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First published online March 18, 2005; 10.1104/pp.104.051243 Plant Physiology 137:1445-1455 (2005) © 2005 American Society of Plant Biologists Aphid Resistance in Medicago truncatula Involves Antixenosis and Phloem-Specific, Inducible Antibiosis, and Maps to a Single Locus Flanked by NBS-LRR Resistance Gene Analogs1Commonwealth Scientific and Industrial Research Organization Entomology (J.K., L.G., O.R.E.) and Commonwealth Scientific and Industrial Research Organization Plant Industry (J.K., L.G., A.S.C., K.B.S.), Wembley, Western Australia 6913, Australia; School of Animal Biology, University of Western Australia, Crawley, Western Australia 6009, Australia (R.C., H.S.J.); and South Australian Research and Development Institute, Adelaide, South Australia 5001, Australia (R.M.N.)
Aphids and related insects feed from a single cell type in plants: the phloem sieve element. Genetic resistance to Acyrthosiphon kondoi Shinji (bluegreen aphid or blue alfalfa aphid) has been identified in Medicago truncatula Gaert. (barrel medic) and backcrossed into susceptible cultivars. The status of M. truncatula as a model legume allows an in-depth study of defense against this aphid at physiological, biochemical, and molecular levels. In this study, two closely related resistant and susceptible genotypes were used to characterize the aphid-resistance phenotype. Resistance conditions antixenosis since migratory aphids were deterred from settling on resistant plants within 6 h of release, preferring to settle on susceptible plants. Analysis of feeding behavior revealed the trait affects A. kondoi at the level of the phloem sieve element. Aphid reproduction on excised shoots demonstrated that resistance requires an intact plant. Antibiosis against A. kondoi is enhanced by prior infestation, indicating induction of this phloem-specific defense. Resistance segregates as a single dominant gene, AKR (Acyrthosiphon kondoi resistance), in two mapping populations, which have been used to map the locus to a region flanked by resistance gene analogs predicted to encode the CC-NBS-LRR subfamily of resistance proteins. This work provides the basis for future molecular analysis of defense against phloem parasitism in a plant model system.
1 This work was supported in part by the Centre for Legumes in Mediterranean Agriculture and by the Grains Research and Development Corporation (an Honours Scholarship to R.C.). Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.051243. * Corresponding author; e-mail karam.singh{at}csiro.au; fax 61893878991. Received August 5, 2004; returned for revision October 14, 2004; accepted October 20, 2004. Related articles in Plant Physiol.:
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