Oxylipin phytohormones in plant-insect interactions : action and metabolism of jasmonic acid and 12-oxophytodienoic acid in plants and insects
During co-evolution plants have evolved a myriad of different responses to defend themselves against enemies such as insects or pathogens. These defense strategies can either be constitutively expressed or induced in response to attack. The activation of defense mechanisms is mediated by a set of signals in which essential role is played by the compounds from the jasmonate family. The aim of this thesis was to study the importance of jasmonic acid related signaling in plant responses to different stress stimuli and to establish whether, in the course of evolutionary adaptations, insects have evolved ways to perceive this primary defensive plant signal. In the course of the study following observations were made: 1) Jasmonates are universal stress signals in plant kingdom. Levels of jasmonic acid (JA) and its precursor 12-oxophytodienoic acid (OPDA) accumulated after different elicitation events in various plant species. Differences were found in the accumulation of JA in response to mechanical wounding at night and during the day. 2) Continuous mechanical wounding is alone sufficient to upregulate genes hitherto correlated with herbivore-specific responses. 3) Specialist insect Plutella xylostella can influence JA signaling in Arabidopsis thaliana. 4) Generalist insects recognize the plant signaling molecule - OPDA. Generalist insects were found to enzymatically isomerize plant signaling molecule – OPDA to iso-OPDA. The activity of the putative enzyme was correlated with glutathione S-transferases. The expression of candidate genes in E. coli revealed one enzyme which exhibited OPDA isomerization activity. 5) Alternative biosynthetic pathway leading to cis-jasmone can be useful for future crop protection. Defining the plant’s ability to convert iso-OPDA to cis-jasmone and finding a candidate gene for OPDA isomerase allows speculation about genetic modifications, which would enable plants to biosynthesize cis-jasmone along with JA in response to external attack.