Development of molecular markers and identification of gene responsible for prolycopen enriched orange-colored leaves in kimchi-cabbage, Brassica rapa

Abstract

Carotenoids are essential components for photosynthesis process and photoprotection in plant. Furthermore, it is known to promote health, such as prevention of cancers and aging in human. However, most animals including human cant biosynthesize carotenoids by themselves and thus must take them from diets as essential nutrients. Due to this reason, carotenoid itself and its biosynthesis genes have long been a subject of researches. In this study, I analyzed orange-colored (OC) Kimchi cabbage, a cultivar of Chinese cabbage (Brassica rapa) in Korea specialized for Korea traditional food, Kimchi, and identified a gene causing OC phenotype. OC cultivar had inner leaves of deep yellow color, which was changed into orange or reddish color under sunlight, whereas generally cultivated Chinese cabbage had yellow-colored (YE) inner leaves, regardless of sunlight exposure. In addition, OC cultivar showed pale-yellow flower color co-segregated with OC inner leaf trait and the flower color was segregated with 1:3 ratio of OC:YE in F2 progenies of crossing OC and YE inbred lines. This indicated that OC phenotype is controlled by single recessive gene. Through HPLC analysis and comparison with previous studies, OC cultivar was identified to contain high amounts of all-trans-lycopene (lycopene) and 7Z,9Z,7Z,9Z-tetra-cis-lycopene (prolycopene), compared to YE with high β-carotene and none of lycopene and prolycopene, implying that OC phenotype might be due to a mutation in carotenoid isomerase (CRTISO) converting prolycopene into lycopene. I found two genes, BrCRTISO1 and BrCRTISO2, by searching of B. rapa genomic database and determined their expression. RT-PCR analysis revealed that BrCRTISO1 was not normally expressed in OC cultivar whereas BrCRTISO2 was constitutively expressed in both cultivars. In addition, genomic DNA PCR analysis confirmed that BrCRTISO1 of OC cultivar had many sequence alternations such as single nucleotide polymorphisms (SNPs) and insertions and deletions (InDels), compared to that of YE cultivar. Taken together, this result strongly suggests that BrCRTISO1 is a gene responsible for OC phenotype. BrCRTISO1 is a novel B. rapa gene for carotenoid isomerase and will be very useful to study carotenoid biosynthesis pathway as well as to develop new cultivar with unique carotenoid contents in Brassica species. Currently, to assist marker-associated breeding for OC cultivar, DNA molecular markers based on sequence polymorphism found in BrCRTISO1 gene are being examined.