Genome-wide identification and functional analysis of the peach (P. persica) laccase gene family reveal members potentially involved in endocarp lignification

Two members of the 48 laccase genes were highly expressed in peach endocarp and activated by the lignin biosynthesis-related MYB transcription factors, suggesting a role of them in lignification. Peach (P. persica) is an economically important fruit tree. Peach breeds drupe fruits with its seeds encased in an extremely hard lignified endocarp or stone. Laccases (LACs) are copper-containing glycoproteins that are involved in cell elongation processes and widely distributed in plant genomes. LACs are essential for lignin polymerization because laccase is the key enzyme responsible for monolignol polymerization. At present, the research on peach laccase is blank, and the members that play important roles in the polymerization of peach lignin are even unknown. In this study, we identified 48 laccase genes (PpLACs) in the peach genome. All 48 peach laccase genes were unevenly distributed on eight chromosomes and divided into six different phylogenetic groups (I–VI). Among them, 15 were predicted to be potential targets of miR397, a negative regulator of lignin biosynthesis. Phylogenetic analysis, sequence analysis and spatiotemporal expression profile analysis indicated that PpLAC7, 19, 20, 21, 27, 28 and 30 may be related to lignin synthesis and endocarp hardness in fruit. Through expression pattern analysis, PpLAC20 and PpLAC30 were finally screened as most likely important members involved in peach lignin biosynthesis. A peach MYB transcription factor PpMYB63, a homolog of AtMYB58 and AtMYB63, demonstrated the ability to activate the PpLAC20 and PpLAC30 promoters. Moreover, histochemical staining analysis of GUS activity in proPpLAC20::GUS transgenic Arabidopsis seedlings showed the tissue-specific expression pattern in limited tissues or organs, including siliques, inflorescence stems, cauline leaf main veins, and the outer coat of the seed. Our findings revealed that PpLAC20 and PpLAC30 are candidates involved in peach lignin biosynthesis and laid the foundation for subsequent researches on peach lignin synthesis and regulation.