Genome-wide identification and analysis of the laccase gene family in Litchi chinensis Sonn. provides new insights into pericarp browning

Laccases belong to a multigene family and have divergent biological functions. Herein, we explored the litchi Laccase (LcLAC) gene family using genome and transcriptome data to identify key members that play a role in pericarp browning of litchi (Litchi chinensis). LcLAC gene family includes 39 members, which can be divided into seven clusters (C1-C7). The C4 cluster contains 10 LcLAC14s and 2 LcLAC15s, which are aggregate at chromosome 8. Thirty-one LcLACs contain three conserved domains of Cu-oxidase, Cu-oxidase_2 and Cu-oxidase_3, whereas the other eight members have incomplete conserved domains because of their lack of signature sequences (L1-L4) related to copper binding. Twenty-five members contain secretory protein (SP) signal peptides and all members have glycosylation sites, indicating post-translational modifications of the enzymes. RNA-Seq analysis detected 28 LcLAC genes expressed in the litchi pericarp, most of which showed a downregulation trend in the pericarp along with the fruit development and the pericarp browning process after harvest. However, LcLAC14-3 and LcLAC14-4 (previously characterised as anthocyanin degradation enzyme, ADE/LAC) maintained high levels of expression after harvest, and LcLAC7 was highly expressed and upregulated after harvest. Similar to ADE/LAC, LcLAC7 and LcLAC14-3 were located in the endoplasmic reticulum (ER) and vacuole (VAC). We obtained exogenously expressed LcLAC7 from tobacco leaves and found that LcLAC7 enzyme showed high catalytic activity for (-)-epicatechin (EC), whereas it showed relatively lower activity to coniferyl alcohol (ConA); in the presence of EC, its activity toward ConA increased. The high catalytic activity of LcLAC7 toward EC and ConA suggests the important role of proanthocyanin and lignin polymerisation in litchi pericarp browning.