pathways; it was performed by introducing single genes, multiple genes, and gene
silencing by iRNA.
15.4.3.2 Modification of Single Genes
The modification of a metabolic pathway to increase the concentration of a product
of interest using a single gene was the first strategy to create transgenic organisms,
with the goal to have superior characteristics in crops (Capell and Christou 2004).
Molecular biology and genetic engineering in conjunction with transformation and
plant regeneration allow to obtain genetically modified organisms. The manipulation of single genes is of only limited value in metabolic engineering, and
attention has shifted toward more complex and sophisticated strategies in which
several steps in a given pathway are modified simultaneously to achieve optimal
flux (Schwender 2008). Emerging in the early metabolic engineering, the first
generation of transgenic plants, such as Bt cotton, express a toxin of Bacillus
thuringiensis that makes the corn resistant to pests (Qaim 2010). Another example
is the modification of A. thaliana with a gene from Pseudomonas stutzeri PTxd
WM88. This gene encodes the enzyme phosphite oxide reductase which catalyzes
the oxidation of phosphite. It allows the transformed plants to take advantage of a
phosphorous source which is not metabolized by and results toxic for wild type
plants (Lopez-Arredondo and Herrera-Estrella 2012).
15.4.3.3 Modification of Multiple Genes
The simultaneous transfer of multiple genes into plants enables researchers to study
and manipulate entire metabolic pathways, express multimeric proteins or protein
complexes, and study complex genetic control circuits and regulatory hierarchies
(Halpin 2005). Early transformation methods for plants were developed with the
implicit intention to introduce one or two genes, usually a transgene of interest and
a selectable or screenable marker, and have been optimized on that basis (Twyman
et al. 2002). This strategy of metabolic engineering is observed in the modifications
made to the golden rice to express b-carotene (Ye et al. 2000). In this experiment
were introduced three enzymes, phytoene synthase encoded by psy gene of
Narcissus pseudonarcissus, phytoene desaturase encoded by crt1 gene of Erwinia
uredovora and b-Lycopene cyclase encoded in lcy gene of N. pseudonarcissus.
Enzymes take as substrate geranyl diphosphate, an early intermediate in carotenoid
synthesis pathway, which is transformed into lycopene and beta-carotene and
finally deposited on rice endosperm giving it a strong yellow color (Ye et al. 2001).
After the evaluation of the transformed lines it was possible to establish one of
the limiting factors in the synthesis of b-carotene from golden rice, the enzyme
phytoene synthase (psy) of N. pseudonarsissus.
A new strategy was established in order to increase the concentration of provitamin A in the endosperm of rice, various psy genes from different plant sources
15 Plants as Bioreactors for Human Health Nutrients
435
silencing by iRNA.
15.4.3.2 Modification of Single Genes
The modification of a metabolic pathway to increase the concentration of a product
of interest using a single gene was the first strategy to create transgenic organisms,
with the goal to have superior characteristics in crops (Capell and Christou 2004).
Molecular biology and genetic engineering in conjunction with transformation and
plant regeneration allow to obtain genetically modified organisms. The manipulation of single genes is of only limited value in metabolic engineering, and
attention has shifted toward more complex and sophisticated strategies in which
several steps in a given pathway are modified simultaneously to achieve optimal
flux (Schwender 2008). Emerging in the early metabolic engineering, the first
generation of transgenic plants, such as Bt cotton, express a toxin of Bacillus
thuringiensis that makes the corn resistant to pests (Qaim 2010). Another example
is the modification of A. thaliana with a gene from Pseudomonas stutzeri PTxd
WM88. This gene encodes the enzyme phosphite oxide reductase which catalyzes
the oxidation of phosphite. It allows the transformed plants to take advantage of a
phosphorous source which is not metabolized by and results toxic for wild type
plants (Lopez-Arredondo and Herrera-Estrella 2012).
15.4.3.3 Modification of Multiple Genes
The simultaneous transfer of multiple genes into plants enables researchers to study
and manipulate entire metabolic pathways, express multimeric proteins or protein
complexes, and study complex genetic control circuits and regulatory hierarchies
(Halpin 2005). Early transformation methods for plants were developed with the
implicit intention to introduce one or two genes, usually a transgene of interest and
a selectable or screenable marker, and have been optimized on that basis (Twyman
et al. 2002). This strategy of metabolic engineering is observed in the modifications
made to the golden rice to express b-carotene (Ye et al. 2000). In this experiment
were introduced three enzymes, phytoene synthase encoded by psy gene of
Narcissus pseudonarcissus, phytoene desaturase encoded by crt1 gene of Erwinia
uredovora and b-Lycopene cyclase encoded in lcy gene of N. pseudonarcissus.
Enzymes take as substrate geranyl diphosphate, an early intermediate in carotenoid
synthesis pathway, which is transformed into lycopene and beta-carotene and
finally deposited on rice endosperm giving it a strong yellow color (Ye et al. 2001).
After the evaluation of the transformed lines it was possible to establish one of
the limiting factors in the synthesis of b-carotene from golden rice, the enzyme
phytoene synthase (psy) of N. pseudonarsissus.
A new strategy was established in order to increase the concentration of provitamin A in the endosperm of rice, various psy genes from different plant sources
15 Plants as Bioreactors for Human Health Nutrients
435