Biomedical Engineering Reference
In-Depth Information
to tryptamine is a rate-limiting step in the terpenoid
pathway, and this has been addressed by overex-
pressing the enzyme tryptophan decarboxylase in
C. roseus
cell-suspension cultures. However, while
transformed cultures produced much higher levels
of tryptamine, no downstream alkaloids were syn-
thesized (Goddijn
et al.
1995, Canel
et al.
1998). The
simultaneous overexpression of the next enzyme in
the pathway, strictosidine synthase, did increase the
levels of the alkaloid ajmalicine, a useful sedative, in
some cultures, but did not result in the synthesis of
vinblastine or vincristine (Canel
et al.
1998).
Thus it seems that single-step engineering may
remove known bottlenecks only to reveal the posi-
tion of the next. The limited success of single-gene
approaches has resulted in the development of
alternative strategies for the coordinated regulation
of entire pathways using transcription factors. Using
the yeast one-hybrid system (Vidal
et al
. 1996a), a
transcription factor called ORCA2 has been ident-
ified that binds to response elements in the genes
for tryptophan decarboxylase, strictosidine synthase
and several other genes encoding enzymes in the
same pathway. A related protein, ORCA3, has been
identified using insertional vectors that activate
genes adjacent to their integration site. By bringing
the expression of such transcription factors under
the control of the experimenter, entire metabolic
pathways could be controlled externally (see review
by Memelink
et al.
2000).
Apart from the modification of endogenous
metabolic pathways to produce more (or less) of a
specific endogenous compound, plants can also be
engineered to produce heterologous or entirely novel
molecules. An example is the production of the
alkaloid scopolamine, an anticholinergic drug, in
Atropa belladonna
(Hashimoto
et al.
1993, Hashimoto
& Yamada 1994). Scopolamine is produced in
Hyo-
scyamus niger
but not in
A. belladonna
, which accu-
mulates the immediate precursor hyoscyamine.
H.
niger
converts hyoscyamine into scopolamine using
the enzyme hyoscyamine-6-hydroxylase (H6H),
which is absent in
A. belladonna
. Hashimoto and
colleagues isolated a complementary DNA (cDNA)
encoding H6H from
H. niger
and expressed it in
A.
belladonna
. The transgenic
A. belladonna
plants pro-
duced scopolamine because they were able to extend
the metabolic pathway beyond its endogenous
end-point. Another example of the production of
novel chemicals in plants is the diversion of carbon
backbones from fatty acid synthesis to the formation
of polyhydroxyalkanoates, which form biodegrad-
able thermoplastics (Steinbuchel & Fuchtenbusch
1998). In this case, the foreign genes derive not from
other plants, but from bacteria.
Theme 6: Plant breeding in the
twenty-first century
Introduction to theme 6
We have discussed several uses for transgenic plants
earlier in this chapter, i.e. as bioreactors producing
recombinant proteins (p. 285) and novel metabol-
ites (see previous section). Transgenic plants can
potentially express any foreign gene, whether that
gene is derived from bacteria, yeast, other plants or
even animals. The scope for exploitation and improve-
ment is virtually limitless, and gene-manipulation
techniques have therefore given the biotechnology
industry a new lease of life. In the following sections,
we consider the development of plant biotechnology
in two key areas: improvement of agronomic traits
and modification of production traits.
Improving agronomic traits
The initial focus of plant biotechnology was on
improving agronomic traits, i.e. the protection of
crops against pests, pathogens and weeds, and thus
increasing yields. Major crop losses are caused every
year by these so-called 'biotic' constraints, as well
as physical (or 'abiotic') factors, such as flooding,
drought, soil quality, etc. The aims of the biotechno-
logy industry went hand in hand with those of
conventional breeders, but offered the possibility of
importing useful genes from distant species that could
not be used for breeding. It has been found that, in
many cases, single genes transferred from another
organism can provide high levels of protection.
Herbicide resistance
Herbicides generally affect processes that are unique
to plants, e.g. photosynthesis or amino acid biosyn-
thesis (see Table 14.8). Both crops and weeds share
