Signaling Pathway In Arabidopsis Research Articles

The Arabidopsis RPS4 bacterial-resistance gene is a member of the TIR-NBS-LRR family of disease-resistance genes.

Plant-disease resistance (R) genes mediate the specific recognition of invading pathogens carrying cognate avirulence (avr) determinants. RPS4 is a disease-resistance locus on chromosome 5 of Arabidopsis thaliana specifying resistance to strains of Pseudomonas syringae pv. tomato expressing avrRps4. We have isolated the RPS4 gene using a map-based cloning approach. RPS4 encodes a predicted protein of 1217 amino acids that contains an N-terminus with homology to the intracellular domains of the Drosophila Toll protein and the mammalian interleukin-1 receptor (TIR domain), a tripartite nucleotide-binding site (NBS), and leucine-rich repeats (LRR). Incomplete splicing of the RPS4 mRNA was observed, which may give rise to truncated protein products consisting mainly of the TIR and NBS domains. These features classify RPS4 as a member of the TIR-NBS-LRR R gene family founded by N, L6 and RPP5, which determine resistance to viral, fungal and oomycete pathogens, respectively. Previous work has shown that RPS4, like other Arabidopsis TIR-NBS-LRR R genes specifying resistance to oomycetes, is dependent on a functional EDS1 allele for disease-resistance signaling. The characterization of RPS4 presented here thus establishes a role for TIR-NBS-LRR R genes in resistance to bacterial pathogens, and provides evidence for the model that dependence of R genes on EDS1 is determined by R protein structure, and not by pathogen type. The cloning of RPS4 and the previous isolation of avrRps4 provide the molecular tools for a genetic and molecular dissection of the TIR-NBS-LRR R gene signaling pathway in Arabidopsis.

Different requirements for EDS1 and NDR1 by disease resistance genes define at least two R gene-mediated signaling pathways in Arabidopsis.

The Arabidopsis genes EDS1 and NDR1 were shown previously by mutational analysis to encode essential components of race-specific disease resistance. Here, we examined the relative requirements for EDS1 and NDR1 by a broad spectrum of Resistance (R) genes present in three Arabidopsis accessions (Columbia, Landsberg-erecta, and Wassilewskija). We show that there is a strong requirement for EDS1 by a subset of R loci (RPP2, RPP4, RPP5, RPP21, and RPS4), conferring resistance to the biotrophic oomycete Peronospora parasitica, and to Pseudomonas bacteria expressing the avirulence gene avrRps4. The requirement for NDR1 by these EDS1-dependent R loci is either weak or not measurable. Conversely, three NDR1-dependent R loci, RPS2, RPM1, and RPS5, operate independently of EDS1. Another RPP locus, RPP8, exhibits no strong exclusive requirement for EDS1 or NDR1 in isolate-specific resistance to P. parasitica, although resistance is compromised weakly by eds1. Similarly, resistance conditioned by two EDS1-dependent RPP genes, RPP4 and RPP5, is impaired partially by ndr1, implicating a degree of pathway cross-talk. Our results provide compelling evidence for the preferential utilization of either signaling component by particular R genes and thus define at least two disease resistance pathways. The data also suggest that strong dependence on EDS1 or NDR1 is governed by R protein structural type rather than pathogen class.