Transcription Factor CpbHLH3 and CpXYN1 Gene Cooperatively Regulate Fruit Texture and Counteract 1-Methylcyclopropene Inhibition of Softening in Postharvest Papaya (Carica papaya L.)

Papaya (Carica papaya L.) is a climacteric fruit susceptible to postharvest losses attributable to ethylene-induced ripening and softening. In this study, we examined the effect of 1-methylcyclopropene (1-MCP) treatment (1 μL L-1 for 20 h) on the textural properties of "SunUp" papaya fruit and investigated the regulatory mechanisms of molecular profiles. Compared with control, postharvest 1-MCP treatment significantly inhibited fruit softening, which is associated with higher hemicellulose content and lower xylanase activity of papaya fruit. Moreover, RNA-seq and qRT-PCR analyses indicated that CpbHLH3 and CpXYN1 were differentially expressed during storage. Yeast one-hybrid, electrophoretic mobility shift assays, and dual-luciferase reporter assays disclosed that CpbHLH3 activated the transcription of CpXYN1 by binding directly to its promoter. Transient overexpression of CpbHLH3 alleviates the inhibitory effect of 1-MCP on softening by increasing xylanase activity and upregulating the gene expression. Our observations provide new insights into the transcriptional regulatory mechanisms that govern softening of postharvest papaya fruit.