Date Published: February 12, 2019
Publisher: Public Library of Science
Author(s): Xi Cheng, Guohui Li, Chenhui Ma, Muhammad Abdullah, Jinyun Zhang, Hai Zhao, Qing Jin, Yongping Cai, Yi Lin, Keith R. Davis.
The content and size of stone cell clusters affects the quality of pear fruit, and monolignol polymerization and deposition in the cell walls constitute a required step for stone cell formation. Laccase (LAC) is the key enzyme responsible for the polymerization of monolignols. However, there are no reports on the LAC family in pear (Pyrus bretschneideri), and the identity of the members responsible for lignin synthesis has not been clarified. Here, 41 LACs were identified in the whole genome of pear. All Pyrus bretschneideri LACs (PbLACs) were distributed on 13 chromosomes and divided into four phylogenetic groups (I-IV). In addition, 16 segmental duplication events were found, implying that segmental duplication was a primary reason for the expansion of the PbLAC family. LACs from the genomes of three Rosaceae species (Prunus mummer, Prunus persica, and Fragaria vesca) were also identified, and an interspecies collinearity analysis was performed. The phylogenetic analysis, sequence alignments and spatiotemporal expression pattern analysis suggested that PbLAC1, 5, 6, 29, 36 and 38 were likely associated with lignin synthesis and stone cell formation in fruit. The two target genes of Pyr-miR1890 (a microRNA identified from pear fruit that is associated with lignin and stone cell accumulation), PbLAC1 and PbLAC14, were selected for genetic transformation. Interfamily transfer of PbLAC1 into Arabidopsis resulted in a significant increase (approximately 17%) in the lignin content and thicker cell walls in interfascicular fibre and xylem cells, which demonstrated that PbLAC1 is involved in lignin biosynthesis and cell wall development. However, the lignin content and cell wall thickness were not changed significantly in the PbLAC14-overexpressing transgenic Arabidopsis plants. This study revealed the function of PbLAC1 in lignin synthesis and provides important insights into the characteristics and evolution of the PbLAC family.
Pyrus bretschneideri cv. ‘Dangshan Su’ is one of the most important exported fruits in China and is well known throughout the world for its rich nutritional and medicinal value, but one of the disadvantages of this variety is the large diameter of the stone cell clusters (SCCs) and their high abundance in the fruit [1,2]. The content and diameter of SCCs in pear fruit are negatively correlated with the content of sucrose and cause a gritty texture and coarse mouthfeel. An excessive abundance and/or an increased diameter of SCCs affect the fruit flavour and consumer appreciation [3–5]. Therefore, the content and diameter of SCCs are key factors affecting the quality of pear fruit.
The content and size of stone cells are the most important factors affecting fruit quality [2,9,10]. Stone cells not only affect the texture and taste of the flesh but also are negatively correlated with the contents of various nutrients. The content of lignin in the mature stone cells of pear is 20–30% [8,45]. The differentiation of the parenchyma cells of the flesh into stone cells causes the secondary cell walls to thicken and induce the deposition of a large amount of lignin [6,7,16]. Therefore, the development of stone cells is closely related to the synthesis and deposition of lignin. Laccases are responsible for the polymerization of lignin monomers and play an important role in the formation of secondary cell walls [14,22]. Laccases exist in the form of gene families in plants, and their members are numerous and functional [17,46]. Therefore, the screening and identification of PbLACs associated with lignin synthesis are important for the regulation of lignin synthesis and stone cell development in pear.
In conclusion, we screened and identified laccase family members in the pear genome, and the characteristics and evolution of the PbLAC family were systematically analysed. An expression pattern analysis revealed that PbLAC1, PbLAC6, PbLAC29 and PbLAC36 might be lignin-specific PbLACs in pear fruit, and a heterologous expression analysis in Arabidopsis clearly showed that PbLAC1 is involved in lignin metabolism and cell wall development. Thus, this study not only provides target genes for regulating the metabolism of pear lignin but also lays the foundation for clarifying the function of the PbLAC family.