Does engineering enhanced broad-spectrum disease resistance in a legume cause harmful effects to beneficial symbionts?

Manipulation of a plant's own induced defence response pathways, leading to enhanced broad-spectrum disease resistance, offers exciting strategies for genetically modifying plants for disease control in the field. Two such strategies are overexpression of NONEXPRESSOR OF PR (NPRl), a. key regulator of systemic acquired resistance (SAR) and overexpression of ISOFLAVONE METHYL TRANSFERASE (IOMT), an enzyme in phytoalexin biosynthesis. However it has not been tested whether increased defence responses would be harmful to beneficial symbionts, such as nitrogen fixing rhizobia and arbuscufar mycorrhizal fungi (AMF). We have transferred the strategies of overexpression ofNPRl and overexpression of IOMT to the model legume Medicago truncatula to examine the effects on symbionts. Stably transformed transgenic plants of Medicago, overexpressing IOMT and Arabidopsis NPRl, have been produced. In addition a Medicago orthologue ofNPRl and two other Medicago NPRI homologues, an orthologue of NPR~,. a negative regulator of defence responses and one with no Arabidopsis orthologue, have been identified. Transgenic plants overexpressing the Medicago NPRI orthologue showed significantly reduced colonisation by AMF and rhizobia. Characterisation of molecular markers for systemic acquired resistance (SAR) showed a PRI gene would not make a good marker and a PR2 gene strongly induced in shoot tissue in response to root infection with Aphanomyces as a possible defence gene expression marker. Treatment of Medicago with chemical inducers of SAR significantly decreased colonisation by AMF suggesting a role for the induced defence response in AMF colonisation. Most of the transgenic lines produced remain to be tested for their effects on symbionts and for enhanced resistance to a spectrum of pathogens. In addition to assessing the suitability of these strategies for the field, the transgenic plants will be useful tools for understanding the role of induced defence responses in a legume and on beneficial symbionts.EThOS - Electronic Theses Online ServiceGBUnited Kingdo