Up to this point, we have learned about a variety of plant pathogens. These pathogens include bacteria, fungi, oomycetes, and nematods. Today’s lecture covered arthropods.
Arthropods are not like the “classical” plant pathogens. This means that arthropods do not cause diseases in plants. Now I know you may be wondering, “If arthropods do not cause plant diseases then why are they important to the study of plant pathology?” And the answer to that question is that they cause plant damage. Plant damages account for 10 to 30 percent of crop yield losses annually.
There are 3 types of Plant-Arthropod interactions. The first of these interactions is the basic conflict of herbivory. Herbivory is an ongoing conflict in which plants have to defend themselves constantly. In order to defend themselves, plants use chemical and physical defenses. These defenses include thorns, spines, other physical barriers and/or chemicals that deter hungry insects. Two unique compounds that plants make for defense are volicitin and inceptin. What makes these compounds special is the fact that they do not occur anywhere else in nature. Inceptin is derived specifically in plants, while volicitin is a compound that is produced from the conjugation of plant fatty acids and amino acids in the gut of insects. Secondary compounds are also responsible for plant defenses. These compounds attract pollinators, predators or parasitoids. Others are toxic, such as cyanide, or can be anti-nutritives which functions to slowly kill herbivores by slowly hindering digestion of nutrients and by inhibiting other biological processes that are required for an arthropod to develop and reproduce. There are also other secondary compounds that are ingested by humans. These include medicines, stimulants and narcotics.
The second type of Plant-Arthropod interaction is an alliance between plants and carnivorous/parasitoid arthropods. Since plants are constantly being attacked by insects, plants came up with an ingenuous way to protect themselves with the help of other arthropods. Plants release volatile chemical signals that attract the natural enemies of herbivorous arthropods. These chemical signals are released when a plant is injured and the most recognizable is the Green Leaf Volatile that is a combination of compounds that make up “cut grass smell” that we’re used to. When volatiles are released, carnivorous arthropods attack herbivorous insects while the parasitoid arthropods lay their eggs in the herbivorous insects that attack plants. These eggs eventually hatch, killing the herbivorous insect in the process. Other predatory arthropods act as resident body guards that protect plants from herbivores.
The final type of Plant-Arthropod interaction is an alliance between plant and pollinators. This type of interaction is mutually beneficial, the pollinators gets food while the plants gets their pollen carried to other plants and also have the chance to be pollenated. A very special pollinator is the honey bee. Bees are attracted to plants by using olfactory cues, visual cues, and also surface characteristics of the petals of flowers. Bees also have a shifted visual light spectrum compared to humans. Bees see UV, blue and green light. Plants know this and have evolved the pigments of their flowers in such a way that bees can see the UV light that they are attracted to.
There is also another subset of Plant-Arthropod interactions: the cheaters, the thieves, and the deceivers. Cheaters are simply freeloaders that “lie” in order to get additional assistance from carnivorous arthropods. Thieves break mutualistic relationships by taking nectar from plants without the intentions of getting pollen to take to other plants. Deceivers “cry wolf” in a sense. A plant can use a variety of methods to trick pollinators to pollenate them; other plants can produce trichomes that trap prey.