Biochemical characterization and expression analysis of lignification in two pear (Pyrus ussuriensis Maxim.) varieties with contrasting stone cell content.

As lignified stone cells reduce fruit quality, we investigated lignin deposition, phenolic metabolites, and expression of lignin biosynthetic genes during fruit development to elucidate the molecular mechanism of stone cell lignification using histological, biochemical, and transcriptional data from two Ussurian pear varieties (Jianba and Nanguo) with contrasting stone cell content. Lignin content and distribution coincided with stone cell accumulation. As per LC-MS analysis, Jianba exhibited higher levels of lignin monomers and hydroxycinnamates than Nanguo, consistently with lignin amount in each case. However, flavonoid content was much higher in Nanguo. Transcriptional data showed that most monolignol biosynthesis-related genes were particularly upregulated in Jianba during lignin accumulation; especially CCR and LAC, two monolignol biosynthesis-specific genes, were substantially upregulated in Jianba fruits at critical stages. Therefore, differences in stone cell content between "Jianba" and "Nanguo" may result from differential expression of lignin synthase genes located downstream of the lignin biosynthesis pathway. Taken together, our data may provide a deeper understanding of the molecular mechanism for stone cell lignification in pear fruit.