Biosynthesis of β-carotene (provitamin A) in rice endosperm achieved by genetic engineering

To obtain a functioning provitamin A (β-carotene) biosynthetic pathway in rice endosperm, we introduced in a single, combined transformation effort the cDNAs coding for (1) phytoene synthase (psy) and (2) lycopene β-cyclase (β-lcy; both from Narcissus pseudonarcissus and both under control of the endosperm-specific glutelin promoter), with (3) a bacterial phytoene desaturase (crtl, from Erwinia uredovora under constitutive 35S promoter control). This combination covers the requirements for β-carotene synthesis, and yellow, β-carotene-bearing rice endosperm was obtained in the T₀ generation. However, further experiments revealed that the presence of β-lcy was not necessary, since psy and crtI alone were able to drive β-carotene synthesis as well as the formation of further downstream xanthophylls. This finding could be explained if these downstream enzymes are either constitutively expressed in rice endosperm or are induced by the transformation, e.g. by products derived therefrom. Based on results in N. pseudonarcissus as a model system, a likely hypothesis can be developed that trans lycopene or a trans lycopene derivative acts as an inductor in a kind of feedback mechanism stimulating endogenous carotenogenic genes.