Enhanced Tolerance to Chilling Stress in Tomato by Overexpression of a Mitogen-Activated Protein Kinase, SlMPK7

Low temperature is one of the most common environmental stresses affecting plant growth and agricultural production. The mitogen-activated protein kinase (MAPK) cascade plays a pivotal role in diverse signaling pathways related to plant development and stress responses. In this study, a novel MAPK gene, SlMPK7, in tomato (Solanum lycopersicum) belonging to group B MAPK was isolated and functionally characterized. QRT-PCR analysis revealed that transcription of the MAPK SlMPK7 in tomato leaves was triggered by abiotic stresses and exogenous signaling molecules. Moreover, hydrogen peroxide (H2O2) and Ca2+ mediated 4 degrees C-induced upregulated expression of SlMPK7 at the messenger RNA (mRNA) level. SlMPK7 was predominantly localized in the nucleus. Transgenic tomato over-expressing SlMPK7 accumulated less reactive oxygen species (ROS), more superoxide dismutase, peroxidase, catalase activities, more proline and soluble sugar contents, and more stress-responsive gene expression, leading to enhanced chilling stress tolerance compared with the wild-type plants. Collectively, these data demonstrate that SlMPK7 acts as a positive regulator in the response to chilling stress by regulating ROS homeostasis through activation of cellular antioxidant systems and modulating the transcription of stress-associated genes.