Indirect defence of Arabidopsis against herbivorous insects : combining parasitoid behaviour and chemical analyses with a molecular genetic approach

R.M.P. van Poecke

Research output: Thesisinternal PhD, WU

Abstract

<FONT FACE="Garamond"><p>Many plant species are known to defend themselves against herbivorous insects indirectly, by producing volatiles in response to herbivory. These volatiles attract carnivorous enemies of the herbivores, such as parasitoids. Research on the model plant <em>Arabidopsis thaliana</em> (L.) Heynh. (Brassicaceae) has contributed considerably to the unravelling of signal transduction pathways involved in <em>direct</em> plant defence mechanisms against pathogens. In this thesis I demonstrate that <em>Arabidopsis</em> is also a good model plant for studying signal transduction pathways involved in <em>indirect</em> defence mechanisms, by showing that: (a) <em>Arabidopsis</em> plants infested by <em>Pieris rapae</em> caterpillars (Lepidoptera: Pieridae) emit volatiles from several major biosynthetic pathways, including terpenoids, methyl-salicylate (MeSA), nitriles and green leaf volatiles; (b) Adult females of <em>Cotesia rubecula</em> (Hymenoptera: Braconidae), a specialist parasitoid wasp of <em>P. rapae</em> , were attracted to <em>P. rapae-</em> infested <em>Arabidopsis</em> plants; and (c) Genes from major biosynthetic pathways involved in volatile production were induced by caterpillar feeding.</p><p>Moreover, I show that the octadecanoid and the salicylic acid pathways are involved in the induced attraction of <em>C. rubecula</em> by <em>P. rapae</em> -infested <em>Arabidopsis</em> . Besides exogenous application of jasmonic acid (JA) or salicylic acid (SA), I used transgenic <em>Arabidopsis</em> that do not show induced JA levels after wounding (S-12) and transgenic <em>Arabidopsis</em> that do not accumulate SA (NahG). Treatment of <em>Arabidopsis</em> with JA resulted in an increased attraction of parasitoid wasps compared to untreated plants, whereas treatment with SA did not. Transgenic plants impaired in the octadecanoid or the salicylic acid pathway were less attractive than wild-type plants. Chemical analysis of the volatile blends emitted by JA-treated wild-type and <em>P. rapae</em> -infested wild-type, S-12 and <em>NahG</em> plants, demonstrate that JA and SA are indeed involved in the herbivory-induced production of volatiles. Moreover, these data suggest important roles for MeSA and the terpenoid (3 <em>E,7E</em> )-4,8,12-trimethyl-1,3,7,11-tridecate traene as parasitoid attractants <em>.</em> Additionally, these data suggest a signalling role of the plant hormone 12-oxo-phytodienoic acid in induced volatile emissions.</p><p>Although <em>direct</em> defence mechanisms against pathogens and herbivores are often also regulated through JA and SA, these signal transduction pathways differ from those involved in <em>indirect</em> defence of <em>Arabidopsis</em> against <em>P. rapae</em> , as I demonstrated by using the signal transduction mutants <em>npr1-1</em> and <em>jar1-1</em> .</p><p>In this thesis it is also shown that herbivore species with a different way of feeding compared to <em>P. rapae</em> caterpillars - aphids and spider mites - induced no or less attraction of <em>C. rubecula</em> when infesting <em>Arabidopsis.</em> This difference in parasitoid attraction may be the result of different induction of JA and SA signalling pathways by different herbivore species.</p><p>In conclusion, this thesis demonstrates that combing parasitoid behaviour and chemical analysis with a molecular genetic approach can be highly valuable in unravelling signal-transduction pathways involved in indirect defence of plants, a method that so far has been under-exploited.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Dicke, Marcel, Promotor
Award date22 Nov 2002
Place of PublicationS.l.
Print ISBNs9789058086983
Publication statusPublished - 2002

Keywords

  • arabidopsis thaliana
  • defence mechanisms
  • volatile compounds
  • insect pests
  • parasitoids
  • insect attractants
  • host parasite relationships
  • animal behaviour
  • molecular genetics

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