Journal of Plant Physiology 163 (2006) 256—272 Gene-for-gene-mediated recognition of nuclear-targeted AvrBs3-like bacterial effector proteins Sebastian Schornack, Annett Meyer, Patrick Ro ¨mer, Tina Jordan, Thomas Lahaye à Institut fu ¨r Genetik, Martin-Luther-Universita¨t Halle-Wittenberg, 06099 Halle (Saale), Germany Received 9 November 2005; received in revised form 2 December 2005; accepted 2 December 2005 KEYWORDS AvrBs4; Guard model; Receptor–ligand model; Tetratricopeptide repeats; Transcriptional activation Summary Plant disease resistance (R) genes mediate specific recognition of pathogens via perception of cognate avirulence (avr) gene products. The numerous highly similar AvrBs3-like proteins from the bacterial genus Xanthomonas provide together with their corresponding R proteins a unique biological resource to dissect the molecular basis of recognition specificity. A central question in this context is if R proteins that mediate recognition of structurally similar Avr proteins are themselves functionally similar or rather dissimilar. The recent isolation of rice xa5, rice Xa27 and tomato Bs4, R genes that collectively mediate recognition of avrBs3-like genes, provides a first clue to the molecular mechanisms that plants employ to detect AvrBs3-like proteins. Their initial characterization suggests that these R proteins are structurally and functionally surprisingly diverge. This review summarizes the current knowledge on R-protein-mediated recognition of AvrBs3-like proteins and provides working models on how recognition is achieved at the molecular level. & 2005 Published by Elsevier GmbH. Introduction Plants have evolved efficient defense systems to ward off a phylogenetically broad diversity of pathogens including bacteria, fungi, oomycetes, nematodes, viruses and even insects (Dangl and Jones, 2001). Conceptually, we distinguish at present two major layers of resistance, that mediate protection against non-host and ARTICLE IN PRESS www.elsevier.de/jplph 0176-1617/$ - see front matter & 2005 Published by Elsevier GmbH. doi:10.1016/j.jplph.2005.12.001 Abbreviations: 35S, Cauliflower mosaic virus 35S promoter; AD, activation domain; Avr, avirulence; GTF, general transcrip- tion factor; HR, hypersensitive response; LRR, leucine-rich repeat; NB, nucleotide binding site; NLS, nuclear localization signal; PIC, preinitiation complex; PP5, protein phosphatase 5; R, resistance; TFIIA, transcription factor IIA; TPR, tetratrico- peptide repeats; Xcv , Xanthomonas campestris pv. vesicatoria; Xoo, Xanthomonas oryzae pv . oryzae à Corresponding author. Tel.: +49345 5526345; fax: +49 345 55 27151. E-mail address: thomas.lahaye@genetik.uni-halle.de (T. Lahaye).