The effect of bacterial isochorismate synthase on the Brassica rapa metabolome

Abstract

Salicylic acid (SA) is a signal molecule that plays an important role in plant defense against pests and diseases. Introducing the gene(s) that overexpressed SA would be an advantage where the crop performance could be improved by enhancing its resistance to pathogen attack. This thesis aimed at developing transgenic Brassica rapa plants with a bacterial SA pathway gene, the entC gene (encoding for isochorismate synthase) and subsequently investigating the effect of introducing this gene on the B. rapa secondary metabolite profile. The transgenic B. rapa carrying the entC gene was developed by using Agrobacterium tumefaciens-mediated transformation. The level of SA and its glucosides (SAG) were increased in transgenic plants which means that the bacterial entC gene was functional in B. rapa. In contrast, the introduction of the gene was expected to have an effect on other isochorismate derived products, but did not have a significant effect on the phylloquinone accumulation. However, the glucosinolate profiles particularly for indole and aliphatic glucosinolates was altered. The altered profile of the glucosinolates might be due to the increased level of SA produced via isochorismate that leads to activation of plant defense. Metabolome analysis of the transgenic plants showed that some phenylpropanoid compounds and indole glucosinolate (neoglucobrassicin) were increased in comparison to the control plants. These results suggest that the expression of the bacterial entC gene in B. rapa did not affect fluxes into pathways to the other groups of secondary metabolites through competition for the same precursor (chorismate). On the contrary, the biosynthesis of chorismate derived products (SA) seems to induce the competitive pathways via phenylalanine and tryptophan, pathways that are known to play a role in plant resistance. Future studies on the resistance of the transgenic plants are required to proof an increased resistance as it previously has been reported for tobacco plants overproducing salicylic acid