Expression Analysis of Metacaspase (MC) Gene Family in Response to Ethylene Signal During Apple Fruit Ripening

To better understand the relationship between MdMCs and ethylene during apple fruit ripening. “Golden Delicious” apple fruit were treated with ethephon (0.2 mmol L⁻¹) and ethylene action inhibitor 1-methylcyclopropene (1-MCP) (1 µL L⁻¹) after harvest, and the ethylene-responsive transcription factor MdERF2 was transiently over-expressed in apple fruit at commercial maturity. Our results showed that climacteric peaks of respiration rate and ethylene production were advanced by ethephon treatment while delayed by application of 1-MCP. Ethephon treatment increased activity of MC, content of malondialdehyde (MDA), and relative electrical conductivity (REC) compared with controls during ripening, while 1-MCP treatment had the opposite effects. Expression of MdMC04, MdMC07, MdMC12, MdMC14, MdMC16, MdMC18, MdMC19, and MdMC20 was differentially induced by ethephon treatment, while their expression was generally downregulated by application of 1-MCP compared with controls during ripening. MdMC06 expression was induced at the late ripening stage by 1-MCP treatment but reduced by ethephon application during ripening compared to the control group. Transcript levels of MdMC01 and MdMC13 were promoted by both 1-MCP and ethephon treatments during ripening. In contrast, expression of MdMC17, MdMC05, MdMC02, MdMC21, MdMC10, and MdMC09 was differentially reduced after application of ethephon and 1-MCP during ripening compared with controls. Expression patterns of MdMCs varied in fruit transiently over-expressing MdERF2 during ripening. These results indicated that MdMCs expression was modulated by ethylene signal during apple ripening. Our findings would be useful for further unraveling the molecular mechanism of ethylene signaling in modulating MC genes during apple ripening.