A novel linear pentapeptide from Penicillium oxalicum and its antagonism against postharvest decay mold of citrus

Abstract:

[Background] Citrus is an important economic crop in rural areas of South China. Due to the influence of the postharvest fungi, a large number of fruits are rotted and deterioratedannually resulting in huge economic losses and seriously affecting the enthusiasm of citrus farmers. [Objective] The filamentous fungus SG-4 exhibited strong antagonism to the postharvest decay mold Penicillium citrinum and Penicillium digitatum. In order to figure out the fungicidal substance, here we performed biological identification and active ingredient analysis. [Methods] The internal transcribed spacer (ITS) of strain SG-4 nuclear ribosomal DNA was sequenced to identify the strain together with the morphology. The antifungal activities of the mycelium phase and culture liquid were traced and the main active substance was separated by alcohol precipitation and reverse high performance liquid chromatography (RP-HPLC). The structure was determined by high resolution mass spectrometry and nuclear magnetic resonance spectroscopy. The antimicrobial activity was evaluated both by Oxford cup method as in vitro and by inoculation test of citrus as in vivo. The content of the fungicidal substance in fermentation broth was determined by RP-HPLC. [Results] Molecular and morphological identification showed that the strain SG-4 was a Penicillium oxalicum. The active substance inhibiting postharvest decay mold of citrus in the aqueous extract was highly polar. Further studies showed that the substance was a novel linear alcohol-soluble pentapeptide, which contained three hydroxyl amino acids and was rich in methylation modification. The first amino acid was beta-aminobutyric acid (BABA), and the linear structure was O-Me-BABA-N-Me-Thr-N-Me-Val-Ser. The inhibitory effect to postharvest decay mold is significantly better than the positive control, and the initial content in the culture liquid got to 90 mg/L, reached the level of industrial production. [Conclusion] The new linear pentapeptide from P. oxalicum showed strong antagonism to citrus decay fungi. Since it is the first time to report both the structure and the activity at home and abroad, the pentapeptide can be developed as an antibiotic to inhibit fungi in agriculture and food industry in the future.