Transcriptome and Physiological Analysis Highlight Lignin Metabolism of the Fruit Dots Disordering during Postharvest Cold Storage in ‘Danxiahong’ Pear

Abstract

Pear (Pyrus L.) is one of the most important fruits in the world. Fruit dots are an important trait that affects pear quality. Abnormal fruit dots usually reduce the merchantability of pears. In this research, during cold storage, ‘Danxiahong’ pear fruit exhibited protrudent fruit dots on the peels. Microscopy system measurement showed that fruit dots size and height on the abnormal fruit peel were bigger and higher than the normal ones. Likewise, scanning electron microscopy observations indicated that the abnormal fruit peel, in contrast to the normal pear peel, exhibited an abnormal cell structure and fruit dots. Physiological analysis showed that the lignin content in abnormal fruit peel was significantly higher than in normal fruit peel. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analysis revealed that the top-enriched pathways were mainly associated with lignin synthesis and metabolism. The transcripts of lignin biosynthesis-associated genes were analyzed, and the results showed that the expression of a cascade of structural genes, including PpyPAL, PpyCCR, PpyC3H, PpyC4H, PpyHCT, PpyCAD, PpyLAC, and PpyPOD, was significantly induced in the protrudent peels. Furthermore, the expression of regulatory genes involved in lignin biosynthesis, especially the NAC-MYB-based gene regulatory network, was significantly upregulated in the abnormal peels. Real-time quantitative PCR (RT-qPCR) analysis confirmed the induction of lignin biosynthesis genes. Overall, this research revealed that the abnormal fruit surface was caused by fruit dots disorder during cold storage. This research provides insights into the fine regulation pathways in the prevention of fruit dots protrusion, especially in modulating lignin synthesis and metabolism during postharvest storage.