Physiological and Molecular Mechanisms of Nano-Hydroxyapatite (nHAP) in Regulating Chilling Tolerance of Cucumber Seedlings

Abstract

Xishuangbanna (XIS) cucumber (Cucumis sativus L. var. Xishuangbannesis Qi et Yuan) originated from low-altitude southwest China, and shows extremely cold sensitivity. nano-hydroxyapatite (nHAP) is the nanoparticulate form of hydroxyapatite (HAP) and exhibits different physicochemical properties from HAP. nHAP application retained free and bound water even at 24 h during the cold stress treatment, which is detected by Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI). Exposure to cold stress caused a chilling injury index of 73%, while a gradient concentration of nHAP reduced this index by 36.36%. In foliar application of nHAP, electrolyte leakage and MDA content were decreased by 53.87% and 52.23%% respectively; chlorophyll content increased by 43.96%; SOD, POD, and Soluble Protein content increased by 52.81%, 108.80%, and 38.29%, respectively. Soil application of nHAP showed the similar effect reducing chilling injury by 33.33%, but foliar application performed better than soil application with the concentration of 100 mg/L to be the best. Quantitative Real-Time PCR (qRT-PCR) revealed that, with the application of nHAP (1000 mg/L) in the cold treatment, upregulated the gene expression of Superoxide dismutase gene (CsCu/ZnSOD, CsMnSOD), and Major Facility Superfamily genes (CsSPX-MFS1, CsSPX-MFS2). We profiled the transcriptome changes in seedling leaves during the cold treatment and compared the differences between nHAP application and controls. Without nHAP application, cold stress resulted in a total of 6795 Differentially expressed genes (DEGs), which were functionally enriched in Plant Pathogen interaction and Plant Hormone Signal Transduction Pathways. In the case of nHAP application, cold stress only caused 776 DEGs, which were functionally enriched in Plant hormone signal transduction and galactose metabolism. These findings suggest that nHAP could improve cold tolerance of cucumber seedlings through boosting antioxidant activity and plant hormone signaling pathways.