Figure 5: Working model of the regulation of carotenoid accumulation in carrot root.

Upward- and downward-pointing arrows indicate upregulated and downregulated genes, respectively, in the yellow versus white (yellow arrows) and dark orange versus pale orange (orange arrows) comparisons. The orange box delimits the isoprenoid biosynthetic branch that leads to the carotenoid pathway. As shown in the green box, the majority of the upregulated genes in yellow and dark orange roots are involved in the photosynthetic pathway (Supplementary Table 45); genes that are included are involved in the assembly and function of photosystems I and II and plastid development. We hypothesize that loss of the constitutive repression mechanisms conditioned by genes involved in de-etiolation and photomorphogenensis in non-photosynthetic tissue, such as carrot roots, induces overexpression of DXS1 and, consequently, activation of the metabolic cascade that leads to high levels of carotenoid accumulation in carrot roots.