Genetically Engineering Natural Body Guards

The integration of ecology and molecular biology has resulted in many important advances in understanding the complex interactions between organisms and the underlying mechanisms. One of the many advances in combining the two sciences could be seen in the plant Arabidopsis thaliana L., an excellent model for investigating ecological interactions such as induced indirect defense. Induced indirect defense is a technique plants use to defend themselves against the feeding of herbivorous arthropods by releasing substances into the air called volatiles, which attracts natural enemies of the herbivores and the activities of these natural predators benefit the plant's fitness therefore making this technique evolutionarily advantageous for plants. For many plants species after the releasing of volatiles, a process known as herbivory, the compound called (E)-DMNT has been detected in the surrounding of these plants, and of the many species of plants Arabidopsis is one of them. Using Arabidopsis as a model for studying the compound (E)-DMNT, we could pinpoint certain enzymes that are responsible for the synthesis for the compound and apply it in genetic engineering in order to create pest resistance crops.

The first step in studying the ecological relevance of individual compound of the complex volatile is through understanding the chemical pathway that is genetically engineered to the transgenic plant. In the process of genetic engineering the sesquiterpene (3S)-(E)-nerolidol, a precursor for (E)-DMNT is required in order to create transgenic plants that releases the compound (E)-DMNT during herbivory. However earlier attempts to produce relevant amounts of sesquiterpene have failed due to the lack of farnesyl diphostphate (FPP), which is the precursor for (3S)-(E)-nerolidol, therefore targets with an abundance of FPP are required for the synthesis of sesquiterpene. More suitable target for sesquiterpene synthase to target include the cytosol and the plastids due to their abundance in FPPs, the mitochondria a site of ubiquitous biochemical biosynthesis on the other hand is much alike the cytosol and a the compartmentalization of the mitochondria allows it to regulate the production of (E)-DMNT making it the perfect target for sesquiterpene synthase. FaNES1 a strawberry nerolidol synthase, also a form of sesquiterpene synthase is targeted using CoxIV (cytochromes oxidase subunit IV) localizing it in the mitochondria, therefore making up the transgenic plant.

Chemical Pathway of (E)-DMNT with precursors of (3S)-(E)-nerolidol and farnesyl diphostphate (FPP)

In the experiment transgenic Arabidopsis and wild Arabidopsis are grown in containers, where in the transgenic plants CoxIV is fused with green fluorescent protein (GFP) to efficiently target the GFP to the mitochondria. Using a method called solid-phase micro extraction (SPME) the levels of volatile compounds are recorded from the plants during herbivory. The results collected show that the levels of (3S)-(E)-nerolidol detected from the transgenic samples are 20-30 times more than the compounds from wild plant samples, where for the transgenic samples 9 of 12 were recorded detecting the compound (3S)-(E)-nerolidol, and of the 9 transgenic samples 5 were detected with the compound (E)-DMNT during herbivory. The wild plants on the other hand were unable to cause the formation of either (3S)-(E)-nerolidol or (E)-DMNT during herbivory, unlike other plant species whom proved otherwise. The reason for this is because the rate in which (3S)-(E)-nerolidol is converted into (E)-DMNT is inversely proportional to the rate at which they are released during herbivory, however this could be regulated using jasmonic acid, which acts as a mediator.

(E) Undamaged Transgenic (F) Undamaged Wild Type, Jasmonic acid (G) Undamaged Transgenic, Jasmonic acid

Transgenic CoxIV-FaNES1 plants were successful in emitting both (3S)-(E)-nerolidol and (E)-DMNT compounds during herbivory, and from the study both compounds were successful in attracting predatory mites (P. persimilis) when the transgenic Arabidopsis were being eating by herbivorous arthropods such as spider mites (Tetranychus Urticae) and caterpillar (P. rapae). From these conclusions we utilize this defensive mechanism and applying them to improve pest resistance of crops, however there are also several problems concerning with the transgenic CoxIV-FaNES1 plants that were used in the experiment. The problem that was seen in the transgenic Arabidopsis plants was that both the first and second generation of the transgenic species suffered from growth retardation, this is due to the divergence of FPP to be synthesized to (3S)-(E)-nerolidol causing a uneven distribution leading to growth inhibition and greatly impacting the performance of the plant. So although the genetic modification of induced indirect defense could improve crop production and allow crop plants to generate control agents against arthropod pest infestation, at the same time also result harming the performance of the plant, therefore saying that even though genetic engineering could be beneficial there is also equal risks involved.

% of predators CoxIV-FaNES1 vs. Wild Type Plants

Work Cited:
1. Kappers, Iris. "Genetic Engineering of Terpenoid Metabolism Attracts Bodyguards to Arabidopsis." Science. 2005: n. page. Web. <http://stke.sciencemag.org/cgi/content/full/sci;309/5743/2070

2. M, Dicke. "ISOLATION AND IDENTIFICATION OF VOLATILE KAIROMONE THAT AFFECTS ACARINE PREDATOR-PREY INTERACTIONS - INVOLVEMENT OF HOST PLANT IN ITS PRODUCTION." JOURNAL OF CHEMICAL ECOLOGY. 02 1990: n. page. Web. <http://cel.webofknowledge.com/InboundService.do?SID=S1AGmejh7c5GN75K46d&product=CEL&UT=A1990CT40300008&SrcApp=Highwire&Init=Yes&action=retrieve&Func=Frame&customersID=Highwire&SrcAuth=Highwire&IsProductCode=Yes&mode=FullRecord>.

3. Kessler, Andre. "Silencing the Jasmonate Cascade: Induced Plant Defenses and Insect Populations." Science. 30 07 2004: n. page. Web. 7 May. 2012. <http://www.sciencemag.org/content/305/5684/665.abstract?ijkey=65037872d5afd6050cc21f838fe7bc11bd65feee&keytype2=tf_ipsecsha>.