Exogenous application and endogenous elevation of salicylic acid levels by overexpressing a salicylic acid-binding protein 2 gene enhance nZnO tolerance of tobacco plants

Salicylic acid (SA) plays vital roles in the stress response of plants. However, the relationship between SA signaling and plant tolerance to nZnO stress has not been reported. The effects of different sizes and concentrations of nZnO on tobacco plants grown in MS medium or soil were evaluated. The effects of exogenous and endogenous SA on the morphological and biochemical parameters of tobacco plants by exogenous application of SA and overexpression of LcSABP2 gene were also investigated in this study. The endogenous SA content and the expression of salicylic acid-binding protein 2 (SABP2) gene increased in tobacco individuals with nZnO treatment. Exogenously applied SA increased the germination rate of tobacco seeds, and enhanced the root length and chlorophyll content of tobacco seedlings. Overexpression of LcSABP2 gene in transgenic tobacco significantly increased SA content in plants with nZnO treatment. Compared to the wild type plants, transgenic tobacco with nZnO treatment showed higher chlorophyll content, net photosynthetic rate, related gene expression, activities of SOD, POD and CAT, and less contents of H2O2, O2− and MDA, which suggested that LcSABP2 might play important regulatory roles in plant resistance to nZnO toxicity by elevating the level of endogenous SA, promoting the action of photosynthetic system, and enhancing the activities of antioxidant enzymes. This study illustrated that exogenous and endogenous SA might all be involved in alleviating the toxicity of nZnO on plants.