Unlike animals, plants are sessile organisms, which are not able to move by themselves. They do not have wings to fly nor legs to walk. However, plants are anywhere, and they are transported from an area to another area with the help of a disperser, or a pollinator. The dispersers/pollinators can be rain or wind, or it can be organisms like insects, animals, and humans. The wild nature is about being prey and predator. Prey finds a way to escape from a predator like an insect hides in the shaded area from the bird. How do the plants voice out to protect themselves under such circumstances? Fortunately, plants have evolved various defenses against herbivores, which can be categorized into physical or mechanical, chemical, and co-evolved defense mechanisms as known as a plant protection mechanism.
Once a viable seed is set, it germinated into a plant and standstill regardless of the change in the environment full of abiotic and biotic stresses. One example of an abiotic factor is the wind that can uproot a tree or snap the main shoot of a small plant. However, due to the high regenerative property, the axillary buds of the tree give the plant a second chance as they grow out to replace the lost shoot. On the other hand, the biotic factor is more detrimental to plants than abiotic factors. Start from the infestation by pests and then followed by the infection of diseases like bacteria, fungus which tap into nutrient resources of plants, the virus can even employ the DNA-replicating mechanisms of plants to self-replicate. Some may even kill the plant cells immediately leading to necrosis. The attacking threat can be enhanced from other plants such as non-native invasive species, who have no natural predators in the new environment.
One of cell structure that animal lack of is the cell wall which allow plant to build their first-line defense. As the outermost surface of the plant leaves, the dermal tissue system contains a lipid material (wax or cutin, cuticle layer) that not only preventing the plant from dehydration but also attack from biotic factors. Some plants may have other specialized epidermal cells like suberin, trichomes, bark, thorns, as well as idioblasts like pigmented cells, sclereids, crystalliferous cells, and silica cells. These mechanical defenses reduce the attack from the herbivores, but these exterior defenses can be penetrated then lead to mechanical wounds and increase the possibilities of microbial entry. Bacteria can cause damage because they provide the sites for ice nucleation. Moreover, parasitic nematodes use their sharp mount parts to get through the plant cell walls, forming tumors on roots (root nematodes). Pests like aphids obtain the nutrients from plants through the sap by inserting a tubular structure into the deeper layer of the plant tissues, thus leaving an entry for infection. Fungi seek out the weak spot in the dermal system or the “free entry,” known as stomata, to enter the plant. The phases of fungal invasion are normally transmitted through the air, referred to as the air-borne disease. The windblown spore lands on the leaves, which then germinates and forms an adhesion pad. The hyphae grow through the cell walls and press against the cell membrane that differentiates into haustoria. The fungal invasion is pretty similar to the invasion of the parasitic plant onto a host plant.
Thanks to the built in powerful biochemistry factory of plants, the second line of defense is the chemical defense. Many plants produce toxins that kill herbivores or make them sick. They also have essential oils that produce strong flavors or odors which repel the insects. Primary metabolites are the building blocks or precursors for many growth substances like starch to provide energy for development. To sustain their life, they have a metabolic pathway that employs primary metabolites and leads to secondary metabolites. Secondary metabolites not only affect herbivores but humans as well. Protective secondary metabolites include alkaloids such as caffeine and nicotine, tannins, and oils. Each compound is found to have different impacts on humans. For instance, manihotoxin from cassava (Manibot esculenta) will release lethal cyanide if consumed. Taxol or simply known as terpenoid obtained from Pacific yew (Taxus brevifolia), is used as an anticancer drug. Besides, morphine, one of the alkaloids extracted from the opium poppy (Papavier somniferum), is used as a narcotic pain killer in the medicinal field. Moreover, wild species of tobacco have elevated levels of nicotine that are lethal to tobacco hornworms (Manduca sexta).
Plants produce toxic substances within their system, but how they protect themselves from the toxins? The toxin is sequestered in a membrane-bound structure and such a compound is not toxic until the attack of the animals metabolizes it. For example, cyanogenic glycosides break down only into cyanide when animals ingest it. The toxicity level of the compounds produced by the plants is species dependent. The castor bean plant, which is scientifically known as Ricinus communis, produces an alkaloid known as ricin. It is six times more lethal than cyanide and twice as lethal as the cobra venom! It functions as a ribosome-binding protein that inhibits the translation in our body. Furthermore, it is found that some murderous plants have specialized glands to protect themselves like glands that produced sticky fluids that kill the little insects. Some plants possess sap containing resin or latex that produce an awful taste for consumption. It is also used to trap and kill any herbivores like insects from further attacking the plants. However, resin and latex are known as self-healing mechanisms. They solidify easily when exposed to the air, thus preventing any contact between the wound and the air from reducing air-borne diseases.
Plants protect themselves from the attacks by animals, but some animals are their lifesavers! A complex coevolution of plants and animals has resulted in mutualistic associations that benefit both parties—for example, acacia trees and the “Antman” comprised of small armies of ants. The ants will make the nest on the bull horn acacia (Acacia cornigera) by using the large thorns as the supportive structure. This ant swarm defends any animals that eat the tree. Not only that, but the ants also clear an area around the base of the tree to reduce the competition of other plants for nutrients. In return, bull horn acacia provides nectar to the ants as their main food source. Similarly, Daemonorops verticullaris and Korthalsia scaphigera used the same way as acacia trees that provide ants with “housing estate”. Moreover, there is a complex relationship between the plant, caterpillar, and parasitic wasp. As the caterpillar chews away the leaves, the plat releases volatile compounds through the wound to attract female parasitic wasp. She is then lay fertilized eggs in the caterpillar, and it helps to grow her babies. Once the eggs are hatched, the larvae kill the caterpillar.
Sessile by itself is a disadvantaged condition, which means nowhere to hide. Thus it’s important for plants to stay energetic downside when the static response to invaders occurs. Because the secondary metabolites are maintained in the presence or absence of a threat. The energy resources and primary metabolites would be conserved if the plant responses were inducible and the defenses launched only when it is needed. Traditionally, the plants produce proteinase inhibitors throughout the whole plant when there is a wound response. It is used to bind to the digestive enzymes in the gut of the herbivores. Other molecules like salicylic acid and jasmonic acid also play pivotal roles in the systemic response to the invaders. The specific defense response at the molecular level is the gene-for-gene hypothesis whereby there is a plant resistance gene from plant and a virulence gene from pathogens. The successful binding of the genes is known as a protective hypersensitive response. If there is no binding occurs, the plants succumb to the diseases. Both jasmonic and salicylic acids are inducer that triggers systemic acquired resistance as a wider range of alarm, which allows the plant to respond more quickly to a second attack. Still, it is neither specific nor long-lasting as compared to mammalian responses.
At a physiological aspect, plants get stimulated when it is attacked. If there is any diseases infection, the endogenous plant growth regulator like abscisic acid is produced in the response of environmental stresses. It is used to stimulate the abscission region of a particular leaf or branch which allows the plant to shed off the infected part and avoid the spread of the infection. The mechanism is to inhibit the division of cells in the vascular cambium by suspending primary and secondary growth. Plants must be taken in horticultural practices so that they experience less stress thus having lesser concentrations of the toxic compounds in their system. Moreover, it is essential to provide sufficient nutrients for the plants to keep them healthy as they are feasible to diseases when they are not healthy. Infected plants are suggested to be sanitized by burning to prevent and reduce the spread of the diseases in a whole plantation. Soil sanitization also important to reduce the loss to the minimum. With biotechnology, many crops are now genetic modified to resist the diseases that saved the troubles of the farmers.
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