Role of the Tomato Gene Mi in Conferring Specific Resistance against Nematodes and Aphids

University Of California-Davis, Davis CA

Investigators

Abstract

The tomato gene, Mi, confers resistance against several species of root-knot nematodes (Meloidogyne spp.) as well as against specific isolates of potato aphid (Macrosiphum euphorbiae). It is the only major gene for resistance to an aphid to be cloned so far and the first discovered that can mediate gene-for-gene resistance to two unrelated pests. Mi is a member of a diverse class of plant resistance-genes (R-genes) involved in gene-specific resistance against pathogens including viruses, bacteria and fungi. Members of this class of plant proteins mediate specific pathogen recognition and signaling of host defense responses. More than 20 such genes have been identified, yet how the pathogen is recognized and resistance conferred is not known. The Mi clone constitutes a unique resource for gaining insights into R-gene functions associated with specific pest recognition and signaling in plants and for determining the mechanism(s) by which a single gene can mediate resistance to multiple organisms. Chimeric constructs of Mi that produce a lethal phenotype when transiently expressed in Nicotiana benthamiana leaves are already available. Specific in vitro mutations in Mi and in constitutively lethal constructs of Mi will be produced. The phenotypes of the mutations will be examined in two different assays to delineate the function in pathogen recognition and signal transmission of particular regions of the gene. These assays will determine the effects of mutated alleles on cell death in a leaf infiltration assay and on nematode resistance in transformed roots. The phenotypes of in vitro-generated alterations in Mi on the signalling of resistance will also be examined in transgenic plants to identify the functional domains associated with nematode and/or aphid recognition and downstream defense signaling. In addition, proteins that interact with Mi will be identified using the yeast 2-hybrid system and the role of these proteins in resistance will be examined. Root- knot nematodes cause billions of dollars in damage to the world's crops. Current agricultural control measures include application of chemical pesticides or the use of resistant plants. A single gene present in some tomato varieties can confer effective resistance against nematodes as well as resistance against some aphids. This project explores the question of how a single gene, which was bred into tomato from a wild plant species, can mediate recognition of the nematode pest and trigger an effective defense. An increased understanding of how plants defend themselves from pathogens and pests should result from this work. Such understanding is essential for rational design of strategies to improve pest and pathogen resistance in plants.