Plants have evolved many defense mechanisms against insect herbivory in the 350 million years in which they have co-evolved. Such defenses can be broadly classified into two categories: (1) permanent, constitutive defenses, and (2) temporary, inducible defenses.Karban R, Baldwin IT. Induced responses to herbivory. Chicago: Chicago University Press; 1997. These differ in that constitutive defenses are present before an herbivore attacks, while induced defenses 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 defenses, initiation of specific defense responses to herbivory is an important strategy for plant persistence and survival.
Benefits of induced defences
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.Karban R, Agrawal AA, Mangel M. The benefits of induced defenses against herbivores. Ecology. 1997;78:1351-5.Shelton AL. Variation in chemical defenses of plants may improve the effectiveness of defense. Evolutionary Ecological Research 2004;6:709-26. 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.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.
In addition to chemical defenses, herbivory can induce 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.
Cost of induced defences
Central to the concept of induced defences is the cost involved when stimulating such defences in the absence of insect herbivores. After all, in the absence of cost, selection is expected to favour the most defended genotype.Mole S. Trade-offs and constraints in plant-herbivore defense theory: a life history perspective. Okios. 1994;71:3-12. Accordingly, individual plants will only do so when there is a need to. The cost of induced defences to a plant 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).Strauss SY, Rudgers JA, Lau JA, Irwin RE. Direct and ecological costs of resistance to herbivory. Trends in Ecology & Evolution. 2002;17:278-85.
Allocation cost is related to the channelling of a large quantity fitness-limited resources to form resistance traits in plants.Heil M, Baldwin IT. Fitness costs of induced resistance: emerging experimental support for a slippery concept. Trends in Plant Science. 2002;7:61-7. Such resources might not be quickly recycledBaldwin IT, Ohnmeiss TE. Swords into plowghshares? Nicotiana sylvestris does not use nicotine as a nitrogen source under nitrogen-limited growth Oecologia. 1994;98:385-92. and thus, are unavailable for fitness-relevant process such as growth and reproduction.Herms DA, Mattson WJ. The dilemma of plants: to grow or to defend. Quarterly Review of Biology. 1992;67:283-335. For instance, herbivory on the broadleaf dock (Rumex obtusifolius) by the green dock beetle (Gastrophysa viridula) induces an increased activity in cell wall-bound peroxidase. The allocation of resources to this increased activity results in reduced leaf growth and expansion in R. obtusifolius.Moor JP, Taylor JE, Paul ND, Whittaker JB. Reduced leaf expansion as a cost of systematic induced resistance to herbivory. Functional Ecology. 2003;17:75-81. In the absence of herbivory, inducing such a defence would be ultimately costly to the plant in terms of development.
Ecological cost results from the disruption of the many symbiotic relationships that a plant has with the environment. For example, jasmonic acid can be used to simulate an herbivore attack on plants and thus, induce plant defences. The use of jasmonic acid on tomato (Lycopersicon esculentum) resulted in plants with fewer but larger fruits, longer ripening time, delayed fruit-set, fewer seeds per plant and fewer seeds per unit of fruit weight.Redman AM, Cipollini DF, Schultz JC. Fitness cost of jasmonic acid-induced defense in tomato, Lycopersicon esculentum. Oecologia. 2001;126:380-5. All these features play a critical role in attracting seed dispersers.Giovannoni JJ. Genetic regulation of fruit development and ripening. Plant Cell. 2004;16:S170-S80. Due to the consequences of induced defences on fruit characteristics, L. esculentum are less able to attract seed dispersers and this ultimately results in a reduced fitness.
Sensing herbivory attack
Induced defences require plant sensing the nature of injury, such as wounding from herbivore attack as opposed to wounding from mechanical damage. Plants therefore use a variety of cues, including the sense of touch,Boyd, Jade (2012). "A bit touchy: Plants' insect defenses activated by touch". Rice University. http://news.rice.edu/2012/04/09/a-bit-touchy-plants-insect-defenses-activated-by-touch/ and salivary enzymes of the attacking herbivore. For example, in a study to test whether plants can distinguish mechanical damage from insect herbivory attack, Korth and Dixon (1997) discovered that the accumulation of induce defence transcription products occurred more rapidly in potato (Solanum tuberosum L.) leaves chewed on by caterpillars than in leaves damaged mechanically.Korth KL, Dixon RA. Evidence for chewing insect-specific molecular events distinct from a general wound response in leaves. Plant Physiology. 1997;115:1299-305. Distinct signal transduction pathway are activated in response either to insect damage or mechanical damage in plants. While chemicals released in wounding responses are the same in both cases, the pathway in which they accumulate are separate. Not all herbivore attack begins with feeding, but with insects laying eggs on the plant. The adults of butterflies and moths (order Lepidoptera), for example, do not feed on plants directly, but lay eggs on plants which are suitable food for their larva. In such cases, plants have been demonstrated to induce defences upon contact from the ovipositing of insects.Tamiru (2017). Bruce TJA, Richter A, Woodcock CM, Midega CAO, Degenhardt J, Kelemu S, Pickett JA, Khan ZRA (2017) A maize landrace that emits defense volatiles in response toherbivore eggs possesses a strongly inducible terpene synthase gene. Ecology and Evolution 7: 2835-2845
A mechanism of defence induction: changes in gene transcription rates
Systemically induced defences are at least in some cases the result of changes in the transcription rates of genes in a plant. Genes involved in this process may differ between species,Walling LL. The myriad plant responses to herbivores. Journal of Plant Growth Regulation. 2000;19:195-216. but common to all plants is that systemically induced defences occur as a result of changes in gene expression. The changes in transcription can involve genes which either do not encode products involved in insect resistance, or are involved in general response to stress. In cultivated tobacco (Nicotiana tobacum) photosynthetic genes are down-regulated, while genes directly involved in defences are up-regulated in response to insect attack.Hermsmeier D, Schittko U, Baldwin IT. Molecular interactions between the specialist herbivore Manduca sexta (Lepidoptera, Sphingidae) and its natural host Nicotiana attenuata. 1. Large-scale changes in the accumulation of growth- and defense-related plant mRNAs. Plant Physiology. 2001;125:1875-87. This allows more resources to be allocated to producing proteins directly involved in the resistance response. A similar response was reported in Arabidopsis plants where there is an up-regulation of all genes that are involved in defence.Schenk JM, Kazan K, Wilson I, Anderson JP, Richmond T, Somerville SC, et al. Coordinated plant defence responses in Arabidopsis revealed by microarray analysis. Proceedings of the National Academy of Sciences, USA. 2000;97:11655-60. Such changes in the transcription rates are essential in inducing a change in the level of defence upon herbivory attack.
Classification of induced genes