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BIODIVERSITY
Published: 13 October 2022
doi: 10.3389/frym.2022.739393
WHY ARE SOME PLANTS HAIRY?
Alejandro R. Vasquez 1, Ishveen Kaur 1and Rupesh Kariyat 2*
1Department of Biology, The University of Texas Rio Grande Valley, Edinburg, TX, United States
2Department of Entomology and Plant Pathology, University of Arkansas, Fayetteville, AR, United States
YOUNG REVIEWERS:
DARIO
AGE: 15
SRIJA
AGE: 13
VALERIE
AGE: 14
Whether we live in a sprawling metropolis or on a rural farm, plants
and insects are part of our daily lives, and many of them cannot exist
without the other. Insects help plants reproduce via pollination, and
plants provide insects with food and shelter. However, plants also
require nutrition to grow, develop, and reproduce; and plant-eating
insects can damage them. Over millions of years of living together,
plants have evolved multiple defense strategies to defend themselves
against insects. One such defense is tiny hair-like projections called
trichomes. In this article, we explain how plants use their trichomes
as a creative and unique weapon to protect themselves from
insect herbivores. Trichomes can cause physical injury, release toxic
chemical compounds, and even cause internal injury to insects. We
also discuss how plant-eating insects counter these plant defenses
using their own defenses—leading to a tug-of-war for survival.
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Vasquez et al. Plant Hairs Protect Against Insects
WHY ARE PLANTS SO IMPORTANT?
Plants, the backbone of planet Earth, are widely thought to be nothing
more than stationary, uninteresting organisms. However, studying
plants has revealed how complex and interesting they are, and plant
research has taught us about the vast array of physical and chemical
characteristics plants use to thrive on Earth, where they are surrounded
by organisms that feast on them. Plants are not just food sources
for animals—they also shape the environment around us in many
ways, and most other organisms could not survive without them. For
example, plants take up carbon dioxide (CO2), a gas that animals exhale
or produced when burning fossil fuels, and use it in photosynthesis.
PHOTOSYNTHESIS
Biological process by
which plants make and
store food, in the form
of sugars.
This removes CO2from the atmosphere. Without the removal of CO2,
life on Earth would be impossible. As you may know, the excess of
CO2in the atmosphere is causing environmental problems like global
warming [1]. Also, when it comes to farming, plants called cover crops
minimize the eects of soil erosion, by holding the topsoil in place
via their root structures. This helps to keep soil heathy. However, to
perform these ecosystem services, plants must thrive well in various
ECOSYSTEM
SERVICES
Actions done by
organisms in the
benefit of
the ecosystem.
environments and protect themselves against their mortal enemies,
the herbivorous insects. These insects damage and even kill plants by
HERBIVOROUS
Plant eating.
feeding on plants’ leaves, flowers, fruits, and roots.
INSECTS AND PLANTS
Insects, regardless of their relatively small size, are extremely
important organisms in the animal kingdom. The diversity of insects
is unmatched, with many feeding on animals and others dependent
upon plants. Many species of insects rely solely on plants for food
(Figure 1), and others only feed on plants during certain parts of
their lifecycle. Think about a caterpillar hatching from its egg on a
leaf surface, feeding on the plant until it is ready to form a pupa,
then attaching itself to the plant or burrowing into nearby soil until
it is ready to emerge as a moth or butterfly. To outsmart each other,
plants and insects have been co-evolving for millions of years and, in
many cases, they cannot live or reproduce without each other. While
many insect species feed on plants, plants also depend on insects
for reproduction, through pollination. Without insects, many plant
POLLINATION
Transfer of pollen from
plant’s male
reproductive organs to
the plant’s female
reproductive organs.
species would cease to exist. But how can plants thrive on Earth if the
number of insects, many of which constantly feed on plants, heavily
outweighs them?
PLANT DEFENSES AGAINST INSECTS
Plants are mostly immobile and appear defenseless to the untrained
eye, but a deeper look can show us a wide range of both chemical
and physical defenses that plants employ to protect themselves
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Vasquez et al. Plant Hairs Protect Against Insects
Figure 1
Figure 1
(A) Picture of silverleaf
nightshade (SLN) plants
sprouted in the field as
weed. (B) Magnified
image of SLN leaf. (C)
Scanning Electron
Microscopy (SEM)
image of same leaf at
100X, with visible
trichomes (hairs). (D)
Enlarged image of
single trichome
highlights the pointed
structure of trichome
with one spiled head
and pointed parts
(roughly 25) which are
responsible for poking
holes on insects pests
feeding on plant. (E)
Tobacco hornworm
caterpillar on a
horsenettle plant. (F) A
bordered patch
butterfly caterpillar on a
sunflower plant. (G) A
cabbage butterfly
caterpillar on a thale
cress leaf. (H) A fall
armyworm caterpillar
on a tomato plant. All
the host plants have
trichomes that vary in
type and density.
(Image credits: Rupesh
Kariyat and Ishveen
Kaur).
against the insects ready to feed on and possibly decimate them.
Plants have chemical defenses that are invisible to us, which they
use to deter insects from mindlessly chowing down on them. These
defenses include the emission of volatile compounds that attract
other organisms that feed on insects, indirectly protecting the plant
[2]. Some plants are toxic or taste terrible, thereby directly defending
themselves against insects.
In addition, as you may have noticed, some plants are armed with
thick spines that we can see and—even worse—feel. Plants also
have physical defenses that are not so easily noticed. Herbivorous
insects, like the caterpillar that just hatched from its egg, must
find a spot on the leaf surface to start feeding. Here is where the
plant’s first line of defense is waiting—plant hairs called trichomes
(Figures 2E,F). We often think of animals as the only organisms with
hair or fur, yet an estimate 80% of plants also have hairs on many of
their structures.
Figure 2
Figure 2
SEM image of (A) bottle
gourd at 400×
magnification, showing
non-glandular (1) and
glandular (2) trichomes.
(B,C) A glandular
trichome magnified at
600×and 800×,
respectively. (D) The
same leaf at 450×.
Bulky, globular,
glandular trichomes
housing chemical
toxins (1) and
hook-shaped,
non-glandular
trichomes (2) can be
seen. (E) Sunflower leaf
at 200×.(F) Squash leaf
at 100×.
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Vasquez et al. Plant Hairs Protect Against Insects
HOW PLANTS USE TRICHOMES TO DEFEND AGAINST
INSECTS
Trichomes are present on various parts of plants, such as leaves, stems,
VOLATILE
COMPOUNDS
Chemical compounds
released by plants, in
this case to alert the
predators of the insects
that are attacking
the plants.
TRICHOMES
Hair-like protrusions
used by plants
for defense.
and fruits, and these hairs function as a formidable barrier against
a wide range of herbivorous insects—in truly fascinating ways. But
trichomes have other functions, too. For example, if it is extremely hot,
they act as a canopy to produce shade, reduce water loss, and protect
the plant against the sun’s ultraviolet radiation. But their protection
function against insects is the most interesting part. Plant trichomes
have tremendous variation in size, shape, and numbers (Figure 2). A
mat of trichomes on the leaf surface can delay caterpillar feeding in a
density-dependent manner [3], which means that, if a plant has more
trichomes, it takes caterpillars longer to find a spot or to chew through
the trichomes to get to epidermis—the first nutritious leaf layer. Some
trichomes are also sharp, and they can poke holes in the soft bodies of
caterpillars (Figures 2A,D). Surprisingly, some plants can even increase
the number of trichomes on young leaves if their older leaves are being
eaten—preparing the young leaves against future damage!
Broadly speaking, trichomes are divided into two types: non-glandular
and glandular. Non-glandular trichomes are sharp, pointed, spiny
NON-GLANDULAR
TRICHOMES
Type of trichomes that
primarily deter insects
by mechanically
stopping them from
attacking the plant.
appendages that hinder the movement of herbivorous insects by
acting as a physical barrier (Figures 2A,D) [4, 5]. In many cases
they look like stars. These sharp needles are also fortified with hard
substances like silica and calcium carbonate, which can blunt the
teeth of caterpillars, making it dicult for them to chew. In addition
to restricting the ability of caterpillars to feed, trichomes are also
destructive once they enter the caterpillar’s gut. They poke holes in
the gut wall, which causes the food to mix with the blood. This can
lead to dangerous infections and activate the caterpillar’s immune
system [6].
Glandular trichomes, in contrast, have swollen globular heads (like
GLANDULAR
TRICHOMES
Type of trichomes that
contain toxins or
substances that are
harmful to
insect predators.
water tanks) that contain toxic or sticky compounds that can
either trap herbivores or kill them when ingested (Figures 2A–C). In
some cases, these globular heads can also produce foul-smelling
compounds that repel herbivores. Thus, we can say that these mini
water tanks splash toxins as soon as they get a danger signal from any
kind of insect attack. In summary, trichomes—the harmless-looking
hairs on plant leaves—can protect plants in multiple ways before,
during, and after attack by caterpillars or other insects.
HOW DO INSECTS DEAL WITH THIS “HAIRY” PROBLEM?
Does this mean that plants win the battle against insects very easily?
Not really. Insects have co-evolved in multiple unique ways to battle
plant defenses. Some insects can shave trichomes o plant parts, like
lawn mowing. If you carefully watch a tobacco hornworm caterpillar
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Vasquez et al. Plant Hairs Protect Against Insects
on a tomato plant, you will see that it navigates along the leaf surface
until it finds a good spot with fewer trichomes, or it travels along leaf
edges to avoid them. Some insects, like the tiger clearwing butterfly
caterpillar, weave silk fibers over the trichomes to create a smooth
surface that helps them to walk across a rough patch on the leaf. The
digestive systems of some caterpillars allow them to eat trichomes
without any toxic eects, so they can still feed on those plants [7].
Still other insects form a thick layer of secretions over trichomes,
preventing direct contact of insects to trichomes [8].
WHY DO WE STUDY THESE “HAIRS”
In summary, trichomes have diverse functions, and their unique
structures have the potential to protect plants in several dierent
ways against the herbivores that want to feed on them. However,
some insects have co-evolved unique ways to get around these plant
defenses. This is fascinating because it clearly depicts how plants and
insects outcompete each other in an evolutionary race for survival,
alongside the ways we can manipulate trichomes to protect plants
making it an interesting area of research. Moving forward, it would be
interesting for scientists to study the role of each type of trichomes
in more depth, to understand why there are such wide variations in
the density and type of trichomes across plant families. Maybe this
variation helps plants defend themselves against various insect groups.
For example, what if a stink bug is feeding on a plant, instead of
a caterpillar, would dierent types of trichomes help more against
one insect or the other? As pest species become more prevalent
as a result of global climate change, we need to better understand
how trichomes work together to protect plants, as it will help us to
devise integrated pest management strategies to control harmful pests
feeding on agricultural crops.
FUNDING
This work was supported by the startup funds, College of Sciences
seed grant, and University of Texas Regents Rising Star Award to RK
through The University of Texas Rio Grande Valley.
REFERENCES
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to enhance biological control in agriculture. Neotrop.
Entomol. doi: 10.1007/s13744-013-0147-z
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3. Kaur, J., and Kariyat, R. 2020. “Role of trichomes in plant stress biology,” in
Evolutionary Ecology of Plant-Herbivore Interaction (Cham: Springer
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and Mescher, M. C. 2018. Leaf trichomes aect caterpillar feeding in an
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5. Kariyat, R. R., Raya, C. E., Chavana, J., Cantu, J., Guzman, G., and Sasidharan, L.
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growth and development in tobacco hornworm (Manduca sexta) caterpillars.
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6. Kariyat, R. R., Smith, J. D., Stephenson, A. G., De Moraes, C. M., and Mescher, M.
C. 2017. Non-glandular trichomes of Solanum carolinense deter feeding by
Manduca sexta caterpillars and cause damage to the gut peritrophic matrix.
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7. Kaur, I., Watts, S., Raya, C., Raya, J., and Kariyat, R. 2022. “Surface warfare: plant
structural defenses challenge caterpillar feeding,” in Caterpillars in the Middle
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Annu. Rev. Entomol. 60:393–414. doi: 10.1146/annurev-ento-010814-020932
SUBMITTED: 10 July 2021; ACCEPTED: 20 September 2022;
PUBLISHED ONLINE: 13 October 2022.
EDITOR: Vishal Shah, Community College of Philadelphia, United States
SCIENCE MENTORS: Julia Rittenschober and Anamika Dubey
CITATION: Vasquez AR, Kaur I and Kariyat R (2022) Why Are Some Plants Hairy?
Front. Young Minds 10:739393. doi: 10.3389/frym.2022.739393
CONFLICT OF INTEREST: The authors declare that the research was conducted in
the absence of any commercial or financial relationships that could be construed
as a potential conflict of interest.
COPYRIGHT © 2022 Vasquez, Kaur and Kariyat. This is an open-access article
distributed under the terms of the Creative Commons Attribution License (CC BY).
The use, distribution or reproduction in other forums is permitted, provided the
original author(s) and the copyright owner(s) are credited and that the original
publication in this journal is cited, in accordance with accepted academic practice.
No use, distribution or reproduction is permitted which does not comply with these
terms.
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Vasquez et al. Plant Hairs Protect Against Insects
YOUNG REVIEWERS
DARIO, AGE: 15
My name is Dario. I live in a small village in Austria. It is full of nature so in my freetime
I like to go out with my dogs or climb trees. My parents are both biologist so I got
into biology pretty early.
SRIJA, AGE: 13
Hi! My name is Srija and I am fun loving girl and I love painting and
playing chess!
VALERIE, AGE: 14
I am in 9th grade of a middle school in Austria. My hobbies are horseback riding,
skating and dancing. I have got a very old cat and we are getting a dog soon. I also
like meeting my friends and listening to some music.
AUTHORS
ALEJANDRO R. VASQUEZ
Hi! I am Alejandro, I am a Masters (Entomology focus) student who loves plants and
insects, as well as the environment! I have always been passionate about science
and discovery and understanding the quirky and unique things about the living things
around us, by looking for the unexpected in every aspect of living organisms!
ISHVEEN KAUR
Hi! I am Ishveen. I am a Ph.D. Student in Biological Sciences, and I am passionate
about plants and insects. I am curious to learn about plant based bioactive
compounds, and their roles in defending against environmental vagaries and how
these compounds aect health of living beings, thereby linking plant science to
animal and human health.
RUPESH KARIYAT
I am an Associate Professor of Crop Entomology at University of Arkansas.
As a scientist, I am interested in understanding how insect herbivores defend
against plants and their how they counter plant defenses to successfully feed and
develop. As a long-term goal, my lab is focused on using the knowledge from
studying insect plant interactions to devise sustainable pest management strategies.
*rkariyat@uark.edu
kids.frontiersin.org October 2022 | Volume 10 |Article 739393 |7
