γ-Aminobutyric acid enhances resistance in postharvest mango fruits to Colletotrichum gloeosporioides by activating related defense mechanisms

Abstract

γ-Aminobutyric acid (GABA), as a crucial signaling molecule, participates in a series of defense responses in postharvest fruits to fungal diseases. In this research, we evaluated the effect of exogenous GABA on Colletotrichum gloeosporioides in vitro, and assayed the changes of defense-related enzyme activities, gene relative expressions and secondary metabolite contents in ‘Guifei’ mango fruits after inoculation with C. gloeosporioides. The results showed that GABA had no direct antifungal effect on C. gloeosporioides in vitro, while GABA treatments at 0.4, 2.0 and 10.0 mM significantly inhibited the expansion of lesions in inoculated mango fruits during storage. On day-10 after inoculation, the lesion diameters of mango fruit treated with 0.4, 2.0 and 10 mM GABA were 26.00, 21.00 and 32.30 mm, respectively, which decreased by 42.22 %, 53.33 %, and 28.22 % compared to the control group. The results indicated that the GABA treatment with 2.0 mM had the best inhibitory effect on the expansion of lesion in inoculated mango fruit. Moreover, GABA treatment increased the relative expressions of genes related to phenylpropanoid metabolism and the activities of their corresponding enzymes, including phenylalanine ammonia lyase (PAL), cinnamate-4-hydroxylase (C4H), 4-coumarate coenzyme A ligase (4CL), peroxidase (POD) and polyphenol oxidase (PPO), and increased the levels of the secondary metabolites including total phenolics (TP), total flavonoids (TF), oligomeric proanthocyanidins (OPC) and lignin of mango fruits inoculated with C. gloeosporioides. GABA treatment also increased the activities of β-1, 3-glucanase (GLU) and chitinase (CHT), along with the expression of these corresponding genes in inoculated fruits. The results indicated that the phenylpropanoid metabolism and pathogenesis-related proteins played a crucial role in inducing mango fruit resistance to C. gloeosporioides by GABA treatment.