Comparative transcriptome profiling and morphology provide insights into endocarp cleaving of apricot cultivar (Prunus armeniaca L.)

Background: A complete and hardened endocarp is a typical trait of drupe fruits. However, the ‘Liehe’ (LE) apricot cultivar has a thin, soft, cleavable endocarp that represents 60.39% and 63.76% of the thickness and lignin content, respectively, of the ‘Jinxihong’ (JG) apricot (with normal hardened-endocarp). To understand the molecular mechanisms behind the LE apricot phenotype, comparative transcriptomes of Prunus armeniaca L. were sequenced using Illumina HiSeq™ 2500. Results: In this study, we identified 63,170 unigenes including 15,469 genes >1000 bp and 25,356 genes with Gene Function annotation. Pathway enrichment and expression patterns were used to characterize differentially expression genes. The DEGs encoding key enzymes involved in phenylpropanoid biosynthesis were significantly down-regulated in LE apricot. For example, CAD gene expression levels, encoding cinnamyl alcohol dehydrogenase, were only 1.3%, 0.7%, 0.2% and 2.7% in LE apricot compared with JG cultivar at 15, 21, 30, 49 days after full bloom (DAFB). Furthermore, transcription factors regulating secondary wall and lignin biosynthesis were identified. Especially for SECONDARY WALL THICKENING PROMOTING FACTOR 1 (NST 1), its expression levels in LE apricot were merely 2.8% and 9.3% compared with JG cultivar at 15 and 21 DAFB, respectively. Conclusions: Our comparative transcriptome analysis was used to understand the molecular mechanisms underlie the endocarp-cleaving phenotype in LE apricot. This new apricot genomic resource and the candidate genes provide a useful reference for further investigating the lignification during development of apricot endocarp. Transcription factors such as NST1 may regulate genes involved in phenylpropanoid pathway and affect development and lignification of the endocarp.