Expression of the hypersensitive response-assisting protein in Arabidopsis results in harpin-dependent hypersensitive cell death in response to Erwinia carotovora Ajay-Kumar Pandey 1 , Mang-Jye Ger 2 , Hsiang-En Huang 1 , Mei-Kuen Yip 1 , Jiqing Zeng 1 and Teng-Yung Feng 1, * 1 Institute of Plant and Microbial Biology, Academia Sinica, 115, Taipei, Taiwan (*author for correspondence; e-mail mpmi224@gate.sinica.edu.tw); 2 Department of Life Science National University of Kaohsiung, 811, Kaohsiung, Taiwan Received 9 November 2004; accepted in revised form 15 July 2005 Key words: Arabidopsis thaliana, Erwinia carotovora, harpin, HRAP, hypersensitive response Abstract Active defense mechanisms of plants against pathogens often include a rapid plant cell death known as the hypersensitive cell death (HCD). Hypersensitive response-assisting protein (HRAP) isolated from sweet pepper intensifies the harpin Pss -mediated HCD. Here we demonstrate that constitutive expression of the hrap gene in Arabidopsis results in an enhanced disease resistance towards soft rot pathogen, E. carotovora subsp. carotovora. This resistance was due to the induction of HCD since different HCD markers viz. Athsr3, Athsr4, ion leakage, H 2 O 2 and protein kinase were induced. One of the elicitor harpin proteins, HrpN, from Erwinia carotovora subsp. carotovora was able to induce a stronger HCD in hrap-Arabidopsis than non-transgenic controls. To elucidate the role of HrpN, we used E. carotovora subsp. carotovora defective in HrpN production. The hrpN ) mutant did not induce disease resistance or HCD markers in hrap-Arabidopsis. These results imply that the disease resistance of hrap-Arabidopsis against a virulent pathogen is harpin dependent. Abbreviations: ATP, adenosine tri-phosphate; DTT, dithiothreitol; EGTA, Ethylene glycol-bis(2-amino- ethylether)-N,N,N¢,N¢-tetraacetic acid; HRAP, hypersensitive response-assisting protein; HCD, hypersen- sitive cell death; MAPK, mitogen activated protein kinase; PCR, polymerase chain reaction; SDS-PAGE, sodium dodecyl sulphate-polyacrylamide gel electrophoresis Introduction Plants use several defense pathways to with- stand pathogen attack. The most powerful defense system of plants against invading pathogens is the hypersensitive cell death (HCD). The HCD is a highly concerted complex defense response that results in local accumulation of pathogenesis related (PR) proteins, deposition of callose and lignin in the cell wall near the infection site, and rapid death of plant cells at the site of infection (Dangl et al., 1996; Lamb and Dixon, 1997). These events limit the nutrient availability to the pathogens and thereby restrict the spread of the pathogens (Dixon and Harrison, 1990). The HCD also triggers the accumulation of signal molecules such as jasmonic acid, salicylic acid, protein kinases, activation of transcription factors and degradation of proteins by the polyubiquitin sys- tem (Dangl et al., 1996). The HCD process is often characterized by induction of molecular HCD markers like Athsr (Lacomme and Roby, 1999) in Plant Molecular Biology (2005) 59:771–780 Ó Springer 2005 DOI 10.1007/s11103-005-1002-3