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NONEXPRESSOR OF PR GENES1 (NPR1) is a key regulator of the plant defense response known as systemic acquired resistance. Accumulation of the signal molecule salicylic acid (SA) leads to a change in intracellular redox potential, enabling NPR1 to enter the nucleus and interact with TGACG sequence-specific binding protein (TGA) transcription factors, which in turn bind to SA-responsive elements in the promoters of defense genes. Here, we show that two NPR1-like genes, BLADE-ON-PETIOLE1 (BOP1) and BOP2, function redundantly to control growth asymmetry, an important aspect of patterning in leaves and flowers. Phenotypes in the double mutant include leafy petioles, loss of floral organ abscission, and asymmetric flowers subtended by a bract. We demonstrate that BOP2 is localized to both the nucleus and the cytoplasm, but unlike NPR1, it is highly expressed in young floral meristems and in yeast interacts preferentially with the TGA transcription factor encoded by PERIANTHIA (PAN). In support of a biological relevance for this interaction, we show that bop1 bop2 and pan mutants share a pentamerous arrangement of first whorl floral organs, a patterning defect that is retained in bop1 bop2 pan triple mutants. Our data provide evidence that BOP proteins control patterning via direct interactions with TGA transcription factors and demonstrate that a signaling mechanism similar to that formally associated with plant defense is likely used for the control of developmental patterning. [ABSTRACT FROM AUTHOR]

Arabidopsis nonexpresser of pathogenesis-related (PR) genes (NPR1) is the sole positive regulator that has been shown to be essential for the induction of systemic acquired resistance. In npr1 mutant plants, salicylic acid (SA)-mediated PR gene expression and pathogen resistance are abolished completely. NPR1 has been shown to interact with three closely related TGA transcription factors--TGA2, TGA5, and TGA6--in yeast two-hybrid assays. To elucidate the biological functions of these three TGA transcription factors, we analyzed single and combined deletion knockout mutants of TGA2, TGA5, and TGA6 for SA-induced PR gene expression and pathogen resistance. Induction of PR gene expression and pathogen resistance by the SA analog 2,6-dichloroisonicotinic acid (INA) was blocked in tga6-1 tga2-1 tga5-1 but not in tga6-1 or tga2-1 tga5-1 plants. Loss of INA-induced resistance to Peronospora parasitica Noco2 cosegregated with the tga6-1 mutation in progeny of multiple lines that were heterozygous for tga6-1 and homozygous for tga2-1 tga5-1 and could be complemented by genomic clones of wild-type TGA2 or TGA5, indicating that TGA2, TGA5, and TGA6 encode redundant and essential functions in the positive regulation of systemic acquired resistance. In addition, tga6-1 tga2-1 tga5-1 plants had reduced tolerance to high levels of SA and accumulated higher basal levels of PR-1 under noninducing conditions, suggesting that these TGA factors also are important for SA tolerance and the negative regulation of the basal expression of PR-I. [ABSTRACT FROM AUTHOR]

Plants have evolved sophisticated defense mechanisms against pathogen infections, during which resistance (R) genes play central roles in recognizing pathogens and initiating defense cascades. Most of the cloned R genes share two common domains: the central domain, which encodes a nucleotide binding adaptor shared by APAF-1, certain R proteins, and CED-4 (NB-ARC), plus a C-terminal region that encodes Leu-rich repeats (LRR). In Arabidopsis, a dominant mutant, suppressor of npr1-1, constitutive I (snc1), was identified previously that constitutively expresses pathogenesis-related (PR) genes and resistance against both Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2. The snc1 mutation was mapped to the RPP4 cluster. In snc1, one of the TIR-NB-LRR-type R genes contains a point mutation that results in a single amino acid change from Glu to Lys in the region between NB-ARC and LRR. Deletions of this R gene in snc1 reverted the plants to wild-type morphology and completely abolished constitutive PR gene expression and disease resistance. The constitutive activation of the defense responses was not the result of the overexpression of the R gene, because its expression level was not altered in snc1. Our data suggest that the point mutation in snc1 renders the R gene constitutively active without interaction with pathogens. To analyze signal transduction pathways downstream of snc1, epistasis analyses between snc1 and pad4-1 or eds5-3 were performed. Although the resistance signaling in snc1 was fully dependent on PAD4, it was only partially affected by blocking salicylic acid (SA) synthesis, suggesting... [ABSTRACT FROM AUTHOR]

Plant immune responses depend on dynamic signaling events across the nuclear envelope through nuclear pores. Nuclear accumulation of certain resistance (R) proteins and downstream signal transducers are critical for their functions, but it is not understood how these processes are controlled. Here, we report the identification, cloning, and analysis of Arabidopsis thaliana modifier of snc1, 7 (mos7-1), a partial loss-of-function mutation that suppresses immune responses conditioned by the autoactivated R protein sncl (for suppressor of npr1-1, constitutive 1). mos7-1 single mutant plants exhibit defects in basal and R protein-mediated immunity and in systemic acquired resistance but do not display obvious pleiotropic defects in development, salt tolerance, or plant hormone responses. MOST is homologous to human and Drosophila melanogaster nucleoporin Nup88 and resides at the nuclear envelope. In animals, Nup88 attenuates nuclear export of activated NF-κB transcription factors, resulting in nuclear accumulation of NF-κB. Our analysis shows that nuclear accumulation of sncl and the defense signaling components Enhanced Disease Susceptibility 1 and Nonexpresser of PR genes 1 is significantly reduced in mos7-1 plants, while nuclear retention of other tested proteins is unaffected. The data suggest that specifically modulating the nuclear concentrations of certain defense proteins regulates defense outputs. [ABSTRACT FROM AUTHOR]

Presents a study the revealed the effects of the isolation and characterization of phyCmutants in arabidopsis on phytochrome signaling pathways. Background on arabidopsis; Control of flowering in arabidopsis by phyC; Differential crosstalk among phytochromes in the photoregulation of multiple processes throughout the plant's life cycle.