Gene-for-gene disease resistance without the hypersensitive response in Arabidopsis dnd1 mutant

Abstract

The cell death response known as the hypersensitive response (HR) is a central feature of gene-for-gene plant disease resistance. A mutant line of Arabidopsis thaliana was identified in which effective gene-for-gene resistance occurs despite the virtual absence of HR cell death. Plants mutated at the DND1 locus are defective in HR cell death but retain characteristic responses to avirulent Pseudomonas syringae such as induction of pathogenesis-related gene expression and strong restriction of pathogen growth. Mutant dnd1 plants also exhibit enhanced resistance against a broad spectrum of virulent fungal, bacterial, and viral pathogens. The resistance against virulent pathogens in dnd1 plants is quantitatively less strong and is differentiable from the gene-for-gene resistance mediated by resistance genes RPS2 and RPM1. Levels of salicylic acid compounds and mRNAs for pathogenesis-related genes are elevated constitutively in dnd1 plants. This constitutive induction of systemic acquired resistance may substitute for HR cell death in potentiating the stronger gene-for-gene defense response. Although cell death may contribute to defense signal transduction in wild-type plants, the dnd1 mutant demonstrates that strong restriction of pathogen growth can occur in the absence of extensive HR cell death in the gene-for-gene resistance response of Arabidopsis against P. syringae.

Figure 1

HR cell death defect in dnd1 mutant. Leaves of wild-type parent (Col) and dnd1 mutant (dnd1) plants were inoculated with a high dose (2 × 10⁸ cfu/ml) of avirulent, HR-stimulating P. syringae pv. glycinea Race 4 pV288 (Psg avrRpt2⁺) or the isogenic, nonavirulent control strain P. syringae pv. glycinea Race 4 pVSP61 (Psg). At 24 h postinoculation, leaves were harvested, fixed, and examined for autofluorescent dead cells by using a fluorescence microscope. (Upper Right) The edge of an inoculated zone, revealing confluent cell death in response to bacteria only on the left (inoculated) side.

Figure 2

Growth of bacteria within plant leaves. (A) Arabidopsis lines Col (Col-0 wild-type, RPS2/RPS2; DND1/DND1), rps2 (Col-0 rps2–201/rps2–201; DND1/DND1), and dnd1 (Col-0 RPS2/RPS2; dnd1/dnd1) inoculated with P. syringae pv. tomato DC3000 pV288 (avrRpt2⁺). (B) Arabidopsis lines Col-0 and dnd1 inoculated with isogenic P. syringae pv. tomato DC3000 differing by the presence (pAvrRpm1, filled symbols) or absence (pVSP61, open symbols) of avirulence gene avrRpm1 carried on plasmid pVSP61. Both plant lines are RPM1/RPM1 genotype. All data points are mean ± SD.

Figure 3

Pathogenesis-related gene expression monitored by RNA blot analysis of Col-0 wild-type (Col) and Col-0 dnd1/dnd1 mutant (dnd1) plants. (A) β-glucanase expression 72 h after treatment of leaves with 10 mM MgCl₂ containing no pathogen (φ), the nonavirulent control strain P. syringae pv. tomato DC3000 pVSP61 (vir), or the isogenic avrRpt2-expressing strain P. syringae pv. tomato DC3000 pV288 (avr). (B) PR-1 expression 24 h after treatment as in A. (C) Phosphorimager quantification of PR-1 expression from blot shown in B, normalized to level of constitutive β-ATPase mRNA. Similar results were obtained in multiple experiments.

Figure 4

Levels of salicylic acid and glucoside-conjugated salicylic acid compounds in Col-0 and mutant Col-0 dnd1–1/dnd1–1 plants.