Inducible plant defenses against herbivory

Plants and herbivores have co-evolved together for 350 million years. Plants have evolved many defense mechanisms against insect herbivory. Such defenses can be broadly classified into two categories: (1) permanent, constitutive defenses, and (2) temporary, inducible defenses. Both types are achieved through similar means but differ in that constitutive defenses are present before an herbivore attacks, while induced defenses are activated only when attacks occur. In addition to constitutive defenses, initiation of specific defense responses to herbivory is an important strategy for plant persistence and survival.

Inducible defenses allow plants to be phenotypically plastic. This may confer an advantage over constitutive defenses for multiple reasons. First, it may reduce the chance that attacking insects adapt to plant defenses. Simply, inducible defenses cause variations in the defense constituents of a plant, thereby making the plant a more unpredictable environment for insect herbivores. This variability has an important effect on the fitness and behaviour of herbivores. For example, the study of herbivory on radish (Raphanus sativus) by the cabbage looper caterpillar (Trichoplusia ni) demonstrated that the variation of defensive chemicals (glucosinolates) in R. sativus, due to induction, resulted in a significant decrease in the pupation rates of T. ni. In essence, defensive chemicals can be viewed as having a particular dosage-dependent effect on herbivores: it has little detrimental effect on herbivores when present at a low or moderate dose, but has dramatic effects at higher concentrations. Hence, a plant which produces variable levels of defensive chemicals is better defended than one that always produces the mean level of toxin.

Second, synthesizing a continually high level of defensive chemicals renders a cost to the plant. This is particularly the case where the presence of herbivorous insects is not always predictable. For example, the production of nicotine in cultivated tobacco (Nicotiana tabacum) has a function in plant defence. N. tabacum plants with a higher constitutive level of nicotine are less susceptible to insect herbivory. However, N. tabacum plants that produce a continually high level of nicotine flower significantly later than plants with lower levels of nicotine. This results suggest that there is a biosynthetic cost to constantly producing a high level of defensive chemicals. Inducible defences are advantageous as they reduce the metabolic load on the plant in conditions where such biological chemicals are not yet necessary. This is particularly the case for defensive chemicals containing nitrogen (e.g. alkaloids) as if the plant is not being attacked it is able to divert more nitrogen to producing rubisco and will therefore be able to grow faster and produce more seeds.

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