Defense Response in Plants Induced by HarpinXoo，An Elicitor of Hypersensitive Response From Xanthomonas oryzae pv. oryzae
Wen Weigang Shao Min Chen Gongyou Wang Jinsheng **
（Key Laboratory of Monitoring and Management of Plant Diseases and Insects, Ministry of Agriculture, College of Plant Protection, Nanjing Agricultural University, Nanjing 210095, China）
Abstract：Abstract: Harpin proteins with the elicitor activity are encoded by particular genes in hrp gene clusters of Gram negtive plant pathogenic bacteria. As HarpinEa of Erwinia amylovora, HarpinXoo (15.6 kD) is encoded by gene hrfAXoo (420 bp) positioned in hrp cluster of Xanthomonas oryzae pv. oryzae and shows the ability for induction of hypersensitive reaction of plants. HarpinXoo could induce plant resistance against some diseases and the relevant mediated mechanisms by the expressed product of hrfAxoo. HarpinXoo was expressed in recombinant bacteria E.coli BLHR4/pHRF4(pET30a(+)::hrfAxoo) and the protein expression was at peak level after IPTG induction for 16 h. After applying HarpinXoo to tobacco (Nicotiana tabacum L), rape (Brassica campestris L.) and tomato (Lycopersicon esculentum Mill.), the resistance was induced with remarkable control effects to tobacco mosaic virus (TMV), sclerotinia stem blight of rape (Sclirotinia sclerotiorum (Lib.) de Bary) and tomato gray mold rot (Botrytis cinerea Pers.) respectively. Comparing with water treatment control, 66.5% for TMV control efficiency, 79.39% for contol of Sclerotinia sclerotiarum of rape and more than 75% for control of Botrytis fruit rot of tomato were achieved. Biochemical and molecular technologies were used to reveal resistance mechanisms in tobacco (Xanthi) as model plant. The activities of defense-related enzymes involving phenylalanine ammonia lyase (PAL), peroxidase (PO) and polyphenoloxidase (PPO) were determined in tobacco leaves treated with HarpinXoo and the result showed that their activites were 110.1%, 211.2% and 273.0% higher than that treated with water control respectively. Two pathogenesis-related genes PR-1b and PR-1a were expressed conspicuously at 24 h after induction with HarpinXoo and showed the highest level of expression at 36 h after treatments by RT-PCR methodology. These results indicated that HarpinXoo similar to HarpinEa could induce the resistance of plants by various signal transduction pathways.
