metabolic pathways for the vitamins b-carotene, ascorbate, and folate. Constructs
using these cDNAs were transferred to Z. mays. Analyses of stable transgenic
pants showed that b-carotene in the transgenic kernels was increased to 169-fold,
ascorbate levels up to 6-fold, while folate levels were doubled. This approach
provides a potential use of engineered plants to deal with vitamin deficiency in the
world’s poorest regions (Ye et al. 2000).
Changing the expression of genes using RNA silencing has also proved to be a
successful strategy to modulate metabolic pathways. This type of strategy has been
recently used to alter the expression of MYB-type transcription factors to activate
the anthocyanin pathway in salvia by silencing PAP1 expression (Zhang et al. 2010)
and Medicago by downregulating LAP1. In both cases the results show that
silencing these MYB transcription factors generate transgenic plants with high
anthocyanin contents.
Another interesting application of RNAi in metabolic engineering is to knock out
a native pathway and then use the transgenic cells as biotransformation factories;
Runguphan et al. (2009) used a Catharanthus roseus hairy root culture system in
which the tryptamine biosynthesis pathway silenced. Under such conditions if the
culture is provided with a synthetic tryptamine analog, the mutated system generates
a variety of non-natural products. This novel approach can be used not only for the
successful synthesis of non-natural alkaloids, as in this case, but also to synthesize
modified metabolites of diverse molecular nature with potential applications.
15.7.1.2 Production of Heterologous Proteins and Nonprotein
Molecules Using Plant-Based Systems
Either using whole-plant stable transgenics or transient expression systems,
involving cell cultures and hairy roots, several strategies have succeeded in the
production of nonplant proteins and other exogenous molecules.
Diverse therapeutic products have been produced using plant-based systems; an
interesting and successful case is the production of antibodies in plants, the
‘‘plantibodies’’ approach (De Jaeger et al. 2000; Ma et al. 2005).
It has been a challenge for diverse research groups to produce antibodies in
plants since these molecules need to be assembled in the proper manner and folded
correctly in order to be recognized by antigens. Another challenge to overcome is
the glycosylation pattern of the proteins, which is different depending on the biological system used for their production and the glycosylation pattern of a given
peptide is a major characteristic for the function and efficiency of the antibody
(Sethuraman and Stadheim 2006). A compendium of the first plantibodies and other
pharmaceuticals generated in plants is part of the excellent review by Ma et al.
(2005). Here we mention in chronological order a few of the most recent examples:
human serum albumin (Sijmons et al. 1990), human interferon alpha (Zhu et al.
1994), antimalaria epitope vaccine (Turpen et al. 1995), secretory antibody (IgA)
(Ma et al. 1995), Clostridium xylanase (Herbers et al. 1995), Glucocerebrosidase
(Cramer et al. 1996), anticancer therapeutic protein (McCormick et al. 1999),
444
L. A. Cruz-Ramírez et al.
using these cDNAs were transferred to Z. mays. Analyses of stable transgenic
pants showed that b-carotene in the transgenic kernels was increased to 169-fold,
ascorbate levels up to 6-fold, while folate levels were doubled. This approach
provides a potential use of engineered plants to deal with vitamin deficiency in the
world’s poorest regions (Ye et al. 2000).
Changing the expression of genes using RNA silencing has also proved to be a
successful strategy to modulate metabolic pathways. This type of strategy has been
recently used to alter the expression of MYB-type transcription factors to activate
the anthocyanin pathway in salvia by silencing PAP1 expression (Zhang et al. 2010)
and Medicago by downregulating LAP1. In both cases the results show that
silencing these MYB transcription factors generate transgenic plants with high
anthocyanin contents.
Another interesting application of RNAi in metabolic engineering is to knock out
a native pathway and then use the transgenic cells as biotransformation factories;
Runguphan et al. (2009) used a Catharanthus roseus hairy root culture system in
which the tryptamine biosynthesis pathway silenced. Under such conditions if the
culture is provided with a synthetic tryptamine analog, the mutated system generates
a variety of non-natural products. This novel approach can be used not only for the
successful synthesis of non-natural alkaloids, as in this case, but also to synthesize
modified metabolites of diverse molecular nature with potential applications.
15.7.1.2 Production of Heterologous Proteins and Nonprotein
Molecules Using Plant-Based Systems
Either using whole-plant stable transgenics or transient expression systems,
involving cell cultures and hairy roots, several strategies have succeeded in the
production of nonplant proteins and other exogenous molecules.
Diverse therapeutic products have been produced using plant-based systems; an
interesting and successful case is the production of antibodies in plants, the
‘‘plantibodies’’ approach (De Jaeger et al. 2000; Ma et al. 2005).
It has been a challenge for diverse research groups to produce antibodies in
plants since these molecules need to be assembled in the proper manner and folded
correctly in order to be recognized by antigens. Another challenge to overcome is
the glycosylation pattern of the proteins, which is different depending on the biological system used for their production and the glycosylation pattern of a given
peptide is a major characteristic for the function and efficiency of the antibody
(Sethuraman and Stadheim 2006). A compendium of the first plantibodies and other
pharmaceuticals generated in plants is part of the excellent review by Ma et al.
(2005). Here we mention in chronological order a few of the most recent examples:
human serum albumin (Sijmons et al. 1990), human interferon alpha (Zhu et al.
1994), antimalaria epitope vaccine (Turpen et al. 1995), secretory antibody (IgA)
(Ma et al. 1995), Clostridium xylanase (Herbers et al. 1995), Glucocerebrosidase
(Cramer et al. 1996), anticancer therapeutic protein (McCormick et al. 1999),
444
L. A. Cruz-Ramírez et al.