An alternative pathway to β-carotene formation in plant chromoplasts discovered by map-based cloning of Beta and old-gold color mutations in tomato

Abstract

Carotenoid pigments in plants fulfill indispensable functions in photosynthesis. Carotenoids that accumulate as secondary metabolites in chromoplasts provide distinct coloration to flowers and fruits. In this work we investigated the genetic mechanisms that regulate accumulation of carotenoids as secondary metabolites during ripening of tomato fruits. We analyzed two mutations that affect fruit pigmentation in tomato (Lycopersicon esculentum): Beta (B), a single dominant gene that increases ␤-carotene in the fruit, and old-gold (og), a recessive mutation that abolishes ␤-carotene and increases lycopene. Using a map-based cloning approach we cloned the genes B and og. Molecular analysis revealed that B encodes a novel type of lycopene ␤-cyclase, an enzyme that converts lycopene to ␤-carotene. The amino acid sequence of B is similar to capsanthin-capsorubin synthase, an enzyme that produces red xanthophylls in fruits of pepper (Capsicum annum). Our results prove that ␤-carotene is synthesized de novo during tomato fruit development by the B lycopene cyclase. In wild-type tomatoes B is expressed at low levels during the breaker stage of ripening, whereas in the Beta mutant its transcription is dramatically increased. Null mutations in the gene B are responsible for the phenotype in og, indicating that og is an allele of B. These results confirm that developmentally regulated transcription is the major mechanism that governs lycopene accumulation in ripening fruits. The cloned B genes can be used in various genetic manipulations toward altering pigmentation and enhancing nutritional value of plant foods.

Genome sequence of the hot pepper provides insights into the evolution of pungency in Capsicum species

“…CCS exhibits lycopene β-cyclase activity 54 and has an orthologous relationship with chromoplast-specific lycopene β-cyclase (CYC-B) 55 , which exhibits ethylene-dependent repression 44 during tomato ripening. CCS expression was extremely high during pepper ripening (Fig.…”

Biofortified crops to alleviate micronutrient malnutrition

“…Reconstitution of the pathway in other crops depends on the metabolic bottleneck, as illustrated by canola, tomato and potato [33][34][35][36][37]. The Grand Challenges in Global Health initiative of the Bill & Melinda Gates Foundation is funding biofortification projects on banana, cassava, and sorghum for Africa.…”

Nucleotide diversity of a major carotenoid biosynthetic pathway gene in wild and cultivated Solanum (Section Lycopersicon) species

“…The sequence similarity between the putative wild species CYCB genomic amplicons with the mRNA sequence available on GenBank indicates this gene is intronless. Translation of this sequence and analysis of the predicted polypeptide showed identity to the AF 254793 sequence (Ronen et al, 2000) varying from 95.5% (S. neorickii) to 100% (S. lycopersicum 'Santa Clara'). The results support the specificity of the pair of primers for the CYCB gene sequences, since no other PCR product was obtained in the described assay conditions.…”

“…Two lycopene beta-cyclases have also been described in tomato; one is LCYB and the other is the chromoplastspecific lycopene beta-cyclase (CYCB) (EC 1.14.-.-). The characterization of two yellow/orange fruit mutants (delta and Beta) revealed that these traits are controlled by distinct lycopene cyclase genes (Ronen et al, 1999(Ronen et al, , 2000. The Beta mutation controls the up regulation of the CYCB gene, so that the fruit accumulates the orange pigment beta-carotene.…”

“…The delta corresponds to an upregulated LCYE, which results in the accumulation of the yellow pigment delta-carotene. A third mutation (oldgold) controls lycopene accumulation in fruits and petals due to a frame-shift mutation in the CYCB gene (Ronen et al, 1999(Ronen et al, , 2000.…”