Transcription factors of the NAC family are known to be involved in various growth or developmental processes and in regulation of response to environmental stresses. In the present study, we report that Arabidopsis ATAF1 is a negative regulator of defense responses against both necrotrophic fungal and bacterial pathogens. Expression of ATAF1 was downregulated after infection with Botrytis cinerea or Pseudomonas syringae pv. tomato or after treatment with salicylic acid (SA), jasmonic acid, and 1-amino cyclopropane-1-carboxylic acid (the precursor of ethylene biosynthesis). Transgenic plants that overexpress the ATAF1 gene (ATAF1-OE) showed increased susceptibility while expression of an ATAF1 chimeric repressor construct (ATAF1-SRDX) exhibited enhanced resistance to P. syringae pv. tomato DC3000, B. cinerea, and Alternaria brassicicola. The ataf1 mutant plants showed no significant resistance against the pathogens tested. After inoculation with B. cinerea or P. syringae pv. tomato DC3000, expressions of defense-related genes PR-1, PR-5. and PDF1.2 were upregulated in the ATAF1-SRDX plants but attenuated or unchanged in the ATAF1-OE plants. In ATAF1-OE plants, SA-induced expression of pathogenesis-related genes and disease resistance against P. syringae pv. tomato DC3000 was partially suppressed. Increased levels of reactive oxygen species (i.e., H(2)O(2) and superoxide anion) accumulated only in the ATAF1-OE but not in the ATAF1-SRDX plants after Botrytis spp. infection. Our studies provide direct genetic evidence for the role of ATAF1 as a negative regulator of defense response against different type of pathogens.

RING finger proteins comprise a large family and play key roles in regulating growth/developmental processes, hormone signaling and responses to biotic and abiotic stresses in plants. A rice gene, OsBIRF1, encoding a putative RING-H2 finger protein, was cloned and identified. OsBIRF1 encodes a 396 amino acid protein belonging to the ATL family characterized by a conserved RING-H2 finger domain (C-X2-C-X15-C-X1-H-X2-H-X2-C-X10-C-X2-C), a transmembrane domain at the N-terminal, a basic amino acid rich region and a characteristic GLD region. Expression of OsBIRF1 was up-regulated in rice seedlings after treatment with benzothaidiazole, salicylic acid, l-aminocyclopropane-1-carboxylic acid and jasmonic acid, and was induced differentially in incompatible but not compatible interactions between rice and Magnaporthe grisea, the causal agent of blast disease. Transgenic tobacco plants that constitutively express OsBIRF1 exhibit enhanced disease resistance against tobacco mosaic virus and Pseudomonas syringae pv. tabaci and elevated expression levels of defense-related genes, e.g. PR-1, PR-2, PR-3 and PR-5. The OsBIRF1-overexpressing transgenic tobacco plants show increased oxidative stress tolerance to exogenous treatment with methyl viologen and H2O2, and up-regulate expression of oxidative stress-related genes. Reduced ABA sensitivity in root elongation and increased drought tolerance in seed germination were also observed in OsBIRF1 transgenic tobacco plants. Furthermore, the transgenic tobacco plants show longer roots and higher plant heights as compared with the wild-type plants, suggesting that overexpression of OsBIRF1 promote plant growth. These results demonstrate that OsBIRF1 has pleiotropic effects on growth and defense response against multiple abiotic and biotic stresses.

Programmed cell death (PCD) is a precise, genetically controlled cellular process with important roles in plant growth, development, and response to biotic and abiotic stress. However, the genetic mechanisms that control PCD in plants are unclear. Two Arabidopsis genes, DAL1 and DAL2 (for Drosophila DIAP1 like 1 and 2), encoding RING finger proteins with homology to DIAP1 were identified, and a series of experiments were performed to elucidate their roles in the regulation of PCD and disease resistance. Expression of DAL1 and DAL2 genes was induced in Arabidopsis plants after inoculation with virulent and avirulent strains of Pseudomonas syrinage pv. tomato (Pst) DC3000 or after infiltration with fumonisin B1 (FB1). Plants with mutations in the DAL1 and DAL2 genes displayed more severe disease after inoculation with an avirulent strain of Pst DC3000, but they showed similar disease severity as the wild-type plant after inoculation with a virulent strain of Pst DC3000. Significant accumulations of reactive oxygen species (ROS) and increased cell death were observed in the dal1 and dal2 mutant plants after inoculation with the avirulent strain of Pst DC3000. The dal mutant plants underwent extensive PCD upon infiltration of FB1 and displayed higher levels of ROS accumulation, callose deposition, and autofluorescence than the wild-type plants. Our data suggest that DAL1 and DAL2 may act as negative regulators of PCD in Arabidopsis.