ABSTRACT
Previously, we identified Arabidopsis thaliana mutant rhd1-4 as hypersusceptible to the sugar beet cyst nematode Heterodera schachtii. We assessed rhd1-4 as well as two other rhd1 alleles and found that each exhibited, in addition to H. schachtii hypersusceptibility, decreased root length, increased root hair length and density, and deformation of the root epidermal cells compared with wild-type A. thaliana ecotype Columbia (Col-0). Treatment of rhd1-4 and Col-0 with the ethylene inhibitors 2-aminoethoxyvinylglycine and silver nitrate and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid suggests that the rhd1-4 hypersusceptibility and root morphology phenotypes are the result of an increased ethylene response. Assessment of known ethylene mutants further support the finding that ethylene plays a role in mediating A. thaliana susceptibility to H. schachtii because mutants that overproduce ethylene (eto1-1, eto2, and eto3) are hypersusceptible to H. schachtii and mutants that are ethylene-insensitive (etr1-1, ein2-1, ein3-1, eir1-1, and axr2) are less susceptible to H. schachtii. Because the ethylene mutants tested show altered susceptibility and altered root hair density and length, a discrimination between the effects of altered ethylene signal transduction and root hair density on susceptibility was accomplished by analyzing the ttg and gl2 mutants, which produce ectopic root hairs that result in greatly increased root hair densities while maintaining normal ethylene signal transduction. The observed normal susceptibilities to H. schachtii of ttg and g12 indicate that increased root hair density, per se, does not cause hypersusceptibility. Furthermore, the results of nematode attraction assays suggest that the hypersusceptibility of rhd1-4 and the ethylene-overproducing mutant eto3 may be the result of increased attraction of H. schachtii-infective juveniles to root exudates of these plants. Our findings indicate that rhd1 is altered in its ethylene response and that ethylene signal transduction positively influences plant susceptibility to cyst nematodes.
Subject(s)
Arabidopsis/metabolism , Beta vulgaris/parasitology , Ethylenes/metabolism , Glycine/analogs & derivatives , Nematoda/growth & development , Plant Roots/growth & development , Signal Transduction , Amino Acids, Cyclic/pharmacology , Animals , Arabidopsis/genetics , Arabidopsis/parasitology , Ethylenes/antagonists & inhibitors , Female , Glycine/pharmacology , Lyases/antagonists & inhibitors , Lyases/metabolism , Mutation , Nematoda/pathogenicity , Plant Proteins/antagonists & inhibitors , Plant Proteins/metabolism , Plant Roots/parasitology , Receptors, Cell Surface/antagonists & inhibitors , Receptors, Cell Surface/metabolism , Signal Transduction/drug effects , Silver Nitrate/pharmacologyABSTRACT
Genetic approaches are a powerful means to elucidate plant-pathogen interactions. An in vitro screening protocol was developed to identify Arabidopsis thaliana mutants with altered susceptibility to Heterodera schachtii, the sugar beet cyst nematode. In an initial screen of approximately 5,200 ethyl methanesulfonate-generated mutant plants, two stable mutations were identified. Both mutant lines were backcrossed and were found to harbor single recessive mutant alleles. Mutant line 2-4-6 shows an approximately two-fold increase in sedentary and developing nematodes, while mutant line 10-5-2 exhibits a significant decrease in susceptibility that manifests itself only after nematodes become sedentary. Analyses of progeny from crosses of lines 2-4-6 and 10-5-2 indicated that the two mutations are not allelic. However, the mutant gene in line 2-4-6 was found to be allelic to the previously identified mutant rhd1 and was termed rhd1-4. The mutant gene in line 10-5-2 was called asc1 for altered susceptibility to cyst nematodes. Our results demonstrate the feasibility of a nematological mutant screen and strengthen A. thaliana and H. schachtii as a model pathosystem.
