Sodium nitroprusside as a priming agent induces drought stress tolerance in Citrus

Priming is a process whereby exposure to a mild stress or specific chemical stimulus enhances plants' resilience to future biotic and abiotic stresses. Signalling molecules such as hydrogen peroxide (H₂O₂) and nitric oxide (NO) function as priming agents. In this study, Bitters (C22) citrus rootstock was treated with the NO donor sodium nitroprusside (SNP) and subjected to drought stress. Malondialdehyde (MDA) and H₂O₂ levels were measured to assess oxidative stress. Primed plants showed significantly higher tolerance to water scarcity than non-primed ones. RNA-seq analysis revealed that priming, followed by drought stress, regulated a broad spectrum of stress responses, enhancing the expression of genes involved in photosynthetic efficiency and antioxidant activity, reallocating energy, and reinforcing external barriers and xylem vessels. As concerns phytohormones, analysis of gene expression clearly indicated that auxin biosynthesis and signalling were activated, whereas those involving ethylene were repressed. Moreover, the application of weighted gene co-expression network analysis (WGCNA) enabled the identification of genes whose expression showed positive or negative correlations with the levels of MDA and/or H₂O₂. This study provides insights into the role of priming in improving Citrus adaptability to water scarcity and identifying molecular strategies and candidate genes to enhance drought tolerance. To our knowledge, this is the first study correlating transcriptomic data with priming-induced drought tolerance in Citrus.