Carotenoid biosynthetic genes in Brassica rapa: comparative genomic analysis, phylogenetic analysis, and expression profiling

Peirong Li, Shujiang Zhang, Shifan Zhang, Fei Li, Hui Zhang, Feng Cheng, Jian Wu, Xiaowu Wang, Rifei Sun

Carotenoids are isoprenoid compounds synthesized by all photosynthetic organisms. Despite much research on carotenoid biosynthesis in the model plant Arabidopsis thaliana, there is a lack of information on the carotenoid pathway in Brassica rapa. To better understand its carotenoid biosynthetic pathway, we performed a systematic analysis of carotenoid biosynthetic genes at the genome level in B. rapa. We identified 67 carotenoid biosynthetic genes in B. rapa, which were orthologs of the 47 carotenoid genes in A. thaliana. A high level of synteny was observed for carotenoid biosynthetic genes between A. thaliana and B. rapa. Out of 47 carotenoid biosynthetic genes in A. thaliana, 46 were successfully mapped to the 10 B. rapa chromosomes, and most of the genes retained more than one copy in B. rapa. The gene expansion was caused by the whole-genome triplication (WGT) event experienced by Brassica species. An expression analysis of the carotenoid biosynthetic genes suggested that their expression levels differed in root, stem, leaf, flower, callus, and silique tissues. Additionally, the paralogs of each carotenoid biosynthetic gene, which were generated from the WGT in B. rapa, showed significantly different expression levels among tissues, suggesting differentiated functions for these multi-copy genes in the carotenoid pathway. This first systematic study of carotenoid biosynthetic genes in B. rapa provides insights into the carotenoid metabolic mechanisms of Brassica crops. In addition, a better understanding of carotenoid biosynthetic genes in B. rapa will contribute to the development of conventional and transgenic B. rapa cultivars with enriched carotenoid levels in the future.