes" generally refers to outgrowths ranging from small hairs to larger outgrowths like thorns, it is typically used to refer to the tiny hairs that can be seen emerging from the surfaces of leaves and other epidermal surfaces of plants.

They are either unicellular or multicellular (epidermal cells), which means that some require a microscope to take a closer look.




** The word trichome originated from the Greek word "Trichoma" that refers to hair growth.


Development

Recent studies suggest that these cells differentiate from a pool of equivalent cells. This means that like other cells, they differentiate with the growth of the plant and become specialized cells that serve a number of functions important to the plant.

The development and specialization of these cells is largely regulated by a number of transcription factors which include the R2R3 MYB, basic helix-loop-helix protein and WD40 repeat protein among others.

These developmental factors, among others, are responsible for regulating the parts of the plant where the trichomes will develop as well as the number of trichomes for a given plant. For instance, whereas the miR156 causes ectopic trichomes to develop on such parts as the floral organs of a plant, high expression of SPL, a resistant form of miR156 results in reduced reduction of trichomes.

Such regulation varies from one plant to another.


Some of the other factors that regulate the development and differentiation of trichomes include:

 

  • phytohormones - regulate the differentiation process
  • cytokinins - influence increased formation of trichomes
  • jasmonic acid and salicylic acid - these acids contribute to the formation of these cells in Arabidopsis (small flowing plants found in Eurasia)

Scanning electron micrograph of trichome: a leaf hair of thale cress by Heiti Paves (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia CommonsScanning electron micrograph of trichome: a leaf hair of thale cress by Heiti Paves (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons


Types of Trichomes

As unicellular and multicellular epidermal appendages, there are a variety of trichomes that exist. These vary in size, morphology, origin as well as where they are located and the ability to secrete.


While the classification of trichomes has been found to be somewhat challenging, they are categorized as follows:



Glandular and Non-Glandular Trichomes

Trichomes are either glandular or non-glandular.

 

Glandular Trichomes

 

These types of trichomes are known to secrete various substances including water, nectar, resins, mucilage and terpens among others. Apart from their capacity to secrete, glandular trichomes can also be grouped in accordance to the number of cells.

 

* Glandular trichomes is the term used to refer to an array of glands.

 

Glandular trichomes not only vary in the type of substances they secrete and their location, but also with regards to the mode through which they produce these secretions.

There are both unicellular and multicellular glandular trichomes. Moreover, they can also be uniseriate (they are arranged in single series or layer) or multiseriate (arranged in several series).


Effect of external stress on density and size of glandular trichomes in full-grown Artemisia annua, the source of anti-malarial artemisininEffect of external stress on density and size of glandular trichomes in full-grown Artemisia annua, the source of anti-malarial artemisinin

https://academic.oup.com/aobpla/article/doi/10.1093/aobpla/pls018/176290/Effect-of-external-stress-on-density-and-size-of


Unicellular glandular - For these types of hairs, it's possible to see morphological differences between apical and basal part of the cells. They may also occur branched or un-branched.

 

Multicellular glandular trichomes - these types of trichomes appear as outgrowths of the epidermis with a head consisting of cells that secrete and store great amounts of specialized metabolites.

 

The glandular trichomes include the stinging hairs and glandular hairs. The Urtica dioica is a good example of plants that have stinging hairs. The hairs of these plants have in place a basal bulb that gives rise to a protruding stiff and slender structure. The hairs are capable of secreting a poisonous substance that can irritate the skin when people come in contact with the plant and the sharp hair part penetrates the skin.

As for the glandular hairs (unicellular and multicellular) they can produce a variety of substances including oils and resins which makes them important in various industries including pharmaceuticals.

 

Examples of glandular trichomes include:


  • Saltbush (Atriplex) (secretes water)
  • Docks and Sorrels (secretes water)
  • Basil plants (secretes lipids) 


Secretions

 

For the glandular trichomes, there are a wide variety of secretions that are produced by various plants. For this reason, glandular trichomes can be classified on the basis of the type of substances that they secrete. The following is one of the classifications that have been suggested with regards to secretions:

 

Unmodified or slightly modified substances - This includes such substances as salts and nectar

 

Substances that are synthesized by constituent cells - This includes:


  • Hydrophilic substances
  • Stinging hairs
  • Lipophilic substances

 

* One of the main weaknesses of this method of classification is that there are glandular hairs that are capable of secreting a variety of substances.

 

Non-Glandular Trichomes

 

The non-glandular trichomes vary in anatomy, morphology and microstructure. Despite this diversity, they are mostly grouped on the basis of their morphology.

Like the glandular trichomes, non-glandular trichomes also exist as either unicellular or multicellular. However, they can also exist as either branched or un-branched. A majority of these have been shown to be branched, simple and shaped like a star. As for the un-branched non-glandular trichomes, they can exist as uniseriate, biseriate or multiseriate. They also vary in shape, size and length and can be found in a variety of plants.

Different types of non-glandular trichomes can be found in different locations of a single plant. For instance, whole some of these can be found on the leaf of the plant (unicellular non-glandular trichomes on Coridothymus capitatus) two-celled trichomes can be located at the base of abaxial side of the leaf.


The following are examples of non-glandular trichomes:

 

  • Gossypium with vesicular hair (non-flattened)
  • Styrax
  • T- shaped Corokia


Trichomes Functions

There different types of trichomes that serve different functions for plants. There functions are largely dependent on the type of trichome as well as their location on the plant. While the function of some trichomes remains unknown, we will look at some of the known functions of trichomes.

 

Protection

 

Like animals, plants have also evolved over time. This has seen plants developing a range of mechanism for protection. This enhances the life span of the plant and ensures their survival.

Trichomes are some of the means through which some plants protect themselves from animals, extreme environmental conditions and thus enhancing their chances of survival. For instance, the trichomes (glandular) that develop in cannabis plants secrete a bitter substance and a strong aroma that prevents some animals from eating it. In particular, some glandular hairs tend to secrete lipophilic substances that prevent animals from consuming the leaves of the plant.

Apart from chemicals that wade off animals and pathogens, some trichomes secrete a type of mucilage that serves to trap insects when they come in contact with the plant leaves. This substance also helps prevent excessive water loss from the leaves as well as protecting the plant from excessive moisture.   The non-glandular trichomes develop to form a thick and dense surface around the leaves which serves to protect leaves and the plant in general from harsh environmental conditions as well as protection from pathogens.

With some plants such as Tragia cannabina stinging hairs develop to protect the plant from herbivores. When an animal comes in contact with the hair, they break off and penetrate the body of the animal causing irritations.


Absorption of Water and Moisture

 

Non-glandular trichomes can also be found in the roots of the plant as hair-like structures where they support the absorption of water and other minerals required by the plant. These trichomes are more likely to exist as tubular structures that grow outwards to absorb water and minerals from the soil. However, they are very small with a thin cell wall, and thus can not last long. Once they die, they become lignified and are unable to absorb water and minerals any longer. However, new ones continue to form.

In some of the plants (such as apples and sorghum) the unicellular hairs can secrete mucilaginous droplets, which ensures that the plant does not dry out. Therefore, ensuring that the plant can survive for longer.


Some of the other functions of trichomes include:

 

  • Elimination of excess toxic substances and salts from such plants as the Atriplex
  • Waxes protect plants from extreme heat and sunlight
  • Oils from such plants as Cymbopogon act as insect repellent,
  • Protection of developing buds by such trichomes as the Ephemeral trichomes


Marijuana Trichomes

Also referred to as cannabis, marijuana is a plant that contains a mind altering substance. Recent studies have shown that certain chemicals from the plant have medicinal benefits for various patients.

Like a number of other plants, trichomes develop on the plant when it starts growing and develop fully when the plant starts flowering.

For marijuana, the trichomes exist as resin glands that produce various oils that protect the plant by acting as deterrents. At the same time, these substances protect the plant from extreme conditions and fungal growth.


There are three types of trichomes that develop in the cannabis plant. These include:

 

  • Bulbous trichomes - These trichomes appear as small pointed structures on the surface of the plant. They are the smallest trichomes and are responsible for secreting resins.

 

  • Capitate-sessile - This type of trichomes is bigger compared to bulbous trichomes and tend to develop before the plant starts flowering. They are flattened and contain cannabinoids.

 

  • Capitate stalked trichomes - These are the largest of the three types of trichomes and form during flowering. They are largely involved in the synthesis of cannabinoids and terpenoid synthesis.


Cannabis Trichomes by Psychonaught [Public domain], via Wikimedia CommonsCannabis Trichomes by Psychonaught [Public domain], via Wikimedia Commons


Harvesting

Given that some of the trichomes (capitate-stalked trichomes) in marijuana contain the substance that produces the psychoactive effect, they influence the time of harvesting.

Here, the coloration of the glandular heads of the trichomes rather than the amount of trichomes is the determining factor for harvesting.

Using a microscope, it's possible to view the head of the capitate-stalked glandular trichomes. This coloration varies between different marijuana strains as well as depending on the maturity stage of the plant.

As the concentration peak of THC (Tetrahydrocannabinol) nears, trichomes have a translucent coloration. This color is the result of trichome glands producing the resin. However, the levels of CBD (Cannabidiol) is still low at this point. At the same time, the aromatic molecules are also at their peak. 

In most cases, the plant leaves are harvested when the head of the glandular trichome starts turning opaque. And so there is a higher level of THC that can produce the best effects. 


Trichomes Microscopy

While some trichomes can be seen with the naked eye (as hairs and spikes) others are too small that they require a microscope to see and study them. A range of microscopes can be used to view trichomes and identify the differences in their shapes and sizes etc.

 

* Such magnifying devices as the jeweler's Loupe can be used, but do not provide clear details of trichomes.


Pocket/Handheld Microscope

This is one of the most popular tools for those who grow cannabis for monitoring trichomes.

Pocket microscopes are some of the cheapest and simplest microscopes that can provide high magnifications of 100x. One of the biggest advantages of a pocket microscope is that it can be used out in the field.

The user can use it in the farm to view and determine the color of the trichomes. This is particularly due to the fact that most of these microscopes come with a built-in light that makes it possible for this device to be carried out in to the field for use.

The process simply involves pointing the microscope at a leaf (or any other part of the plant with trichomes) and focusing to view and determine the color.





Stereo Microscope

A stereo microscope offers dual magnification. As such, it is like viewing an object using naked eyes. Using a stereo microscope, it is possible to observe both the glandular and non-glandular trichomes on the surface of a plant leaf and stem among other parts.

 

Requirements 

  • Stereo microscope
  • Plant leaf or stem

 

Procedure

 

Place the leaf/stem under the stereo microscope.

Increase magnification while focusing and record observations.

 

Observations

 

Under the stereo microscope, students will be able to see the fine hairs on the surface of the leaf if the leaf/stem has non-glandular, hair-like trichomes on its surface. In addition, it's possible for students to see the heads of capitate glandular hair and secretions (such as oils) on the cuticle surface from sunken peltate hairs.




Compound Microscope (Epi-Fluorescence)

The epifluorescence microscope is ideal for viewing trichomes given that it allows for enhanced specificity and contrast. This makes it ideal for viewing and differentiating different parts to identify trichome structures as well as the density of trichomes on any given part of the plant.

 

Requirements

 

  • epifluorescence microscope
  • young leaves/stems
  • 4% paraformaldehyde
  • phosphate buffered saline
  • gel mount
  • blade

 

Procedure

 

Obtain a young leaf/stem from a young shoot and fix in 4% paraformaldehyde for about 2 days.

Wash the leaf/stem using phosphate buffered saline three times.

Cut the leaf/stem to obtain a cross-section.

Mount the leaf/stem on a microscope glass slide and cover with cover slip using gel mount.

Place under the microscope for observation.

 

* repeat this produce using a young tomato leaf for comparison

 

Observation

 

When viewed under the microscope, the trichomes are clearly visible as with a bluish background





Related:  Microscopy Applications

Check out Leaf Structure under the Microscope

Research Epidermal Cells

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References

David G. Hall, El-Desouky Ammar, Kim D.Bowman and Ed Stover (2017) Epifluorescence and stereomicroscopy of trichomes associated with resistant and susceptible host plant genotypes of the Asian citrus psyllid (Hemiptera: Liviidae), vector of citrus greening disease bacterium.

 

 Ed Rosenthal (2001) The Big Book of Buds: Marijuana Varieties from the World's Great Seed Breeders.

 

J. A. Callow, D. L. Hallahan and J. C. Gray (2000) Plant Trichomes.

 

Jang-Sean Choi and Eun-Soo Kim (2013) Structural Features of Glandular and Non-glandular Trichomes in Three Species of Mentha.



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