De novo transcriptome analysis of an albino mutant Pasphiopedilum pacific shamrock reveals reduced expression of genes related to chloroplast biosynthesis and division

Paphiopedilum pacific shamrock is an orchid with high ornamental value. Understanding the mechanisms responsible for leaf color in albino mutants is important for ornamental development and breeding. In this study, we compared the leaf photosynthetic pigment content and transcriptome of albino mutants ppa01 and wild-type P. pacific shamrock. Photosynthetic pigment in mutants was less than 2% of the wild type and chl a/b was 60% less than the wild type. Transcriptome sequencing yielded 6.27 Gb and 5.67 Gb clean data from the mutant and wild-type leaves, respectively. De novo assembly yielded 104,763 unigenes with 15,400 greater than 1 kb in length. In unigene expression analysis, 3170 differentially expressed genes (DEGs) were identified with 2231 (70.38% of total DEGs) down-regulated. Results from GO and KEGG enrichment analysis, KEGG pathway analysis and qPCR suggest that the reduction of chloroplast biosynthesis and division in the mutant was due to low expression levels of ppGLK1 and ppFtsz. Mutants were associated with fewer chloroplasts in leaf cells, abnormal chloroplast structures, impaired chlorophyll biosynthesis, and thus reduced total chlorophyll and carotenoid contents. Furthermore, down-regulated expression of ppNYC1 reduced transformation of chlorophyll b into chlorophyll a, leading to a chl a/b decline. The research will guide future studies of leaf pigment mutations and the breeding of P. pacific shamrock.