Path analysis suggests phytoene accumulation is the key step limiting the carotenoid pathway in white carrot roots

Abstract

Two F 2 carrot (Daucus carota L.) populations (orange rooted Brasilia x very dark orange rooted High Carotene Mass -HCM cross and the dark orange rooted cultivated variety B493 x white rooted wild carrot Queen Anne's Lace -QAL cross) with very unrelated genetic backgrounds were used to investigate intrinsic factors limiting carotenoid accumulation in carrots by applying phenotypic correlation and path analysis to study the relationships between major root carotenes, root color and several other morphological traits. Most of the correlations between traits were close and agreed in sign between the two populations. Root weight had a moderate to highly significant positive correlation with leaf length, root length and top and middle root diameter. Although phenotypic correlations failed to identify the order of the substrates and products in the carotenoid pathway the correct order of substrates and products (phytoene → zeta-carotene → lycopene) was identified in the causal diagram of beta-carotene for the Brasilia x HCM population. Path analysis of beta-carotene synthesis in the B493 x QAL population suggested that selection for root carotenes had little effect on plant morphological traits. Causal model of beta-carotene and lycopene in the B493 x QAL population suggested that phytoene synthesis is the key step limiting the carotenoid pathway in white carrots. Path analysis, first presented by Sewall Wright to study quantitative traits, appears to be a powerful statistical approach for the identification of key compounds in complex pathways. IntroductionVitamin A deficiency is not only widespread in developing countries but is also is found in developed countries in poor urban populations and among the elderly, heavy drinkers and smokers (Giuliano et al. 2000). Consumption of horticultural crops provides more than 70% of vitamin A for the world population (Simpson, 1983), with carrots accounting for 30% of the total vitamin A precursor in countries such as the United States of America (Simon, 1992).The carotenoid biosynthetic pathway is a well established biochemical pathway which has been studied in many plants (Cunningham and Gantt, 1998;Sandmann, 1998), fungi and microorganisms (Sandmann, 1998;Armstrong, 1994). The formation of the colorless carotene phytoene from two molecules of geranylgeranyl diphosphate (GGDP) or geranylgeranyl pyrophosphate (GGPP) is the first committed step in the carotenoid pathway. Phytoene undergoes a series of four desaturation reactions that result in the sequential formation of phytofluene, zeta-carotene (ζ-carotene), neurosporene and then the redcolored lycopene. A single gene product, lycopene betacyclase (β-cyclase) (LCYB), catalyzes the formation of the bicyclic β-carotene (with two β rings) from the linear symmetrical lycopene in plants and cyanobacteria (Cunningham and Gantt, 1998) as demonstrated in studies with Erwinia herbicola and tomatoes (Sandmann, 1998). In the case of alpha-carotene (α-carotene), with one β and one epsilon (ε) ring, two different enzymes, LCYB...

Carotene Hydroxylase Activity Determines the Levels of Both α-Carotene and Total Carotenoids in Orange Carrots

“…PSY levels/activity are key to carotenoid accumulation in nongreen plant tissues (Ye et al, 2000;Lindgren et al, 2003;Ducreux et al, 2005;Welsch et al, 2010), including carrot roots (Santos et al, 2005;Maass et al, 2009). The context described here between CYP97A3 overexpression leading to reduced PSY protein levels suggests the presence of a negative feedback regulation, stemming from carotenoids downstream of a-carotene or metabolites thereof, acting on PSY levels.…”

Path Analysis of Biomarkers of Exposure and Early Biological Effects among Coke-Oven Workers Exposed to Polycyclic Aromatic Hydrocarbons

“…In this analysis, a network of causes and effects is seen as a series of steps in a path with a coefficient assigned to each step to quantify their interrelationships (21). With the advances of path analysis methodology, this method has been used more extensively in different fields (22)(23)(24)(25)(26). Santos et al first applied the path analysis to elucidate a biochemical pathway of carotenoid and had obtained some interesting results (25).…”

“…With the advances of path analysis methodology, this method has been used more extensively in different fields (22)(23)(24)(25)(26). Santos et al first applied the path analysis to elucidate a biochemical pathway of carotenoid and had obtained some interesting results (25). Their work again showed the efficacy of path analysis for identifying key compounds in complex pathways.…”

Carotenoids in Carrot

“…Additionally, higher PSY levels and activity have been shown to promote the accumulation of carotenoids in crystals and the differentiation of chromoplasts not only in the root of white carrot cultivars but also in other plant systems, therefore improving the capacity of plastids to store carotenoids [14,[48][49][50]. Phytoene synthesis and desaturation appear to be limiting for the production of carotenoids in roots of white carrot, but not in chromoplast-containing orange carrots [14,51]. The activity of multienzyme complexes involving carotenoid biosynthetic enzymes like those found for PDS in carrot [52] might also change during chromoplast differentiation, contributing to a dramatic increase in carotenoid levels observed in mature roots.…”