Trichome

Trichomes on plants are epidermal outgrowths of various kinds. The terms emergences or prickles refer to outgrowths that involve more than the epidermis. This distinction is not always easily applied (see Wait-a-minute tree). Also, there are nontrichomatous epidermal cells that protrude from the surface.

A common type of trichome is a hair. Plant hairs may be unicellular or multicellular, branched or unbranched. Multicellular hairs may have one or several layers of cells. Branched hairs can be dendritic (tree-like) as in kangaroo paw (Anigozanthos), tufted, or stellate (star-shaped), as in Arabidopsis thaliana.

Another common type of trichome is the scale or peltate hair, that has a plate or shield-shaped cluster of cells attached directly to the surface or borne on a stalk of some kind. Common examples are the leaf scales of bromeliads such as the pineapple, Rhododendron and sea buckthorn (Hippophae rhamnoides).

Any of the various types of hairs may be glandular, producing some kind of secretion, such as the essential oils produced by mints and many other members of the family Lamiaceae.

Many terms are used to describe the surface appearance of plant organs, such as stems and leaves, referring to the presence, form and appearance of trichomes. Examples include:

• glabrous, glabrate – lacking hairs or trichomes; surface smooth
• hirsute – coarsely hairy
• hispid – having bristly hairs
• articulate – simple pluricellular-uniseriate hairs
• downy – having an almost wool-like covering of long hairs
• pilose – pubescent with long, straight, soft, spreading or erect hairs
• puberulent – minutely pubescent; having fine, short, usually erect, hairs
• pubescent – bearing hairs or trichomes of any type
• strigillose – minutely strigose
• strigose – having straight hairs all pointing in more or less the same direction as along a margin or midrib
• tomentellous – minutely tomentose
• tomentose – covered with dense, matted, woolly hairs
• villosulous – minutely villous
• villous – having long, soft hairs, often curved, but not matted

The size, form, density and location of hairs on plants are extremely variable in their presence across species and even within a species on different plant organs. Several basic functions or advantages of having surface hairs can be listed. It is likely that in many cases, hairs interfere with the feeding of at least some small herbivores and, depending upon stiffness and irritability to the palate, large herbivores as well. Hairs on plants growing in areas subject to frost keep the frost away from the living surface cells. In windy locations, hairs break up the flow of air across the plant surface, reducing transpiration. Dense coatings of hairs reflect sunlight, protecting the more delicate tissues underneath in hot, dry, open habitats. In addition, in locations where much of the available moisture comes from fog drip, hairs appear to enhance this process by increasing the surface area on which water droplets can accumulate.

Glandular trichomes have been vastly studied, even though they are only found on about 30% of plants. Their function is to secrete metabolites for the plant. Some of these metabolites include:

• terpenoids, which have many functions in the plant related to growth and development[6]
• phenylpropanoids, which have a role in many plant pathways, such as secondary metabolites, stress response, and act as the mediators of plant interactions in the environment[7]
• flavonoids
• methyl ketones
• acylsugars[8]

Non-glandular trichomes are important for plant protection against UV light.[4]

The model plant, Cistus salvifolius, is found in areas of high-light stress and poor soil conditions, along the Mediterranean coasts. It contains non-glandular, stellate and dendritic trichomes that have the ability to synthesize and store polyphenols that both affect absorbance of radiation and plant desiccation. These trichomes also contain acetylated flavonoids, which can absorb UV-B, and non-acetylated flavonoids, which absorb the longer wavelength of UV-A. In non-glandular trichomes, the only role of flavonoids is to block out the shortest wavelengths to protect the plant, which differs from in glandular trichomes.[4]

Non-glandular trichomes in the genus Cistus were found to contain presences of ellagitannins, glycosides, and kaempferol derivatives. The ellagitannins have the main purpose of helping adapt in times of nutrient-limiting stress.[4]

Plant phytohormones have an effect on the growth and response of plants to environmental stimuli. Some of these phytohormones are involved in trichome formation, which include gibberellic acid (GA), cytokinins (CK), and jasmonic acids (JA).[12]

GA stimulates growth of trichomes by stimulating GLABROUS1 (GL1).

However, both SPINDLY and DELLA proteins repress the effects of GA, so less of these proteins create more trichomes.

Some other phytohormones that promote growth of trichomes include brassinosteroids, ethylene, and salicylic acid. This was understood by conducting experiments with mutants that has little to no amounts of each of these substances. In every case, there was less trichome formation on both plant surfaces, as well as incorrect formation of the trichomes present.[12]

Arabidopsis thaliana trichomes are classified as being aerial, epidermal, unicellular, tubular structures.[16]

Stinging trichomes vary in their morphology and distribution between species, however similar effects on large herbivores implies they serve similar functions. In areas susceptible to herbivory, higher densities of stinging trichomes were observed. In Urtica, the stinging trichomes induce a painful sensation lasting for hours upon human contact. This sensation has been attributed as a defense mechanism against large animals and small invertebrates, and plays a role in defense supplementation via secretion of metabolites. Studies suggest that this sensation involves a rapid release of toxin (such as histamine) upon contact and penetration via the globular tips of said trichomes.[24]