PbSQE3 and PbSQE6 regulates the formation of triterpenoid compounds in the russet mutant of ‘Dangshansuli variety’ pear

The outer layer of russet pear fruit skin is coated with suberin, a key protective barrier that deters pests and minimizes water loss, making it an important trait for fruit cultivation. This study investigates the mechanisms underlying russet formation in the pear exocarp, focusing on the ‘Dangshansuli variety’ with yellow-green skin and its russeted mutant ‘Dangshanjinsu variety’ through proteomic analysis. The results revealed numerous differentially expressed proteins associated with the phenylpropanoid biosynthesis pathway, suberin and wax synthesis pathways, and sesquiterpenoid and triterpenoid synthesis pathways. Specifically, six PbSQE genes, encoding squalene epoxidases involved in triterpenoid synthesis, showed distinct expression patterns in the exocarp of ‘Dangshansuli variety’ and ‘Dangshanjinsu variety’ during fruit development. Notably, PbSQE4, PbSQE5, and PbSQE6 displayed a similar expression pattern across the three stages, with lower expressions observed at 25 and 175 DAFB but higher expressions at 100 DAFB in the exocarp of ‘Dangshanjinsu variety’. The expression of PbSQE3 in the exocarp of ‘Dangshanjinsu variety’ at 100 and 175 DAFB was significantly lower than that in ‘Dangshansuli variety’. Based on this, selected PbSQE3 and PbSQE6 to stably transform Arabidopsis thaliana. Overexpression of PbSQE3 and PbSQE6 in transgenic Arabidopsis significantly increased triterpenoid compounds, including ursolic acid, oleanolic acid, α-amyrin, and β-amyrin. These findings highlight the roles of PbSQE3 and PbSQE6 in triterpenoid biosynthesis and suggest they contribute to the downregulation of suberin deposition in the russet pear exocarp.