Differential expression of genes encoding phenylpropanoid enzymes in an apricot cultivar (Prunus armeniaca L.) with cleavable endocarp

Key message Phenylpropanoid pathway related genes were identified and analyzed by co-expression network and expression pattern. NST1 may regulate the expression of CAD to affect the lignin deposition of 'Liehe' apricot endocarp. Apricot (Prunus Armeniaca L.) is a typical Rosaceae stone fruit tree. The extensive lignification of its endocarp is an important metabolic event during fruit ripening. There are abundant apricot germplasm resources in China, including a special apricot cultivar, 'Liehe' (LE), which has a thin, soft, cleavable endocarp with a hardness 45.84% that of the 'Jinxihong' (JG) apricot (with normal hardened-endocarp). To understand the molecular mechanisms behind the LE phenotype, differentially expressed genes (DEGs) encoding key enzymes involved in phenylpropanoid biosynthesis were mined from transcriptome data by the co-expression network and expression patterns. DEGs encoding key enzymes involved in phenylpropanoid biosynthesis were significantly down-regulated in LE, and the activity of these enzymes exhibited similar results. NST1 (NAC secondary wall thickening promoting factor 1) expression levels in LE were only 13.7%, 2.8%, 9.4%, and 82.5% that in JG at 9, 15, 21, and 30 days after full bloom (DAFB), respectively. CAD (Cinnamyl alcohol dehydrogenase) expression levels in LE were 1.3%, 0.7%, 0.2% and 2.7% that in JG at 15, 21, 30, 49 DAFB, respectively. CAD activity in LE was 46.4% and 63.6% that in JG at 42 and 49 DAFB, respectively. We thus used homologous cloning to determine the sequence differences of CAD and NST1 in LE and JG. Our results will help understand the molecular mechanisms underlying endocarp cleavage in LE apricot and provide a useful reference for further investigation of lignification during endocarp development. NST1 may regulate CAD which involved in the phenylpropanoid pathway and affect lignin deposition in LE endocarp.