624 Transcriptional activation of plant defense genes Qun Zhu*, Wolfgang Dr6ge-Laser?, Richard A Dixont and Chris Lambs Significant progress has been made in the characterization of disease resistance genes and receptors for pathogen avirulence signals and non-specific elicitors. Some components involved in elicitor-induced signal transduction have been identified. Phosphorylation of transcription factors has been found to be one of the mechanisms regulating their cellular localization, DNA binding and transcription activities for defense gene activation. Addresses *gPlant Biology Laboratory, Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, California 92037, USA tBiologie VI (Genetik), Universitlt Bielefeld, Postfach 100131, D-33501 Bielefeld, Germany $Plant Biology Division, Samuel Roberts Noble Foundation, 2510 Sam Noble Parkway, Ardmore, Oklahoma 73402, USA Correspondence: Chris Lamb, e-mail: chrislamb@qm.salk.edu Current Opinion in Genetics & Development 1996, 6:624-630 0 Current Biology Ltd ISSN 0959-437X Abbreviations avr pathogen avirulence gene CRER C-rich elicitor-responsive element f. sp. forma specialis HR hypersensitive response LRRs leucine-rich repeats MAP mitogen-activated protein PAL phenylalanine ammonia-lyase PKC protein kinase C pm9 Phytophthora megasperma gfycinea PR pathogenesis-related R disease resistance gene SA salicylic acid SAR systemic acquired resistance Introduction Plants exhibit natural resistance to pathogen attack by a combination of constitutive and induced defenses [l]. According to the gene-for-gene and elicitor-receptor mod- els [2,3*], induced defense is the outcome of molecular recognition through ligand : receptor interactions specified by the direct or indirect products of paired plant disease resistance (R) genes and pathogen avirulence (avr) genes. Attempted infection of a plant by an avirulent pathogen or treatment of plant cells with pathogen-derived elicitors triggers the activation of induced defense responses, including reinforcement of cell walls, synthesis of anti- biotics, production of pathogenesis-related (PR) proteins, and rapid generation of oxidants, often accompanied by programmed cell death of challenged cells in the so-called hypersensitive response (HR) at the infection site [1,3’]. Within this overall syndrome, the battery of the induced responses may vary between different R and av~ gene combinations [4”]. With the exception of callose production and oxidative responses, including cross-linking of cell wall structural proteins and activation of programmed cell death, most of these defense responses depend on the transcriptional activation of a large set of plant defense genes [5]. In a compatible interaction with a virulent pathogen, no HR occurs, induction of other defense responses is slower and/or weaker, and disease develops [1,3*]. In this review, we focus on the mechanisms underlying activation of transcription-dependent defenses including elicitor-responsive and pathogen-responsive elements, and trans factors as targets for specific recognition signals. Transcription of plant defense genes There is a temporal and spatial hierarchy for defense gene activation, with some genes exhibiting rapid localized activation at the site of attempted attack, whereas some acidic PR-proteins and acidic peroxidase undergo slower activation, both locally and then systemically throughout the plant [6,7*]. Prominent among the class of locally induced defense genes are those encoding enzymes of phenylpropanoid biosynthesis that are involved in the syn- thesis of lignin precursors and a number of phytoalexins [8*]. The early activation of phenylpropanoid biosynthetic genes is correlated with the expression of hypersensitive resistance in tissues when challenged with avirulent pathogens or non-pathogens [7*,8’]; in parsley and bean cell suspension cultures, transcription of these genes is activated transiently within lo-20 minutes of elicitor treatment, with maximum rates of transcription after -1 hour and maximum accumulation of transcripts after 3-4 hours [9-111. Recent studies show that the activation kinetics of each gene within such a functionally related battery is controlled precisely. In elicited alfalfa cells, thus, genes encoding phenylalanine ammonia-lyase (PAL) and chalcone synthase are activated rapidly, whereas cin- namic acid 4-hydroxylase, chalcone isomerase, isoflavone reductase and caffeic acid 3-0-methyltransferase genes are activated more slowly and transcription of chalcone 2’-0-methyltransferase is delayed [12*]. In parsley cells the activation of defense genes in response to elicitors is concomitant with the repression of cell cycle related genes [13*], suggesting that arrest of cell division may be required for full commitment of these cells to defense gene transcription. PAL is the key enzyme controlling metabolic flux into the phenylpropanoid pathway [14]. In addition to the synthesis of lignin monomers and phytoalexin antibiotics, a major function of PAL is the production of salicylic acid (SA), required, for example, for resistance of Arabidopsis to the fungal pathogen Peronospora parasitka [15”]. SA