The tomato mitogen-activated protein kinase SlMPK1 is as a negative regulator of the high temperature stress response

High temperature (HT) stress is a major environmental stress that limits plant growth and development. Mitogen-activated protein kinase (MAPK) cascades play key roles in plant growth and stress signaling, but their involvement in the HT stress response are poorly understood. Here, we describe a 47-kDa MBP-phosphorylated protein (p47-MBPK) activated in tomato (Solanum lycopersicum) leaves under HT, and identify it as SlMPK1 by MS/MS analysis. Silencing of SlMPK1 in transgenic tomato plants resulted in enhanced tolerance to HT, while overexpression resulted in reduced tolerance. Proteomic analysis identified a set of proteins involved in antioxidant defense that are significantly more abundant in RNAi-SlMPK1 plants than non-transgenic plants under HT stress. RNAi-SlMPK1 plants also showed changes in membrane lipid peroxidation and antioxidant enzyme activities. Furthermore, using yeast two-hybrid (Y2H) screening, we identified a Serine-Proline-Rich protein Homolog, SlSPRH1, which interacts with SlMPK1 in yeast, plant cells, and in vitro. We demonstrate that SlMPK1 can directly phosphorylate SlSPRH1. Furthermore, the serine residue Ser-44 of SlSPRH1 is a crucial phosphorylation site in the SlMPK1-mediated antioxidant defense mechanism activated during HT stress. We also demonstrate that heterologous expression of SlSPRH1 in Arabidopsis led to a decrease in thermotolerance and lower antioxidant capacity. Taken together, our results suggest that SlMPK1 is a negative regulator of thermotolerance in tomato plants. SlMPK1 acts by regulating antioxidant defense, and its substrate SlSPRH1 is involved in this pathway.