Sweating treatment enhances citrus fruit disease resistance by inducing the accumulation of amino acids and salicylic acid-induced resistance pathway

To clarify the mechanism of fruit disease resistance activated by sweating treatment, Guoqing NO.1' Satsuma mandarin (Citrus unshiu Marc.) fruits were treated by sweating, which is a traditional prestorage treatment in China. Subsequently, we performed inoculation and physiological characterization, two-dimensional gel electrophoresis (2-DE) proteomics analysis and metabonomics analysis based on gas chromatography coupled to mass spectrometry (GC-MS) and high-performance liquid chromatography/electrospray ionization-time of flight-mass spectrometry (HPLC-qTOF-MS). The results showed that sweating treatment significantly inhibited pathogen infection without negatively affecting the fruit commercial quality. In addition, sweating treatment rapidly promoted the accumulation of amino acids (such as proline and serine). Meanwhile, hydrogen peroxide (H2O2) and salicylic acid (SA) were significantly accumulated in the sweating-treated fruit. Thereafter, some stress-response proteins and metabolites [such as ascorbate peroxidase (APX), -1,3-glucanase, vanillic acid and rutin] which can be induced by SA were also significantly increased in the sweating-treated fruit. Taken together, the disease resistance induced by sweating treatment might be attributed to: (1) the induction of the accumulation of amino acids; and (2) the accumulation of SA and subsequent activation of SA-induced resistance pathway, which can induce the stress-response proteins and metabolites that can directly inhibit pathogen development.