Ben F. Holt

bZIP10-LSD1 antagonism modulates basal defense and cell death in Arabidopsis following infection

Plants use sophisticated strategies to balance responses to oxidative stress. Programmed cell death, including the hypersensitive response (HR) associated with successful pathogen recognition, is one cellular response regulated by reactive oxygen in various cellular contexts. The Arabidopsis basic leucine zipper (bZIP) transcription factor AtbZIP10 shuttles between the nucleus and the cytoplasm and binds consensus G‐ and C‐box DNA sequences. Surprisingly, AtbZIP10 can be retained outside the nucleus by LSD1, a protein that protects Arabidopsis cells from death in the face of oxidative stress signals. We demonstrate that AtbZIP10 is a positive mediator of the uncontrolled cell death observed in lsd1 mutants. AtbZIP10 and LSD1 act antagonistically in both pathogen‐induced HR and basal defense responses. LSD1 likely functions as a cellular hub, where its interaction with AtbZIP10 and additional, as yet unidentified, proteins contributes significantly to plant oxidative stress responses.

Tissue-specific expression patterns of arabidopsis NF-Y transcription factors suggest potential for extensive combinatorial complexity.

All aspects of plant and animal development are controlled by complex networks of transcription factors. Transcription factors are essential for converting signaling inputs, such as changes in daylength, into complex gene regulatory outputs. While some transcription factors control gene expression by binding to cis-regulatory elements as individual subunits, others function in a combinatorial fashion. How individual subunits of combinatorial transcription factors are spatially and temporally deployed (e.g. expression-level, posttranslational modifications and subcellular localization) has profound effects on their control of gene expression. In the model plant Arabidopsis (Arabidopsis thaliana), we have identified 36 Nuclear Factor Y (NF-Y) transcription factor subunits (10 NF-YA, 13 NF-YB, and 13 NF-YC subunits) that can theoretically combine to form 1,690 unique complexes. Individual plant subunits have functions in flowering time, embryo maturation, and meristem development, but how they combine to control these processes is unknown. To assist in the process of defining unique NF-Y complexes, we have created promoter:β-glucuronidase fusion lines for all 36 Arabidopsis genes. Here, we show NF-Y expression patterns inferred from these promoter:β-glucuronidase lines for roots, light- versus dark-grown seedlings, rosettes, and flowers. Additionally, we review the phylogenetic relationships and examine protein alignments for each NF-Y subunit family. The results are discussed with a special emphasis on potential roles for NF-Y subunits in photoperiod-controlled flowering time.

An evolutionarily conserved mediator of plant disease resistance gene function is required for normal Arabidopsis development.

Plants recognize many pathogens through the action of a diverse family of proteins called disease resistance (R) genes. The Arabidopsis R gene RPM1 encodes resistance to specific Pseudomonas syringae strains. We describe an RPM1-interacting protein that is an ortholog of TIP49a, previously shown to interact with the TATA binding protein (TBP) complex and to modulate c-myc- and beta-catenin-mediated signaling in animals. Reduction of Arabidopsis TIP49a (AtTIP49a) mRNA levels results in measurable increases of two R-dependent responses without constitutively activating defense responses, suggesting that AtTIP49a can act as a negative regulator of at least some R functions. Further, AtTIP49a is essential for both sporophyte and female gametophyte viability. Thus, regulators of R function overlap with essential modulators of plant development.

Resistance gene signaling in plants — complex similarities to animal innate immunity

During the past year several important publications have significantly enhanced our current understanding of plant disease resistance. Among the most important discoveries are the role of SGT1 in resistance (R) gene mediated defenses, mounting support for the so-called ‘guard hypothesis’ of R gene function, and providing evidence for intramolecular interactions within R proteins as a mode of signaling control. There are many emerging parallels between the plant R genes and animal innate immunity receptor complexes. Plant SGT1 shows similarity to co-chaperones of the animal Hsp90 complex, and many receptor-like R gene products appear to interact indirectly with their pathogen-derived signal. Considering these and other similarities, researchers from both fields should be looking carefully over each other’s shoulders.

Recognition of pathogens by plants

Biotic interactions. Curr Opin Plant Biol 1999, 2:255–338. Galan JE, Collmer A: Type III secretion machines: bacterial devices for protein delivery into host cells. Science 1999, 284:1322–1328.Jones DA, Jones JDG: The role of leucine-rich repeat proteins in plant defences. Adv Bot Res 1997, 24:89–167.Kobe B, Deisenhofer J: Mechanism of ribonuclease inhibition by ribonuclease inhibitor protein based on the crystal structure of its complex with ribonuclease A. J Mol Biol 1996, 264:1028–1043.Kajava AV: Structural diversity of leucine-rich repeat proteins. J Mol Biol 1998, 277:519–527.Michelmore RW, Meyers BC: Clusters of resistance genes in plants evolve by divergent selection and a birth-and-death process. Genome Res 1998, 8:1113–1130.