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Transcriptional repression of pathogen defense-related genes is essential for plant growth and development. Several proteins are known to be involved in the transcriptional regulation of plant defense responses. However, mechanisms by which expression of defense-related genes are regulated by repressor proteins are poorly characterized. Here, we describe the in planta function of CBNAC, a calmodulin-regulated NAC transcriptional repressor in Arabidopsis. A T-DNA insertional mutant (cbnac1) displayed enhanced resistance to a virulent strain of the bacterial pathogen Pseudomonas syringae DC3000 (PstDC3000), whereas resistance was reduced in transgenic CBNAC overexpression lines. The observed changes in disease resistance were correlated with alterations in pathogenesis-related protein 1 (PR1) gene expression. CBNAC bound directly to the PR1 promoter. SNI1 (suppressor of nonexpressor of PR genes1, inducible 1) was identified as a CBNAC-binding protein. Basal resistance to PstDC3000 and derepression of PR1 expression was greater in the cbnac1 sni1 double mutant than in either cbnac1 or sni1 mutants. SNI1 enhanced binding of CBNAC to its cognate PR1 promoter element. CBNAC and SNI1 are hypothesized to work as repressor proteins in the cooperative suppression of plant basal defense.

The mitogen-activated protein kinases (MAPKs) are key signal transduction molecules, which respond to various external stimuli. The MAPK phosphatases (MKPs) are known to be negative regulators of MAPKs in eukaryotes. We screened an Arabidopsis cDNA library using horseradish peroxidase-conjugated calmodulin (CaM), and isolated AtMKP1 as a CaM-binding protein. (MEKK). The MAPKs are activated by MEK via the phosphorylation of threonine and tyrosine residues within a conserved TXY motif (14, 15). MAPKs are inactivated by protein phosphatases such as tyrosine-specific phosphatase, serine/threonine-specific phosphatase, and dual specificity phosphatase, through specific dephosphorylation of threonine and tyrosine residues within the TXY consensus sequence (16, 17). MAPK phosphatases (MKPs) belong to the dual specificity phosphatase (DsPTP) family and dephosphorylate both phosphotyrosine and phosphoserine/phosphothreonine residues (18). Only five members of the MKPs, including the previously described AtDsPTP1, IBR5, PHS1, and AtMKP1, are predicted to exist in the Arabidopsis genome, and these five MKPs are responsible for the modulation of 20 potential MAPKs (18,19). The MKP/MAPK ratio in Arabidopsis is disproportionately low compared with other eukaryotes, implying that plants may possess integrated and complicated mechanisms for the regulation of MAPK signaling through MKPs.Plant MKPs play important roles in various biotic and abiotic stress responses. AtMKP1 was reported to be involved in genotoxic stress signaling (20) and AtMKP2 in the regulation of the cellular response to oxidative stress in Arabidopsis (21).