Integrated function of proanthocyanidin and lignin polymerization mediated by LAC/PRXs in pericarp browning of longan fruit

Longan can be regarded as a distinct dry fruit with fleshly aril, whose pericarp browning may follow an unclear browning mechanism as dry fruits. Here, we integrated metabolome and transcriptome analysis, validated the function of the candidate metabolites and genes, to dissect the underlying mechanism. With the pericarp water content of longan fruit decreased from 51% to 20%, the pericarp completely turned brown after 8 days of storage at 25 celcius and relative humidity 70%. Instead of the decline of soluble proanthocyanidins (PAs), such as (-)-epicatechin (EC), (+)-catechin (CT), and procyanidin A2/B2 etc., as well as lignin monomers, coniferyl alcohol (Con_A) and sinapyl alcohol (Sin_A), the contents of their polymers, insoluble PAs and lignin, increased. Gene expression of three Laccases (LAC14-4, 14-8, 7) and four Class III Peroxidase (PRX52-1, 52-2, 53-2, 4-15) were highly negatively correlated to metabolites EC and Con_A. Exogenously expressed LAC7 and LAC14-8, PRX4-15 and PRX52-1 could catalyze the polymerization and browning of EC, CT, Con_A or Sin_A in vitro. Compared to single substrate (EC or Con_A), the polymerization and browning were dramatically accelerated when double substrates (EC and Con_A) were present, by either LACs or PRXs. In addition, these LACs and PRXs were found to be located in endoplasmic reticulum (ER), correlated to their 'secretion protein' signal peptides. Based on these results, we propose that the water-loss in pericarp induces the polymerization reaction of PAs and lignin monomers mediated by LACs/PRXs, leading to the browning of longan pericarp. The hypothesis is distinct to the 'polyphenol oxidases-phenols' classical browning mechanism established for fleshly fruits.