Transposon inactivation of Arabidopsis MAP kinase 4 produced the mpk4 mutant exhibiting constitutive systemic acquired resistance (SAR) including elevated salicylic acid (SA) levels, increased resistance to virulent pathogens, and constitutive pathogenesis-related gene expression shown by Northern and microarray hybridizations. MPK4 kinase activity is required to repress SAR, as an inactive MPK4 form failed to complement mpk4. Analysis of mpk4 expressing the SA hydroxylase NahG and of mpk4/npr1 double mutants indicated that SAR expression in mpk4 is dependent upon elevated SA levels but is independent of NPR1. PDF1.2 and THI2.1 gene induction by jasmonate was blocked in mpk4 expressing NahG, suggesting that MPK4 is required for jasmonic acid-responsive gene expression.

Animal SGT1 is a component of Skp1-Cullin-F-box protein (SCF) ubiquitin ligases that target regulatory proteins for degradation. Mutations in one (SGT1b) of two highly homologous Arabidopsis SGT1 genes disable early plant defenses conferred by multiple resistance (R) genes. Loss of SGT1b function in resistance is not compensated for by SGT1a. R genes differ in their requirements for SGT1b and a second resistance signaling gene, RAR1, that was previously implicated as an SGT1 interactor. Moreover, SGT1b and RAR1 contribute additively to RPP5-mediated pathogen recognition. These data imply both operationally distinct and cooperative functions of SGT1 and RAR1 in plant disease resistance.

We have identified the Arabidopsis ortholog of barley RAR1 as a component of resistance specified by multiple nucleotide binding/Leu-rich repeat resistance ( R ) genes recognizing different bacterial and oomycete pathogen isolates. Characterization of partially and fully defective rar1 mutations revealed that wild-type RAR1 acts as a rate-limiting regulator of early R gene-triggered defenses, determining the extent of pathogen containment, hypersensitive plant cell death, and an oxidative burst at primary infection sites. We conclude that RAR1 defense signaling function is conserved between plant species that are separated evolutionarily by 150 million years. RAR1 encodes a protein with two zinc binding (CHORD) domains that are highly conserved across eukaryotic phyla, and the single nematode CHORDcontaining homolog, Chp , was found previously to be essential for embryo viability. An absence of obvious developmental defects in null Arabidopsis rar1 mutants favors the notion that, in contrast, RAR1 does not play a fundamental role in plant development. INTRODUCTIONIn countering attack by microbial pathogens or insects, plants have evolved resistance ( R ) genes that specifically recognize corresponding pathogen avirulence ( avr ) genes to trigger plant defenses (Dangl and Jones, 2001). Two plant R gene-encoded proteins, tomato Pto and rice Pi-ta, have been shown to interact physically with their pathogen Avr counterparts, AvrPto and Avr-Pita, respectively, in in vitro assays (Scofield et al., 1996;Tang et al., 1996;Jia et al., 2000). Other plant R proteins may associate with pathogen Avr proteins indirectly within a protein complex . In the absence of a corresponding R gene, the pathogen is able to colonize its host. Some Avr proteins are virulence factors that facilitate pathogen growth or interfere with basal plant defenses (Nimchuk et al., 2000;Staskawicz et al., 2001). R-Avr protein recognition commonly involves localized programmed plant cell death (the hypersensitive response [HR]), an oxidative burst producing reactive oxygen intermediates (ROI), and the accumulation of salicylic acid (SA), a phenolic molecule necessary for the induction of systemic immunity (systemic acquired resistance) (Feys and Parker, 2000).Plant R proteins share a limited repertoire of motifs with animal proteins that control innate immunity (Staskawicz et al., 2001). The most prevalent R gene class encodes predicted cytosolic proteins with a central nucleotide binding (NB) domain and C-terminal Leu-rich repeats (LRRs) (Dangl and Jones, 2001). At least one NB-LRR-type protein, Arabidopsis RPM1, is tethered to the plasma membrane, where it may encounter bacterial Avr proteins that are secreted into the plant cell (Boyes et al., 1998;Nimchuk et al., 2000). NB-LRR proteins fall into two subclasses based on their different N-terminal motifs. One group possesses an N-terminal coiled-coil (CC) domain. The second group has N-terminal similarity to the cytoplasmic Toll Interleukin-1 Receptor (TIR) domains of human and Drosophila Toll-like r...