Abstract
In pomegranate (Punica granatum), seed hardness is an important trait directly affecting fruit marketability. However, seed formation in pomegranate has not been well studied. We investigated the genetic mechanism underlying pomegranate seed hardness by comparing protein expression profiles between soft- and hard-seeded varieties 60 and 120 days after flowering. We identified 1940 proteins, of which 399 were differentially expressed. Most of the differentially expressed proteins were involved in posttranslational modification and carbohydrate metabolism. Cell wall biosynthesis, which showed positive correlations with seed hardness, was selected as the candidate pathway. The mRNA levels of 14 proteins involved in cell wall biosynthesis were further analyzed by qPCR. Lignin biosynthesis-related differentially expressed proteins showed lower expression at protein and gene levels in a soft-seeded variety at the early stages. Moreover, cellulose biosynthesis-related differentially expressed proteins showed higher expression levels in the soft-seeded variety at 60 days after flowering. Thus, the soft-seeded variety showed lower lignin but higher cellulose biosynthesis at the early fruit developmental stage, suggesting that lignin and cellulose play opposing roles in cell wall formation in pomegranate seeds. Moreover, differentially expressed proteins involved in cell wall degradation showed higher expression levels in the soft-seeded variety at both developmental stages. These results suggested that differences in seed hardness between soft- and hard-seeded pomegranates might result from cell wall biosynthesis and also be affected by cell wall degradation. The present proteome-wide profiling of pomegranate genotypes with contrasting seed hardness adds to the current knowledge base of the molecular basis of seed hardness development.
Highlights
Pomegranate (Punica granatum L.) has been cultivated for more than 2000 years, and it possesses high economic, nutritional, medicinal, and ornamental value (Cao et al 2015)
Quantitative proteomics was used to identify the DEPs between soft- and hard-seeded pomegranate varieties at 60 and 120 days after flowering (DAF)
We found that UGP2 and sucrose synthase 3 (SUS3) were upregulated at 60 DAF and downregulated at 120 DAF at both gene and protein expression levels in Tunisia compared with Sanbai (Figs. 3 and 6)
Summary
Pomegranate (Punica granatum L.) has been cultivated for more than 2000 years, and it possesses high economic, nutritional, medicinal, and ornamental value (Cao et al 2015). Tree Genetics & Genomes (2018) 14: 14 cellulose content in pomegranate seeds (Zarei et al 2016). The cell wall, which is mainly composed of lignin and cellulose, is a key factor affecting seed hardness (Cosgrove 2005). Several studies have identified that cell wall biosynthesis-related genes are differentially expressed between soft- and hard-seeded varieties in fruit trees. Genes associated with lignin biosynthesis, such as caffeic acid 3-Omethyltransferase (COMT), ferulate-5-hydroxylase (F5H), and shikimate O-hydroxycinnamoyl transferase (HCT), were found to be significantly downregulated in soft-seeded hawthorn compared with hard-seeded hawthorn (Dai et al 2013). Dang suggested that high phenylalanine ammonia lyase and peroxidase (PRX) activity and gene expression were associated with pericarp hardening of mangosteen (Dang et al 2008). High accumulation of UDP-glucose pyrophosphorylase (UGP) and sucrose phosphate synthase were found to be involved in cell wall synthesis in seeded fruit than in parthenocarpic fruit during fruit development (Rounis et al 2015)
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