Plants usually produce a certain quantity of a chemical defense, a sort of background amount. This is called a constitutive defense. After a plant is attacked, however, the amount of these chemicals usually increases. In other cases, entirely new compounds are produced after an attack. Such a reaction, known as an induced defense to herbivore attack, can be thought of as a parallel to the immune system in animals.
According to Karban and Myers (1989), any change in a plant following herbivory can be thought of as an induced response. Such changes include not only allelochemical induction (Baldwin 1994), but also increases in physical defenses such as thorn length (Young 1987), spine density, production of trichomes, emission of volatiles that attract predators and parasites, reduction in plant nutritional quality for herbivores, and even increases in extrafloral nectar in plants protected by ants (Agrawal 2000). Induced defenses are not limited to plants. Phytoplankton, rotifers, ciliated protozoans, cladocerans, and even carp (Cyprinus carpio) are known to respond morphologically or behaviorally to the presence of herbivores or predators (Tollrian and Harvell 1999). Even marine bryozoans have been shown to have inducible physical defenses. Bryozoans produced spines, which were effective in reducing mortality, after being attacked by nudibranchs (Harvell 1984).
To qualify as an induced defense (or induced resistance), the response must result in a decrease in herbivore or predatory damage, and an increase in fitness must be demonstrated as compared to non-induced controls (Karban and Baldwin 1997). Harvell and Tollrian (1999) identified the following as conditions necessary for the evolution of an inducible defense: (i) the selective pressures should be variable and unpredictable, but sometimes strong; if the inducing species is constant, then permanent, constitutive defenses should be present; (ii) a reliable cue is necessary to activate the defense; (iii) the defense must be effective; (iv) the inducible defense must save energy as compared to a constitutive defense or no defense at all.
The basic hypothesis is that while defenses increase plant fitness when herbivores are present, the energy invested in such defenses results in lowered plant fitness when herbivores are absent (Agrawal et al. 1999). Inducible defenses allow an organism to invest in defense when necessary, but avoid costly allocations to defense when the herbivore or predator is absent.
In the 1980s Schultz and Baldwin (1982) and Rossiter et al. (1986) showed that defoliation by gypsy moth (Lymantria dispar) larvae stimulated oaks to increase the phenolic content of leaves. Rossiter et al. found a significant negative correlation between phenolic content of leaves and the size of female gypsy moth pupae. Assuming reproductive output of a female gypsy moth is positively correlated with size, the induction of a high phenolic content in oak leaves would be expected to eventually cause a decline in gypsy moth populations. However, a complication to this seemingly straightforward picture is the fact that another control agent, the gypsy moth nuclear polyhedrosis virus, is inhibited by high levels of phenolic compounds in oak leaves. Even though leaf phenolics depress gypsy moth reproduction, survivorship is enhanced due to suppression of the virus. The two potential control agents work in opposition to each other, making it increasingly difficult to predict the dynamics of gypsy moth populations (Foster et al. 1992). Furthermore, throughout the 1990s gypsy moth populations declined to very low levels, apparently due to a fungus. The moth populations in the eastern United States increased during the period 2000-02, but did not reached the population levels common in the early 1990s.
Agrawal (2000) exposed the leaves of the herbaceous plant Lepidium virginicum (Brassicaceae or mustard family) to herbivory by the larvae of Pieris rapae (Pieridae). Induced plants responded by producing more trichomes per leaf and increasing the glucosinolates (mustard oil) content of the leaves. While induction did not affect the feeding behavior of the specialist P. rapae, feeding by generalist caterpillars in the family Noctuidae was reduced.
In a related field study, Agrawal (2000) again induced a response by allowing leaves to be consumed by larvae of P. rapae. However, in addition to undamaged controls, he damaged leaves by clipping with scissors. In the field, aphids were important herbivores. Controlled herbivory by P. rapae induced resistance to attack by the aphids, but clipping with scissors did not. The number of aphids feeding on control and clipped plants averaged five per plant, while the average number on induced plants was just three per plant. Furthermore, plant survivorship was lowest in the clipped plants. Previous studies have suggested that the saliva of herbivores is necessary for successful defensive induction (Bodnaryk 1992, Mattiacci et al. 1995). Plants therefore may respond differently to leaf losses from herbivores as opposed to leaf damage from storms or other physical causes.
Other research has focused on the ability of plants to produce proteinase inhibitors that inhibit the major digestive enzymes of insects. For example, when attacked by herbivores, sagebrush (Artemisia tridentate) produces a compound known as jasmonic acid. Under the influence of jasmonic acid, tobacco (Nicotiana sylvestris), tomato (Lycopersicon escu-lentum) (Farmer and Ryan 1990, 1992), and alfalfa (Medicago sativa) plants all were induced to produce proteinase inhibitors. More recently it was found that injury to a plant tissue causes the production of a peptide hormone. The hormone stimulates the release of linolenic acid, a fatty acid common to plant cell membranes. Linolenic acid is then converted to jasmonic acid, which in turn stimulates proteinase inhibitors (Chen 1990). In another study, jasmonic acid stimulated the production of nicotine in tobacco plants (Ohnmeiss and Baldwin 2000).
Some induced responses to wounds are considered "systemic." This means that damaged plant tissue may transmit a signal to other areas of the plant, resulting in the induced reaction. Karban and Baldwin (1997) have outlined the requirements for a hypothetical signal to be taken seriously.
1 The signal must be rapidly generated at the wound site;
2 the inducer must be known;
3 the signal must travel through the plant in a time course consistent with the induced response;
4 the signal must stimulate the induced response at concentrations consistent with those known from damaged plants.
Signals that have been proposed include: oligosaccharide fragments from cell walls, systemin (a polypeptide), salicylic acid, ethylene, abscisic acid, jasmonic acid/methyl jasmonate, and electrical signals (Karban and Baldwin 1997). Of these, jasmonic acid and methyl jasmonate are the most likely signal compounds in that they meet the requirements listed above (Karban and Baldwin 1997). Jasmonic acid is derived from common fatty acids and it, along with its methyl ester relative (methyl jasmonate), are commonly found in plants. Both compounds elicit a multitude of responses in plants. Mechanical wounding increases the levels of jasmonic acid, which then move rapidly through the phloem.
Moreover, methyl jasmonate is very volatile. Minute concentrations of gaseous methyl jasmonate can induce the synthesis of proteinase inhibitors, as described above. Other chemicals involved in plant defense such as ethylene, systemin, and several alkaloids increase after plants are exposed to methyl jasmonate. The possibility that the gas methyl jasmonate is a signaling compound provides a potential mechanism for communication among plants (Karban and Baldwin 1997).
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