Mechanism Study Of Exogenous γ-Aminobutyric Acid And Melatonin Alleviates Chilling Injury In Postharvest Peach Fruit

Peach fruit(Prunus Persica), as a typical tropical fruit, quickly reaches the peak of respiration and ethylene after harvest at room temperature, and thus accelerates fruit senescence and deterioration. Cold storage is one of the most widely used technologies to retard respiration and other metabolic processes and then extend harvested life of peach fruit. However, the fruit is very sensitive to chilling injury(CI) when stored at temperatures below 10 ℃. The symptoms of CI include internal browning and flesh mealiness, which shortens storage life and reduces consumer acceptance. Therefore, the better understanding of physiological response to low temperature stress of harvested peach fruit and developing techniques to induce chilling tolerance is in urgent demand for its storage and transportation. In this paper, the effects of exogenous GABA, 1-MCP and melatonin treatment on proline, polyamines, endogenous GABA content and gene expression of their metabolism key enzymes in peach fruit were investigated to reveal the mechanism of exogenous GABA, 1-MCP and melatonin improving cold resistance in postharvest peach fruit. The results are as follows:(1) Full-length c DNAs of Δ1-pyrroline-5-carboxylate synthetase(Pp P5CS)ornithine aminotransferase(Pp OAT), glutamate decarboxylase(Pp GAD) and polyamine oxidase(Pp PAO) were isolated from peach fruit using degenerate RT-PCR and RACE(rapid amplification of c DNA ends) method, and amino acid sequence encoded by them were analyzed by bioinformatics. Phylogenetic analysis showed that both of Pp OAT and Pp P5 CS had the highest homology with Malus hupehensis, which might be the direct homologous protein; both of Pp GAD and Pp PAO had the highest homology with Malus domestica, which might be the direct homologous protein. Domains and multiple sequence alignment analysis showed Pp OAT belonged to pyridoxal phosphate dependent transaminases, and the primary structure included a non-conserved N-terminal, PLP-binding domain and a C-terminal; N-terminal assumed the existence of a non-conserved sequence of mitochondrial transit peptide, PLP-binding domain playeda key role in the catalytic converter ammonia process. N-terminal of Pp P5 CS polypeptide was amino acid kinase superfamily G5 K domain, C-terminal was C-glutamate phosphate reductase domain of aldehyde dehydrogenase superfamily,P5 CS conserved protein domain comprised five main functional domains: conserved Glu-5-kinase, conserved GSA-DH domain, a hypothetical leucine conserved domains and ATP binding site and NADPH binding site. Pp GAD belonged to transaminase superfamily, which is a pyridoxal phosphate-dependent protein, including conserved regions and active sites, and the consensus motif for the binding of pyridoxal5’-phosphate and the C-terminal peptide extension; Pp PAO might be a model of spanning multiple domains, and did not belong to any family domains.(2) Exogenous GABA treatment(5 mmol/L) significantly inhibited the decline of extractable juice, and alleviated chilling injury in peach fruit during cold storage. In addition, GABA treatment could increase proline biosynthesis and accumulation by up-regulating the expression levels of Pp OAT and Pp P5 CS and inhibiting Pp PDH expression. Meanwhile, exogenous GABA treatment could maintain higher levels of endogenous polyamines such as Put, Spd and Spm and GABA contents in postharvest peach fruit stored at 0 ℃ by enhancing the expression levels of Pp ODC and Pp ADC and inhibiting the transcripts of Pp PAO, which was contributed to improve the chilling tolerance against low temperature. However, 1-MCP treatment(10 μL/L) decreased endogenous GABA and proline content by inhibiting the expression of Pp OAT and Pp GAD and up-regulating expression of Pp PDH, but had no significant impact on Pp P5 CS expression; Whereas the increase of Pp PAO expression promoted the degradation of polyamines, which might result in the decrease of Spd and Spm content;but 1-MCP treatment had no significant effect on the expression of Pp ADC and Pp ODC and Put content. The results suggest that 1-MCP treatment could enhance chilling injury incidence in postharvest peach fruit, which still need further study to confirm this conclusion.(3) The effect of exogenous melatonin on chilling injury in peach fruit after harvest was investigated. To explore the optimum concentration of melatonin for chilling tolerance induction, peach fruit were treated with 50, 100 or 200 μmol/L melatonin for120 min and then stored for 28 d at 4 °C. The results showed that application of melatonin at 100 μmol/L was most effective in reducing chilling injury of peach fruit after harvest. Peaches treated with melatonin at this concentration displayed higherlevels of extractable juice rate and total soluble solids than the non-treated ones. In addition, melatonin treatment enhanced expression of Pp ADC, Pp ODC and Pp GAD and consequently increased polyamines and γ-aminobutyric acid(GABA) contents.Meanwhile, the up-regulated transcripts of Pp ADC and Pp ODC and inhibited Pp PDH expression resulted into the higher proline content in melatonin-treated fruit as compared to the control fruit. Our results revealed that melatonin treatment may be a useful technique to alleviate chilling injury in cold-stored peach fruit. The chilling tolerance of harvested peaches induced by melatonin treatment is associated with higher levels of polyamine, GABA and proline.