Genetic Control of Carotenoid Synthesis and Fruit Color Inheritance in Tomatoes
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The genetics of carotenoid synthesis have been studied through inheritance experiments on traits linked to carotenoid synthesis, with the most thorough work carried out on the tomato. These studies trace how individual nuclear genes govern the amount and type of pigment a fruit accumulates.
How do genes control the total amount of carotenoids in tomato fruit?
Two genes set the overall quantity of carotenoids in tomato fruit: the r⁺/r gene and the hp⁺/hp gene. Red-fruited plants carry the dominant allele r⁺, while yellow tomatoes are homozygous for the recessive allele r. The r⁺/r gene controls the total quantity of carotenoids formed.
The r/r genotype synthesizes only about 5% of the pigment quantity found in normal fruit. The hp⁺/hp gene likewise governs carotenoid amount, but in contrast to the rr gene, the recessive hp allele stimulates the synthesis of all pigments to 100%.
Which genes produce apricot and tangerine fruit colours?
Apricot-coloured fruit is homozygous for the recessive allele at and contains only trace amounts of acyclic carotenoids — lycopene in particular — while remaining comparable to the control in its β-carotene content. 
The effects of r and t are intensified by at. In the yellow-apricot phenotype, lycopene synthesis is fully suppressed and β-carotene synthesis is reduced, while in the tangerine-apricot phenotype the synthesis of cis-carotenes is stimulated. In the orange phenotype obtained by backcrossing L. esculentum × L. hirsutum to L. esculentum, the principal carotenoid is β-carotene rather than lycopene.
How is the shift from lycopene to β-carotene controlled?
Orange fruit accumulating β-carotene is homozygous or heterozygous for the dominant allele b⁺, whereas normal fruit is homozygous for the recessive allele b. The effect of the b⁺/b gene is partly regulated by an independently inherited modifier gene, mo_B⁺/mo_B. When mo_B⁺ is present in either the homozygous or heterozygous state, the amount of β-carotene falls by almost half, while the amount of lycopene rises markedly and reaches the level of β-carotene.
Delta lines of tomato, which accumulate a high concentration of β-carotene in place of lycopene, carry the del⁺ allele, which is either partially dominant or dependent on a modifier gene.
What happens in the gh mutant?
Tomato plants homozygous for the recessive allele gh arise spontaneously among red-fruited lines. These plants have green cotyledons, but they rapidly lose chlorophyll thereafter.
Grafting normal scions onto a gh/gh rootstock produces plants whose unripe fruit is milky-white, turning yellowish as it ripens. This fruit contains large amounts of phytoene but no coloured carotenoids; its yellow colour comes not from carotenoids but from an alkali-soluble pigment.
How is carotenoid biosynthesis controlled in the cell?
The carotenoid mutations studied in tomato all result from changes in nuclear structural genes, as their pattern of inheritance demonstrates. Drawing on these mutant studies, T. Goodwin proposed a scheme for the nuclear control of carotenoid biosynthesis in the tomato.
Mutations in other plants that alter carotenoid biosynthesis are likewise inherited as nuclear-encoded traits. At the same time, experiments performed on Chlamydomonas have shown that protein synthesis on plastid ribosomes is required for carotenoids to accumulate — evidence that the process depends on both the nuclear and the plastid genetic systems.
