Genome-wide identification of MAPK family in papaya (Carica papaya) and their involvement in fruit postharvest ripening.

BACKGROUND: Papaya (Carica papaya) is an economically important fruit cultivated in the tropical and subtropical regions of China. However, the rapid softening rate after postharvest leads to a short shelf-life and considerable economic losses. Accordingly, understanding the mechanisms underlying fruit postharvest softening will be a reasonable way to maintain fruit quality and extend its shelf-life. RESULTS: Mitogen-activated protein kinases (MAPKs) are conserved and play essential roles in response to biotic and abiotic stresses. However, the MAPK family remain poorly studied in papaya. Here, a total of nine putative CpMAPK members were identified within papaya genome, and a comprehensive genome-wide characterization of the CpMAPKs was performed, including evolutionary relationships, conserved domains, gene structures, chromosomal locations, cis-regulatory elements and expression profiles in response to phytohormone and antioxidant organic compound treatments during fruit postharvest ripening. Our findings showed that nearly all CpMAPKs harbored the conserved P-loop, C-loop and activation loop domains. Phylogenetic analysis showed that CpMAPK members could be categorized into four groups (A-D), with the members within the same groups displaying high similarity in protein domains and intron-exon organizations. Moreover, a number of cis-acting elements related to hormone signaling, circadian rhythm, or low-temperature stresses were identified in the promoters of CpMAPKs. Notably, gene expression profiles demonstrated that CpMAPKs exhibited various responses to 2-chloroethylphosphonic acid (ethephon), 1-methylcyclopropene (1-MCP) and the combined ascorbic acid (AsA) and chitosan (CTS) treatments during papaya postharvest ripening. Among them, both CpMAPK9 and CpMAPK20 displayed significant induction in papaya flesh by ethephon treatment, and were pronounced inhibition after AsA and CTS treatments at 16 d compared to those of natural ripening control, suggesting that they potentially involve in fruit postharvest ripening through ethylene signaling pathway or modulating cell wall metabolism. CONCLUSION: This study will provide some valuable insights into future functional characterization of CpMAPKs, and hold great potential for further understanding the molecular mechanisms underlying papaya fruit postharvest ripening.

Physiological and transcriptomic analysis of IAA-induced antioxidant defense and cell wall metabolism in postharvest mango fruit.

Mango fruit tend to oxidize and senescence rapidly after harvesting, significantly reducing their commercial value. This study investigated the effect of exogenous auxin indole-3-acetic acid (IAA) on fruit quality, antioxidant system, and cell wall metabolism of mango fruit during storage. The results showed that the 1.0 mM IAA treatment delayed weight loss and maintained the firmness, pH and contents of total soluble solids (TSS) and titratable acidity (TA) of the mango fruit. The 1.0 mM IAA treatment increased the peroxidase (POD) and phenylalanine ammonia-lyase (PAL) activities and the ascorbic acid (AsA) and total phenols (TP) contents but decreased the polyphenol oxidase (PPO) activity in postharvest mango fruit. Moreover, beta-galactosidase (ß-Gal) and polygalacturonase (PG) activities were increased, but the pectinesterase (PME) activity was decreased in the IAA-treated fruit. Transcriptome analysis showed that the differentially expressed genes (DEGs) in the IAA vs. control groups were mainly associated with oxidative stress responses, cell wall metabolism, and transcription factors (TFs). The IAA treatment upregulated the antioxidant-related genes (SOD, CAT1, PODs, GSTs, Prxs, and Trxs) and MYB TFs, and downregulated cell wall metabolism-related genes (PG, PME31 and two PME63) and 11 ethylene-responsive transcription factors (ERFs). These results suggested that exogenous IAA could improve the antioxidant system and maintain the storage quality of mango fruit by regulating gene expression and metabolic pathways. The results provide insights into the mechanisms involved in IAA-mediated delayed ripening and senescence of mango fruit.