Abstract：Despite the greatly agricultural importance of pepper (Capsicum annuum) and the ubiquitously involvement of MAPK (mitogen-activated protein kinases) cascades in a broad range of plant biological processes, the roles of MAPK cascades in pepper remain unclearly. In the present study, a MAPK of pepper, designated as CaMAPK9 (Accession no.: CA04g21490), was expressed and functionally characterized. The results showed that CaMAPK9 shared more than 95% deduced amino acid sequence identity with its homologues in other plant species including tobacco (Nicotiana tabacum), tomato (Solanum lycopersicum) and potato(Solanum tuberosum). By subcellular localization assay through transient overexpression of CaMAPK9-GFP (Green Fluorescent Protein) in leaves of Nicotiana benthamiana plants, CaMAPK9 was found to target to the nuclei. Total RNA was extracted from abiotic stresses imposed pepper plants, as salt, mannitol and foliar application of salicylic acid (SA), methyl jasmonate (MeJA) or abscisic acid (ABA) and used for first-stand cDNA synthesis. cDNA RT-PCR showed that CaMAPK9 was induced under salt or mannitol and foliar application of SA or ABA stresses but impeded by SA, showing that CaMAPK9 may be involved in salt stress pepper response. In addition, two T3 transgenic Arabidopsis lines were developed and the ectopic overexpression of CaMAPK9 was confirmed. The 2 transgenic lines consistently showed a enhanced tolerance to salt stress manifested by the higher germination rate and longer root length under salt stress compared to that of the wild type seeds of seedlings. The transcriptional expression of ABI5, an ABA maker gene which was responsive to signaling in plant under salt-stress tolerance, was higher in two transgenic Arabidopsis types that in wild plants under salt-stress. All these results collectively suggested that CaMAPK9 acted as a positive regulator in the response of pepper to salt stress possibly regulated by signaling mediated by ABA and JA. This study provides a theoretical basis for further research on the molecular breeding mechanism of salt stress response in pepper
