The proline-rich protein MdPRP6 confers tolerance to salt stress in transgenic apple (Malus domestica)

High salinity is a major environmental factor limiting plant growth and crop productivity. Proline-rich proteins (PRPs) are cell wall structural proteins involved in the response of plants to various stresses. Here, we showed that the transcript level and protein abundance of MdPRP6, a PRP,protein from Malus domestica, decreased under salt stress. We treated WT plants and MdPRP6-Ri plants, in which MdPRP6 was repressed via RNA interference, with 150 mM NaCl in a hydroponic system. The results showed that Ri plants under 150 mM NaCl treatments were taller and had more chlorophyll, less electrolyte leakage, and less malondialdehyde compared with wild -type (WT) plants. The photosynthetic performance was improved and the structure of the chloroplast was more complete in Ri plants than in WT plants. Ri plants also accumulated fewer reactive oxygen species (ROS) under salt stress; had higher activities of catalase, superoxide dismutase, and peroxidase; and had higher expression levels of related genes. Furthermore, Ri plants accumulated less Na+ and had lower Na+/K+ratios under salt stress. The proline and glutamate content increased in Ri plants, which might play might play an important role in the resistance of apple plants to salt stress. Overall, our results demonstrated that suppression of MdPRP6 enhanced tolerance to salinity stress in transgenic apple by preserving the structure of the chloroplast, reducing the accumulation of ROS and Na+, and promoting the accumulation of compatible osmolytes.