Genome-wide identification and comparative expression analysis of ascorbate peroxidase (APX) gene family in apple fruit under 1-methylcyclopropene (1-MCP) and ethephon treatments during ripening

This research determined the impacts of the ethylene antagonist 1-methylcyclopropene (1-MCP, 1 μL L−1) and ethephon (0.2 mmol L−1) on expression of ascorbate peroxidase (APX) genes in pulp and peel tissues of apple fruit (Malus × domestica Borkh.) during postharvest ripening at 20 ± 1 °C. The results suggested that application of 1-MCP enhanced AsA content and ratio of AsA/DHA, promoted APX, MDHAR and DHAR activities, and reduced contents of DHA and H2O2 in both tissues during ripening, while ethephon treatment had the opposite results. A total of 13 MdAPXs were identified to be expressed in both tissues. Subcellular localization prediction analysis showed that five APX proteins were localized in cytosol (MdAPX6, MdAPX9, MdAPX13, MdAPX14 and MdAPX21), three in peroxisomal tissue (MdAPX2, MdAPX4 and MdAPX5), three in chloroplast (MdAPX7, MdAPX12 and MdAPX19), one in plasma membrane (MdAPX16) and one in extracellular region/cell wall (MdAPX11). These identified MdAPXs were expressed in a tissue-specific manner during ripening. In peel, compared with the control, both treatments differentially reduced expression of MdAPX7, MdAPX2, MdAPX14, MdAPX12, MdAPX16, MdAPX21, MdAPX9 and MdAPX13 while promoted expression of MdAPX11 during ripening. Expression of MdAPX5 and MdAPX4 was reduced by ethephon application while differentially enhanced by 1-MCP application during ripening. Transcript levels of MdAPX19 and MdAPX6 were differentially induced by ethephon treatment while decreased by 1-MCP application. In pulp, transcript levels of MdAPX13, MdAPX6, MdAPX14 and MdAPX11 were differentially prompted by both treatments during storage. Expression of MdAPX16 and MdAPX9 was differentially reduced by ethephon treatment while up-regulated by 1-MCP treatment during storage. Transcript levels of MdAPX21, MdAPX4, MdAPX2 were differentially reduced by ethephon treatment while enhanced by 1-MCP treatment during the early stage of ripening. Transient overexpression of MdERF2 differentially mediated MdAPXs expression in both tissues during ripening. These results indicated that ethylene signal plays a regulatory role in MdAPXs expression and AsA-GSH cycle in peel and pulp of apple fruit during ripening.