Plant – Insect Relation: Plant Defense Mechanisms against Insect Pests

How the plants react to insects’ threats

Plants and phytophagous insects have lived together on Earth for more than 400 million years (1). Both organisms have evolved strategies to avoid each other’s defense systems in order to survive. 

In every plant organism, the amino-acid phenylalanine helps plant growth and defense. This means the plant uses it for both processes in a competitive manner (2).

In many cases, the domestication of crop species led to the lack or decrease of the plants’ traits related to defense against “enemies” (3). This may have happened because the breeders were focusing more on other commercially important traits like increased productivity. At the same time, using insecticides counterbalanced this loss, helping control crop pests. The susceptibility of modern crop plants, combined with the increased severity of the damage caused by insects and the need to decrease the dependence on chemical control, has led breeders’ attention back to the crops’ wild relatives and their traits related to plants’ defensive mechanisms against pests.

Plant insect defense overview – How is the plant defends itself against insects?

Plants follow two main strategies for defending against their insect enemies:

• Resistance occurs when plant structural or chemical traits prevent insects from feeding on the plant and decrease the digestibility-palatability of plant tissues (due to the production of toxic compounds like alkaloids, terpenoids, or glucosinolates). This minimizes the amount of herbivore damage to the plant. 
• Tolerance occurs when plant traits allow insects to feed on them. Still, the plant controls-restricts the level of herbivorous damage (per unit of insect present) and has strong recovery mechanisms.

Both strategies include two levels of defense:

•  Constitutive or Pre-formed defense includes all plant traits, and physical and chemical barriers, which protect the plant constantly. Such examples include hard plant tissue (pectin, cellulose, and lignin), waxy cuticles, trichomes, etc. Most of these protective mechanisms are effective for a wide variety of pest enemies and are included in the broad spectrum of defense. 

• Induced defense includes all plant traits, activated only when the plant recognizes the threat of insect pests through a surveillance system. This type of defense can save the plant’s resources until they are critically needed. Some examples of induced defense in plants include cell wall reinforcement (callose+lignin deposition), production of novel toxic metabolites, and local cell death – Hypersensitive response/resistance (HR).

Finally, one classification we can make for the types of plant defense is the direct and indirect defense.

• In direct defense, plants use their own tools to defend against pests.
• Indirect defense is the defense in which the plants have the ability to attract natural enemies of herbivorous insects. The natural enemies then reduce the population of pests, so the plants are not in danger anymore. 

Resistance mechanisms of plants against insect pests

1. Structural traits: Plants possess features such as trichomes and waxy cuticles that pose problems in pest attachment, feeding, and oviposition (3). For example, the common pest Tetranychus urticae, was significantly reduced in raspberry genotypes with high leaf trichome densities. Moreover, plants develop some sclerophyll tissues in their organs, making it difficult for the pests to consume and digest them. (due to the high concentration of lignin) 

In this way, pests are deterred from consuming the plant.

Image 1: Different density of  trichomes between cultivars in the species Rubus idaeus. The densier the trichome, the more difficult for spider mites to settle in.

2. Chemical traits: Plants’ chemical defense is a very interesting and rich field for research. Plants have compounds that vary upon species, and they can be toxic or cause problems related to the pests’ behavior or fecundity. The most popular groups of such molecules are alkaloids, glucosinolates, and terpenoids. One well-known molecule is gossypol which exists in cotton and protects the plant.

3. Expression changes: Plants harmed by pests produce Systemic Wound Response Proteins (2), such as lectines and chitinases. The role of these proteins is to inhibit particular enzymes of herbivorous organisms to make the digestion process painful. 

Furthermore, as mentioned above, plants have the ability to emit molecules that can attract natural enemies and, at the same time, act as signals for neighboring plants in order to activate their induced defense.

Image 3: Example of the indirect defense. The plant attracts a natural enemy of the caterpillar (wasp) by releasing particular chemicals in the air. These chemicals inform the wasp for the presence of the caterpillar and it follows the source of their emission.  (Pest and Disease Management in Agriculture)

Tolerance mechanisms of plants against insect pests:

1) Stimulation of Photosynthesis and Growth: Some pests increase the photosynthetic rate and alter the nitrogen physiology inside the plants. Thanks to these mechanisms, plants can counter phytophagy’s negative impact by producing new leaves and stems. As a result, they continue to be productive

2) Phenology cycle: Some plants have evolved to postpone their growth, flower, or fruit production until the threat of attack has passed (3). For example, maize plants seem to put off sending resources to their roots to avoid the pick of the population of the pest Diabrotica virgifera.

Conclusion

Plants can defend themselves against insects. These mechanisms are important for agricultural production. Plant breeding must focus on identifying suitable plant traits for crop protection against specific pests and how environmental conditions affect their expression (3). More research is needed on how certain insects achieve to overcome plants’ defensive mechanisms. 

References

• War, A. R., Paulraj, M. G., Ahmad, T., Buhroo, A. A., Hussain, B., Ignacimuthu, S., & Sharma, H. S. (2012). Mechanisms of plant defense against insect herbivores. PMC, 7(10), 1306–1320.
• Plant Defenses. (n.d.). https://learn.genetics.utah.edu/content/herbivores/defenses
• Mitchell, C. M., Brennan, R. M., Graham, J., & Karley, A. J. (2016). Plant Defense against Herbivorous Pests: Exploiting Resistance and Tolerance Traits for Sustainable Crop Protection. Frontiers in Plant Science, 7.
• Karabourniotis, G., Liakopoulos, G., Nikolopoulos, D., & Agricultural University of Athens. (2012). Plant Stress Physiology: Plant Functions under Adverse Environmental Conditions (Vol. 1). Embryo.