RESUMO
'Whangkeumbae' (Pyrus pyrifolia) is a variety of sand pear fruit well-known for its smooth surface and good taste. However, the fruit quality is adversely affected by postharvest ethylene production. Therefore, improving postharvest shelf life by regulating fruit senescence is critical to promoting the 'Whangkeumbae' fruit industry. Here, we investigated the effect of salicylic acid (SA) spray on fruit senescence in sand pears during room temperature shelf life. Exogenous SA reduced polyphenol oxidase (PPO) activity and malondialdehyde (MDA) content during room temperature shelf life. Additionally, SA effectively maintained the fruit skin coloration and increased the activity of antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). SA treatment inhibited PpPPO1 expression and upregulated PpSOD1, PpAPX6, and PpGST2 expression. Furthermore, SA application downregulated the expression of PpACO2, PpEIN3a, PpNCED1, and PpAOC2, while upregulating PpNPR-1, PpTAR2, and PpCOMT1 during room temperature shelf life. SA treatment also influenced cell wall metabolism and modification genes by inhibiting PpPG1, PpPME2, and PpCEL3 and inducing PpPGIP1 expression. Additionally, SA treatment affected sugar and acid metabolism genes and increased the expression of PpSPS1, PpSUS1, PpSOT1, PpTMT4, PpSWEET15, and PpcyNAD-MDH, but suppressed the expression of PpcyNADP-ME. The Pearson correlation analysis indicated that PPO activity and MDA content were positively correlated with the expression of PpPPO1, PpACO2, PpEIN3a, PpNCED1, PpAOC2, PpPG1, PpPME2, PpCEL3, and PpcyNDA-MDH. Conversely, these factors were negatively associated with the activities of SOD, POD, CAT, and APX, as well as the expression levels of PpSOD1, PpPOD1, PpCAT1, PpAPX6, PpGST2, PpNPR-1, PpTAR2, PpCOMT1, PpPGIP1, PpSPS1, PpSUS1, PpSOT1, PpTMT4, PpSWEET15, and PpcyNAD-MDH. Our results reveal that exogenous SA could delay fruit senescence in sand pear fruit by regulating various biochemical and molecular mechanisms and can be used to effectively extend fruit shelf life during room temperature storage. However, further research is necessary to determine whether the fruits sprayed with SA are suitable for direct human consumption.
RESUMO
Salicylic acid (SA) and ethylene (ET) are crucial fruit senescence hormones. SA inhibited ET biosynthesis. However, the mechanism of SA delaying fruit senescence is less known. ETHYLENE INSENSITIVE 3 (EIN3), a key positive switch in ET perception, functions as a transcriptional activator and binds to the primary ET response element that is present in the promoter of the ETHYLENE RESPONSE FACTOR1 gene. In this study, a gene encoding putative EIN3 protein was cloned from sand pear and designated as PpEIN3a. The deduced PpEIN3a contains a conserved EIN3 domain. The evolutionary analysis results indicated that PpEIN3a belonged to the EIN3 superfamily. Real-time quantitative PCR analysis revealed that the accumulation of PpEIN3a transcripts were detected in all tissues of this pear. Moreover, PpEIN3a expression was regulated during fruit development. Interestingly, the expression of PpEIN3a was downregulated by SA but upregulated by ET, auxin, and glucose. Additionally, the contents of free and conjugated SA were higher than those of the control after SA treatment. While the content of ET and auxin (indole-3-acetic acid, IAA) dramatically decreased after SA treatment compared with control during fruit senescence. The content of glucose increased when fruit were treated by SA for 12 h and then there were no differences between SA treatment and control fruit during the shelf life. SA also delayed the decrease in sand pear (Pyrus pyrifolia Nakai. 'Whangkeumbae') fruit firmness. The soluble solid content remained relatively stable between the SA treated and control fruits. This study showed that SA plays an antagonistic role toward ET, auxin, and glucose in regulating the expression of PpEIN3a to delay fruit senescence.