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Inducible plant defenses against herbivory
Plants and herbivores have co-evolved together for 350 million years. Plants have evolved many defence mechanisms against insect herbivory. Such defences can be broadly classified into two categories: (1) permanent, constitutive defences, and (2) temporary, inducible defences.〔Karban R, Baldwin IT. Induced responses to herbivory. Chicago: Chicago University Press; 1997.〕 Both types are achieved through similar means but differ in that constitutive defences are present before an herbivore attacks, while induced defences are activated only when attacks occur.〔Chen MS. Inducible direct plant defence against insect herbivores: a review. Insect Science. 2008;15:101-14.〕〔Dicke M, van Poecke RMP, de Boer JG. Inducible indirect defence of plants: from mechanisms to ecological functions. Basic and Applied Ecology. 2003;4:27-42.〕〔Gatehouse JA. Plant resistance towards insect herbivores: a dynamic interaction. New Phytologist. 2002;156:145-69.〕 In addition to constitutive defences, initiation of specific defence responses to herbivory is an important strategy for plant persistence and survival.〔
== Benefits of induced defences ==
Inducible defences allow plants to be phenotypically plastic. This may confer an advantage over constitutive defences for multiple reasons. First, it may reduce the chance that attacking insects adapt to plant defences.〔Karban R, Agrawal AA, Mangel M. The benefits of induced defences against herbivores. Ecology. 1997;78:1351-5.〕〔Shelton AL. Variation in chemical defences of plants may improve the effectiveness of defence. Evolutionary Ecological Research 2004;6:709-26.〕 Simply, inducible defences cause variations in the defence 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, synthetising 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.〔Agrawal AA. Induced responses to herbivory in wild radish: effects on several herbivores and plant fitness. Ecology. 1999;80:1713-23.〕 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.〔Steppuhn A, Gase K, Krock B, Halitschke R, Baldwin IT. Nicotine's Defensive Function in Nature. PLoS Biology. 2004 August 01, 2004;2(8):e217.〕 However, ''N. tabacum'' plants that produce a continually high level of nicotine flower significantly later than plants with lower levels of nicotine.〔Preisser EL, Gibson SE, Adler LS, Lewis EE. Underground herbivory and the costs of constitutive defense in tobacco. Acta Oecol-Int J Ecol. 2007;31:210-5.〕 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.
In addition to chemical defenses, herbivory can induced physical defenses, such as longer thorns,〔Milewski, A.V., T.P. Young and D. Madden. 1991. Thorns as induced defenses: experimental evidence. Oecologia 86:70-75.〕〔Young, T.P., M.L. Stanton and C. Christian. 2003. Effects of natural and simulated herbivory on spine lengths of Acacia drepanolobium in Kenya. Oikos 101:171-179.〕 or indirect defenses, such as rewards for symbiotic ants.〔Huntzinger, P.M., R. Karban, T.P. Young and T.M. Palmer. 2004. Relaxation of induced indirect defenses of acacias following exclusion of mammalian herbivores. Ecology 85:609-614.〕
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