Differential Expression Of Carotenoid Biosynthesis Genes In Red Navel Orange And Ordinary Navel Orange Fruits

Carotenoids , which was involved in color and health benefits , contributed greatly to value of citrus fruit. Previous studies were mostly focused on later part of carotenoid biosynthesis pathway, and quite few work was done on early steps. Red navel orange (Citrus sinensis L.), a mutant of ordinary navel orange, accumulated high content of carotenoids, but the underlied reasons have not been unraveled. In this study, gene fragments of 1-deoxy-D-xylulose 5-phosphate synthase (DXS) and 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) were isolated from orange and differential expression of 7 carotenoid biosynthesis genes in red navel orange and ordinary navel orange fruits was analysed.DXS and DXR fragments were obtained with cDNA from mature orange peel as template by conventional PCR. The obtained fragments were 458 bp and 359 bp in size, respectively for DXS and DXR. Both of them were in the region of coding sequences, and encode 152 and 119 amino acids, respectively. BLAST were carried out based on nucleotide sequence, and it was found that both DXS and DXR showed over 80 % homology with corresponding genes from higher plants including tomato, tobacco, etc.Real-time PCR systems for gene expression analysis were established for DXS, DXR, phytoene synthase (PSY), phytoene desaturase (PDS), lycopene e-cyclase (LCY-e), and caroteneβ-ring hydroxylase (BCH). Differential expression of 7 carotenoid biosynthesis genes, 6 genes above-mentioned as well as lycopeneβ-cyclase (LCY-b) established in previous studies, in red navel orange and ordinary navel orange fruits was analysed with citrus citrate synthase (CS) gene as internal reference gene. Expression of BCH w&s strongest among all 7 genes analysed, which probably explained why citrus accumulate xanthophylls as main carotenoids. Expression magnitudes of PSY and DXR were higher than that of DXS, therefore, DXS might be early limiting steps for citrus carotenoid biosynthesis. In general, the expression of almost all genes was stronger in flesh tissue of red navel orange than in ordinary navel orange. It was suggested that higher accumulation of carotenoids in red navel orange might result from coordinated expression of multiple carotenoid biosynthesis genes.