Abstract
Korla pear (Pyrus sinkiangensis Yü) is a landrace selected from a hybrid pear species in the Xinjiang Autonomous Region in China. In recent years, pericarp roughening has been one of the major factors that adversely affects fruit quality. Compared with regular fruits, rough-skin fruits have a greater stone cell content. Stone cells compose sclerenchyma tissue that is formed by secondary thickening of parenchyma cell walls. In this work, we determined the main components of stone cells by isolating them from the pulp of rough-skin fruits at the ripening stage. Stone cell staining and apoptosis detection were then performed on fruit samples that were collected at three different developmental stages (20, 50 and 80 days after flowering (DAF)) representing the prime, late and stationary stages of stone cell differentiation, respectively. The same batches of samples were used for parallel transcriptomic and proteomic analysis to identify candidate genes and proteins that are related to SCW biogenesis in Korla pear fruits. The results showed that stone cells are mainly composed of cellulose (52%), hemicellulose (23%), lignin (20%) and a small amount of polysaccharides (3%). The periods of stone cell differentiation and cell apoptosis were synchronous and primarily occurred from 0 to 50 DAF. The stone cell components increased abundantly at 20 DAF but then decreased gradually. A total of 24,268 differentially expressed genes (DEGs) and 1011 differentially accumulated proteins (DAPs) were identified from the transcriptomic and proteomic data, respectively. We screened the DEGs and DAPs that were enriched in SCW-related pathways, including those associated with lignin biosynthesis (94 DEGs and 31 DAPs), cellulose and xylan biosynthesis (46 DEGs and 18 DAPs), S-adenosylmethionine (SAM) metabolic processes (10 DEGs and 3 DAPs), apoplastic ROS production (16 DEGs and 2 DAPs), and cell death (14 DEGs and 6 DAPs). Among the identified DEGs and DAPs, 63 significantly changed at both the transcript and protein levels during the experimental periods. In addition, the majority of these identified genes and proteins were expressed the most at the prime stage of stone cell differentiation, but their levels gradually decreased at the later stages.
Highlights
Korla pear (Pyrus sinkiangensis Yü) is a landrace selected from a hybrid pear species in the Xinjiang Autonomous Region in China
All the stone cell components were abundant at 20 days after flowering (DAF), after which the levels decreased gradually (Fig. 1C), and the trends of these components were consistent with trends of stone cell contents during fruit development
In this work, we closely focused on the differentially expressed genes (DEGs) and differentially accumulated proteins (DAPs) that were enriched in secondary cell wall (SCW)-related pathways, including those associated with lignin biosynthesis (94 genes and 31 proteins), cellulose and xylan biosynthesis (46 genes and 18 proteins), S-adenosylmethionine (SAM) metabolic processes (10 genes and 3 proteins), apoplastic Reactive oxygen species (ROS) production (16 genes and 2 proteins), and cell death (14 genes and 6 proteins) (Fig. 2B)
Summary
Korla pear (Pyrus sinkiangensis Yü) is a landrace selected from a hybrid pear species in the Xinjiang Autonomous Region in China. While we understand a great deal about the genes involved in lignin synthesis and deposition during stone cell differentiation in pear fruits[4,5,6,7,8,9,10], we know surprisingly little about the underlying physiological and molecular mechanisms of stone cell differentiation because cellulose and hemicellulose account for the largest percentage of secondary cell wall (SCW) biomass. Few studies have linked the formation of stone cells to the synthesis of cell wall polymers and crosslinkages among them To resolve this scientific problem and theoretical inference, in this study, parallel analyses of the transcriptome and proteome of Korla pear fruits at three developmental stages were carried out to identify essential regulators and pathways involved in stone cell formation. The results gained through this study will help us understand the mechanisms underlying stone cell formation in pear fruits
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