Malcolm Rivera
Plants have evolved various defense mechanisms to protect themselves from insect herbivory, which can be broadly categorized into constitutive (permanent) and induced (temporary) defenses. Constitutive defenses are always present in the plant, constantly activated, and do not rely on herbivore attacks. On the other hand, induced defenses are activated only when an attack occurs, and they allow plants to be phenotypically plastic, making it harder for insects to adapt to their defenses. These defense mechanisms have trade-offs and can impact plant growth, reproduction, and survival. The balance between constitutive and induced defenses in plants is an intriguing area of study, providing insights into the diverse strategies plants employ to defend themselves against herbivores.
| Characteristics | Values |
|---|---|
| Classification | Constitutive defenses are permanent and always present in the plant. Induced defenses are temporary and activated in the presence of an attacker. |
| Activation | Constitutive defenses are always activated. Induced defenses are activated in response to an environmental stimulus or attack. |
| Cost | Constitutive defenses are constantly activated, which entails high costs for the plant. Induced defenses are cost-saving strategies, where resources are diverted from defense to growth and reproduction. |
| Trade-offs | There is a trade-off between constitutive and induced defenses, with a negative correlation between the two. |
| Adaptability | Inducible defenses allow plants to be phenotypically plastic, making it harder for insects to adapt to plant defenses. |
| Examples | Constitutive defenses include tannin and flavonoid levels in leaves. Induced defenses can include increased activity in cell wall-bound peroxidase, longer thorns, and indirect defenses like rewards for symbiotic ants. |
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What You'll Learn
- Constitutive defenses are always present in the plant, whereas induced defenses are activated only in the presence of an attacker
- Inducible defenses allow plants to be phenotypically plastic, which may reduce the chance that insects adapt to plant defenses
- Inducible defenses are thought to have evolved as a cost-saving strategy, allowing plants to divert resources from defense to growth and reproduction
- The cost of induced defenses to a plant can be quantified as the resource-based trade-off between resistance and fitness
- The severity of the adverse effects of induced defenses on plant growth and photosynthesis depends on the feeding guild of the phytophagous species
Constitutive defenses are always present in the plant, whereas induced defenses are activated only in the presence of an attacker
Plants have evolved a range of defence mechanisms to protect themselves from insect herbivory. These defence mechanisms can be broadly classified into two categories: constitutive (permanent) and induced (temporary).
Constitutive defences are always present in the plant and do not depend on an attack from an herbivore. These defences are constantly activated, but not always needed, which can be costly for the plant. For example, the Chinese tallow tree (Triadica sebifera) is an invasive species in the southeastern United States. It has constitutive defence traits, such as tannin levels and flavonoid levels in its leaves, which act as effective direct defences against herbivores. However, maintaining these defences constantly can be resource-intensive for the plant, impacting its growth and development.
On the other hand, induced defences are activated only in the presence of an attacker. These defences are initiated after herbivore feeding and can include both chemical and physical responses. For instance, jasmonic acid can be used to simulate an herbivore attack on plants, inducing plant defences. The production of defensive chemicals, such as glucosinolates, can vary in dosage, having a more detrimental effect on herbivores at higher concentrations. Inducible defences allow plants to be phenotypically plastic, making it harder for insects to adapt to their defences.
The balance between constitutive and induced defences is influenced by the probability of attack and the value of the plant organ. Plants that are regularly attacked by herbivores tend to have higher levels of constitutive defences, while those with low herbivore loads invest more in inducible defences.
The interaction between constitutive and induced defences is complex, with trade-offs and negative correlations observed between the two. The expression of both defence mechanisms can be costly for the plant, impacting its growth, reproduction, and symbiotic relationships with the environment.
In summary, constitutive defences are always present in plants, providing constant but costly protection. Induced defences, on the other hand, are activated only when needed, offering a cost-saving strategy that makes plants more unpredictable to herbivores.
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Inducible defenses allow plants to be phenotypically plastic, which may reduce the chance that insects adapt to plant defenses
Plants have evolved defence mechanisms against insect herbivory over the course of 350 million years of co-evolution. These defences can be broadly classified into two categories: constitutive (permanent) and induced (temporary). Constitutive defences are always present in the plant, constantly activated, and do not depend on an attack by herbivores. These defences are always on but come at a high cost to the plant in terms of resources.
Inducible defences, on the other hand, are activated only when an attack occurs. This allows plants to be phenotypically plastic, which may reduce the chance that insects adapt to their defences. Phenotypic plasticity refers to the ability of a plant to alter its phenotype in response to changes in its environment. In the context of inducible defences, this means that plants can vary the constituents of their defence mechanisms, making it harder for insects to adapt.
For example, the production of variable levels of defensive chemicals in plants can be more effective at deterring herbivores than a constant, average level of toxins. This is because defensive chemicals often have a dosage-dependent effect on herbivores, with higher concentrations having a more detrimental impact. Additionally, maintaining a consistently high level of defensive chemicals incurs a cost to the plant, particularly when the presence of herbivores is unpredictable.
Inducible defences also allow plants to avoid the costs associated with constitutive defences. Constitutive defences are always activated, even when there is no threat, which can hinder plant growth and development. By only activating defences when needed, inducible defences allow plants to divert resources that would otherwise be spent on defences towards growth and reproduction.
The balance between constitutive and inducible defences may depend on the probability of attack and the value of the plant organ. Plants or organs that are regularly attacked by herbivores are expected to have high levels of constitutive defences, while those with a lower probability of attack should invest more in inducible defences.
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Inducible defenses are thought to have evolved as a cost-saving strategy, allowing plants to divert resources from defense to growth and reproduction
Plants have evolved defence mechanisms to protect themselves from insect herbivory. These defence mechanisms can be classified as constitutive (permanent) or induced (temporary). Constitutive defences are always present in the plant and do not depend on the presence of herbivores. These defences are constantly activated, which can be costly for the plant.
Inducible defences, on the other hand, are activated only when an attack occurs. This allows plants to divert resources from defence to growth and reproduction. For example, in the absence of herbivores, a plant may allocate resources to producing larger fruits and increasing its reproductive capacity. If the same plant were constantly producing defensive chemicals, it would incur a cost in terms of reduced growth and development.
The evolution of inducible defences can be understood as a cost-saving strategy. By not constantly producing defensive chemicals, the plant saves resources. This is particularly advantageous when the presence of herbivores is unpredictable. Additionally, inducible defences make the plant a more unpredictable environment for insects, as the defence constituents vary. This reduces the chance that insects will adapt to the plant's defences.
The balance between constitutive and inducible defences depends on the probability of attack and the value of the plant organ. Plants or organs that are regularly attacked by herbivores are expected to have high levels of constitutive defences and low levels of induced defences. In populations where herbivory is low, plants are expected to invest more in the inducibility of defence.
Several studies have demonstrated a trade-off between constitutive and induced resistance. This trade-off is thought to contribute to the immense variability among plant species in defence strategies and species diversity.
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The cost of induced defenses to a plant can be quantified as the resource-based trade-off between resistance and fitness
Plants have evolved defence mechanisms against insect herbivory over the course of 350 million years of co-evolution. These defences can be categorised as constitutive (permanent) or induced (temporary). Constitutive defences are always present in the plant and do not rely on the presence of herbivores. These defences are constantly activated, which can be costly for the plant. On the other hand, induced defences are only activated in response to an environmental stimulus, such as herbivory.
The cost of induced defences to a plant can be understood as a resource-based trade-off between resistance and fitness. This trade-off, known as the allocation cost, occurs when a plant channels a large amount of limited resources towards forming resistance traits. These resources are then unavailable for other important processes such as growth and reproduction. For example, in the broadleaf dock plant, an induced defence response to the green dock beetle results in reduced leaf growth and expansion due to the allocation of resources to increased cell wall-bound peroxidase activity.
The allocation cost of induced defences can also be observed in the synthesis of defensive chemicals. Plants that produce variable levels of defensive chemicals are better defended than those that produce a constant mean level of toxins. However, maintaining a consistently high level of defensive chemicals can be costly, especially when the presence of herbivores is unpredictable. This cost is further exacerbated when considering the possibility of insects adapting to plant defences. Inducible defences address this issue by causing variations in defence constituents, making the plant a more unpredictable environment for herbivores.
In addition to the allocation cost, the ecological cost of induced defences should also be considered. The ecological cost refers to the reduced fitness resulting from the disruption of symbiotic relationships between the plant and its environment. For example, the use of jasmonic acid to induce plant defences in tomatoes resulted in plants with larger fruits, longer ripening times, and fewer seeds. These changes negatively impacted the plant's ability to attract seed dispersers, ultimately leading to reduced fitness.
Understanding the trade-offs between resistance and fitness in induced defences is crucial for developing breeding strategies that optimise the balance between growth and defence. By manipulating defence signalling pathways, such as the jasmonic acid pathway, it may be possible to maximise crop yield and meet the increasing global demand for food and biofuel.
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The severity of the adverse effects of induced defenses on plant growth and photosynthesis depends on the feeding guild of the phytophagous species
Plants have evolved defence mechanisms to protect themselves from insect herbivory, which can be broadly classified into two categories: constitutive (permanent) and induced (temporary). Constitutive defences are always present in the plant and do not depend on the attack of herbivores. On the other hand, induced defences are activated only in the presence of an attacker. Inducible defences allow plants to be phenotypically plastic, which may confer an advantage over constitutive defences as it makes the plant a more unpredictable environment for insect herbivores.
Induced defences can be in the form of chemical defences, such as varying levels of defensive chemicals, or physical defences, such as longer thorns. However, activating these defences comes at a cost to the plant, as resources are reallocated from growth and reproduction to defence. The severity of the adverse effects of induced defences on plant growth and photosynthesis depends on the feeding guild of the phytophagous species. For example, the study of herbivory on radish by the cabbage looper caterpillar demonstrated that the variation of defensive chemicals (glucosinolates) in radish, due to induction, resulted in a significant decrease in the pupation rates of the caterpillar.
The cost of induced defences can be quantified as the resource-based trade-off between resistance and fitness (allocation cost) or as the reduced fitness resulting from the interactions with other species or the environment (ecological cost). Allocation cost refers to the large quantity of fitness-limited resources that are channelled to form resistance traits in plants, which may not be quickly recycled and are therefore unavailable for other processes such as growth and reproduction. Ecological cost results from the disruption of the many symbiotic relationships that a plant has with the environment. For example, the use of jasmonic acid on tomato plants to simulate an herbivore attack resulted in plants with fewer but larger fruits, longer ripening time, delayed fruit set, and fewer seeds.
The induction of plant defences by herbivorous arthropods can cause adverse effects on plant growth and photosynthesis, and the severity of these effects depends on the type of plant host. A meta-analysis showed that the cost of induced defences varied depending on whether the plant host was woody, herbaceous, or a crop species. When growth, photosynthesis, and reproduction parameters were measured, the effects of the infestation on plant fitness were always negative, except for a null or slightly positive effect detected for woody plants.
In summary, induced defences in plants come at a cost to the plant's growth and reproduction, and the severity of these adverse effects depends on the feeding guild of the phytophagous species. While induced defences can be effective in protecting plants from herbivory, they also have negative consequences for the plant's fitness and survival.
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Frequently asked questions
Constitutive defenses are always present in plants and do not rely on an attack from herbivores. Induced defenses, on the other hand, are activated only when an attack occurs.
Induced defenses include chemical and physical responses. For example, an increase in jasmonic acid in tomato plants leads to a reduction in the number of fruits, longer ripening time, and a delay in fruit set. Another example is the increased activity in cell wall-bound peroxidase in broadleaf dock plants after an attack by the green dock beetle.
Constitutive defenses are constantly activated, which can be costly for plants. Induced defenses, as a cost-saving strategy, allow plants to divert resources from defense to growth and reproduction when not under attack. However, induced defenses can also be costly when stimulated unnecessarily, such as when there is no attack from herbivores.
