five-carbon building block, which can be assembled into multimers to form complex
molecules. Many familiar plant scents and flavors, such as mint and pine resin, are based
on terpenoids. The five-carbon precursor to terpenoids can be produced via two indepen-
dent pathways, in either the cytoplasm or in plastids. Carotenoids are 40-carbon compounds
produced from the precursor molecule via a biochemical pathway localized in plastids. The
40-carbon backbone ofb-carotene is phytoene, which is assembled by combination of two
20-carbon geranylgeranyl diphosphate (GGPP) molecules by the enzyme phytoene
synthase (Fig. 8.6). Double bonds are then added to phytoene through a series of desatura-
tion steps to produce lycopene, an antioxidant compound found in most plants and that con-
tributes to the red color of tomatoes. Finally, lycopene can be converted tob-carotene by
the enzyme lycopene cyclase. Much of the understanding of how this pathway operates and
could be manipulated came from the laboratories of Dr. Ingo Potrykus in Switzerland (see
Life Box 8.2) and Peter Beyer in Germany. Researchers in these labs led the way in trans-
forming rice with the necessary genes to produce carotenoids in rice grains.
Rice grains naturally produce GGPP, and so the addition of an active phytoene synthase
gene expressed in rice grains under the control of a seed endosperm-specific promoter led to
the production of phytoene in preliminary experiments. Transgenic plants were later
produced via particle bombardment in which genes for phytoene synthase, phytoene
desaturase, and a lycopene cyclase were cotransformed. These transgenic rice plants had
grains with a bright yellow coloring, which was confirmed to come from the presence of
Figure 8.6.The production ofb-carotene in Golden Rice was made possible by high-level, tissue-
specific expression of the necessary enzymes in rice. Rice grains normally produce geranylgeranyl–
diphosphate (GGPP). A gene-encoding phytoene synthase was transferred to rice from daffodil (for
the original Golden Rice) or maize (in Golden Rice 2), and this led to production of phytoene in
rice grains. A desaturase enzyme necessary to add double bonds to the structure was provided by
transfer of a bacterial gene to rice (the two arrows at this step represent the multiple reactions that
are necessary to add all double bonds). Finally, lycopene was converted in rice grains by an endogen-
ous lycopene cyclase activity to the yellow-orange endproduct,b-carotene.
206 GENES AND TRAITS OF INTEREST FOR TRANSGENIC PLANTS