Insects plant defences

Attractive Compounds. Despite the fact that defence chemistry and insect-plant interactions have been extensively investigated in many leaf beetle species, not too much is known about the chemical background of intraspecific communication. 

Recently, it has been demonstrated that ecdysteroid accumulation in spinach is inducible by mechanical [50] or insect [51] damage to roots. Evidence was further provided for the involvement of jasmonates in the induction of de novo ecdysteroid synthesis. Also, short- and long-term labelling of 20E from [2-14C]mevalonic acid in spinach demonstrate that ecdysteroids are metabolically stable in this species, which fits much better with a role in plant defence, rather than a phytohormonal role [52], Most recently, it has been demonstrated that root predation by the fungus gnat Bradysia impatiens results in elevated ecdysteroid levels in spinach and a significant reduction in larval establishment of B. impatiens [53],... 

G. W. Felton J. A. Gatehouse, Antinutritive Plant Defence Mechanisms. In Biology of the Insects Midgut-, M. J. Lehane,... 

In many cases, the volatile compounds emitted from leaves as a result of insect damage allow insect parasitoids and predators to distinguish between infested and uninfested plants, and therefore help to locate hosts or prey [230]. In the majority of plants reported so far, there are remarkable similarities in the structure of VOCs that are emitted from insect-damaged leaves [231]. This structural uniformity suggests the activation of a common set of biosynthetic pathways shared by a wide range of plants, and that the products are detectable by a broad spectrum of insect parasitoids and predators. For instance, nicotine is one of the most broadly effective plant defence metabolites known because it poisons acetylcholine receptors and is thus toxic to most heterotrophic organisms with neuromuscular junctions. 

The role of calystegines in plants has not been elucidated, but the fact that they appear in a limited number of species indicates that they might be a source of carbon and nitrogen to soil bacteria that benefit the rhizosphere of the plant [5, 25]. Calystegines may also play a role in plant defence mechanisms and plant-insect interactions as reported by Nash [10]. 

Terpenes are secondary metabolites synthesized mainly by plants, but also by a limited number of insects, marine micro-organisms and fungi. These corrpounds were first considered as waste products from plant metabolism with no specific biological role, but later, the involvement of some terpenes as intermediates in relevant biosynthetic processes was discovered [1]. Additionally, it has been well demonstrated that many terpenes play important ecological roles [1] as in plant defence, for example as insect repellents, and in symbiotic mechanisms, for example as attractants to specific insect species to stimulate cross pollination. 

Walling, 2000), less is known about plant responses to sucking or piercing insects (Du et al, 1998 Williams HI et al, 2005) (see Section on Regulation of Plant Toxicants by Natural Activators ). A recent study of the interactions between different types of herbivorous insects and plant defence volatiles showed that tobacco, Nicotiana tabacum, releases terpenoids and nicotine after caterpillar regurgitate treatment. Colonisation by western flower thrips, Frankliniella occidentalis, was reduced on the regurgitate treated plants, possibly due to the production of nicotine (Delphia et al, 2007). 

Birkett MA, Campbell CAM, Chamberlain K, Guerrieri E, Hick AJ, Martin JL, Matthes M, Napier JA, Pettersson J, Pickett JA, Poppy GM, Pow EM, Pye PJ, Smart LE, Wadhams GH, Wadhams LJ, Woodcock CM (2000) New roles for Cis-jasmone as an insect semiochemical and in plant defence. Proc Natl Acad Sci USA 97 9329-9334... 

Thompson GA, Goggin FL (2006) Transcriptomics and functional genomics of plant defence induction by phloem feeding insects. J Exp Bot 57 755-766... 

We have, as yet, few clues to how insects overcome plant defences. 

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