By Justin Bauer
In the 350 million years that plants and insects have coexisted, both have evolved strategies to avoid each other’s defense systems. Plants have developed an elegant defense system that can recognize foreign molecules or damaged cells that activate the immune response.1 Plants have two main forms of defense: direct defense and indirect defense. In the direct defense the plants will target the herbivores directly through expression of characteristics that affect the biology of the herbivore. This could be through mechanical protection (thorns, hairs) or through the production of toxic chemicals (phenols, alkaloids). Indirect defenses on the other hand are characterized by the release of volatiles with the purpose of attracting natural enemies of the herbivore.2 We will examine direct defenses in more detail.
In Direct Defense the first barrier is physical. Leaf surface wax, cell wall thickness, and thorns discourage and prevent herbivores from feeding on the plants.3 The second barrier is composed of metabolites that act as toxins, whereas the third barrier is made up of digestibility reducers that will protect the plant from subsequent attacks.4 Oftentimes these different components will work together to enhance the defensive system. For example, in tomatoes alkaloids, phenolics and oxidative enzymes work together synergistically to affect different aspects of the insect. However, when ingested separately they only exhibit reduced effects.5
Any structural defenses, including morphological and anatomical traits, need to provide the plant with a fitness advantage. This can include prominent protrubances or simply be microscopically small changes in cell wall thickness brought upon by lignification and suberization.6 Toughened or hardened leaves (sclerophylly) are an active part of many plants’ defense system. They reduce the palatability and digestibility of the tissues and subsequently reduce herbivore damage.7 Spines, thorns, and prickles are grouped under spinescence and force the herbivore to ear around these structures. Thus, herbivores that lack small mouthparts will be unable to effectively consume these plants (H3). Ironically, while these mechanisms might work against insects they can produce the opposite effect in mammalian herbivores. A study determined that the woodrat Neotoma Albigua was able to clip cactus spines and preferred to eat spiny cacti over de-spined cacti that had lower protein and higher fiber content.8
Trichomes, small hair-like growths, produce both toxic and deterrent effects. Specifically, their density can have negative effects on ovipositional behavior, feeding and larval nutrition of certain pests.7 Similarly, to the structural defenses mentioned above they also have a mechanical effect on the herbivores. Dense trichomes prohibit insects and arthropods from moving easily on the plant surface and thus restrict access to the leaf epidermis.4 Certain glandular trichomes are also able to secrete secondary metabolites (e.g. flavonoids, terpenoids) which can be toxic to insects or repel them.9 Upon insect attack, trichomes can increase in density, but only in leaves that are developing. For example new leaves of the plant Salix Cinera exhibited higher Trichome density following the attack of adult leaf beetles.10 This trend is found in plenty of other plants, including black mustard and wild radish.
Secondary metabolites are important in mediating the direct defense. They have no effect on the growth and development of plants, but rather reduce the palatability of the plant tissues in which they are produced.11 Secondary metabolites are either stored in an inactive form or induced upon insect or microbe attack.
Plant phenols are the most common secondary metabolites. Their role is not just confined to warding off herbivores and microorganisms, but also to defend against competing plants. Typically, upon insect attack, qualitative and quantitative alterations in phenols and increased oxidative enzyme activity can be observed.12 Lignin is a phenol that increases leaf toughness to reduce both feeding by insects and the nutritional value of the lead.13 This compound’s synthesis is induced by herbivory or pathogen attack. Phenols can be oxidated by the enzymes polyphenol oxidase (PPO) and peroxidase (POD) to form Quinones. These quinones inhibit protein digestion in insects through covalent interactions with leaf proteins14 while also exhibiting toxicity to insects.
Tannins are also secondary metabolites. They are a type of bitter polyphenol and act as feeding deterrents to insects. They reduce nutrient absorption efficiency in insects and cause midgut lesions.15 Once ingested, they can precipitate digestive enzymes nonspecifically or chelate metal ions to reduce bioavailability to herbivores. Tannin production is induced by damage, upon which the condensed tannin regulatory gene PtMYB134 is activated.16 Tannin induction can also be stimulated by light stress and through UV light exposure in certain cottonwood trees.17 While they work against a wide range of insects, certain polyphagous insect species have developed a tolerance to tannins. For example, the desert locus Shistocerca Gregaria rapidly hydrolyzes them to avoid damage, restricts tannin passage through adsorption to the thick peritrophic membrane, and inhibits tannin protein complex formation through surfactants located in the midgut.18
Thus, plants have many different options to defend themselves against herbivores. Beneath the typical obvious physical defenses, such as thorns, lies a complicated network of chemicals that works together to deter attacks. However, there is no optimal defense, and with time both herbivores and plants will evolve new defenses and new tricks to bypass them.
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