Proteomic and metabolomic insights into the mechanisms of calcium-mediated salt stress tolerance in hemp

Industrial hemp (Cannabis sativa L.) is a multifaced crop that has the potential to be exploited for many industrial applications, and making use of salt lands is considered to be a sustainable development strategy for the hemp industry. However, no elite salt-tolerant hemp varieties have been developed, and therefore supplementing appropriate exogenous substances to saline soil is one possible solution. Calcium-containing compounds are well-known for their salt tolerance enhancing effects, but the underlying molecular mechanisms remain largely unclear. Here, we first assessed the ameliorative effects of calcium amendments on salt-stressed hemp plants and then investigated these mechanisms on hemp using integrative analysis of proteomics and metabolomics. The stress phenotypes could be lessened by Ca²⁺ treatment. Certain concentrations of Ca²⁺ maintained relative electrical conductivity and the contents of malondialdehyde and chlorophyll. Ca²⁺ treatment also generally led to greater accumulations of soluble proteins, soluble carbohydrates and proline, and enhanced the activities of superoxide dismutase and peroxidase. Through functional classification, pathway enrichment, and network analysis, our data reveal that accumulation of dipeptides is a prominent metabolic signature upon exogenous Ca²⁺ treatment, and that changes in mitochondrial properties may play an important role in enhancing the salt tolerance. Our results outline the complex metabolic alternations involved in calcium-mediated salt stress resistance, and these data and analyses would be useful for future functional studies.