PeMPK7 is induced in an ROS-dependent manner and confers poplar para-hydroxybenzoic acid stress resistance through the removal of ROS

The continuous cropping of poplar plantations will accumulate allelochemicals such as para-hydroxybenzoic acid (pHBA) in the soil, which restricts the growth and biomass of poplar. Mitogen-activated protein kinase (MAPK) cascades are widely involved in environmental stress responses in plants. However, little is known about the role of MAPK cascade genes in the response to pHBA stress until now. Here, a novel group C MAPK gene, PeMPK7, from Populus x euramericana was characterized. The PeMPK7 transcript was induced by exogenous pHBA and H2O2, and its induced transcription by pHBA was blocked by the reactive oxygen species (ROS) manipulators. Overexpression of PeMPK7 in poplar alleviated the phenotypic injury induced by pHBA stress, reduced H2O2 accumulation and alleviated the oxidative injury in leaf cells. The expression of the encoding genes and enzyme activity of the antioxidant enzymes (superoxide dismutase, peroxidase, and catalase) were upregulated in transgenic poplars under pHBA conditions, which will help to remove excessive ROS. In addition, the enhanced soluble sugar and free proline in transgenic poplar seedlings may contribute to maintaining osmotic homeostasis under pHBA stress. Furthermore, the interaction between PeMPKK3 and PeMPK7 was confirmed. A schematic model was proposed to show the regulatory mechanism of PeMPK7 in response to pHBA stress.