Download Plant biotechnology The ins and outs of a new green revolution

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Plant biotechnology
The ins and outs of a new green revolution
Editorial overview
Nam-Hai Chua* and Venkatesan Sundaresan†
Addresses
*Laboratory of Plant Molecular Biology, Rockefeller University,
1230 York Avenue, Ney York, NY 10021, USA;
e-mail: [email protected]
† Institute of Molecular Agrobiology, 1 Research Link, The National
University of Singapore, Singapore 117604;
e-mail: [email protected]
Current Opinion in Biotechnology 2000, 11:117–119
0958-1669/00/$ — see front matter
© 2000 Elsevier Science Ltd. All rights reserved
The past decade has witnessed a revolution in agriculture
arising from plant biotechnology, that is, the application of
the tools of molecular biology to plant breeding and cultivation. The first generation of plant biotechnology has
largely focussed on input traits that benefit mainly the
farmer. Examples of such traits include the introduction of
genes for insect resistance and herbicide tolerance, which
reduce losses due to weeds and pests as well as the quantity of chemical applications. The outstanding success of
these engineered crops is evident from the fact that the area
of farmland devoted to transgenic crops has grown from a
negligible acreage 10 years ago to well over half the acreage
for major crops in agriculturally important countries such as
USA, Canada and Argentina. Although the introduction of
new input traits that benefit the growers will continue, it is
expected that a second generation of plant biotechnology
dealing with output traits that directly benefit the consumers will eventually become prominent. Desirable
output traits might include prolonging shelf-life through
vegetables that are more resistant to rotting and fungal
infections, nutritionally superior foods, such as the vitamin
A enriched ‘golden rice’ [1] or tropical oils that are unsaturated, and agricultural products of medical value, such as
edible vaccines. A long lasting acceptance of genetically
modified crops might well depend upon the emergence of
this second generation of plant biotechnology.
This issue of Current Opinion in Biotechnology covers
advances in plant biotechnology with eight review articles.
The first article describes recent developments in the area
of plant disease-resistance genes, and the following three
articles are devoted to useful output and input traits (i.e.
edible vaccines, and the manipulation of plant growth and
development by hormone biosynthetic and cell-cycle
genes). The fifth and sixth chapters are on advances in
technologies to regulate gene expression, either through
chemical application or RNA-mediated gene silencing.
The final two chapters discuss recent advances in
genomics that permit the rapid identification and functional characterization of genes through large-scale
insertional mutagenesis and by the use of microarrays.
It has been estimated that weeds and pathogens together
are responsible for about 30% of losses in crop yields
worldwide. Early commercial targets of plant genetic modifications therefore focused on the so-called input traits
that can render crops tolerant to these biotic stresses.
Although herbicide- and insect-tolerant transgenic crops
have been commercially planted in the field since 1996,
plants that are resistant to fungal and bacterial pathogens
are still not yet available to farmers. This is because discoveries leading to the isolation and characterization of
plant resistance (R) genes that confer resistance to various
pathogens were made only in the past 5–6 years. Rommens
and Kishore (pp 120–125) review recent work on the
mechanism of action of the R genes as well as genes
encoding downstream responses, and discuss various
strategies to use these genes to produce durable disease
resistance in plants. With the intense activity in this field,
Rommens and Kishore predict that crop plants resistant to
a broad spectrum of diseases will be commercially available within the next five years.
A plant biotechnological innovation that might have
important public health implications is the production of
edible vaccines in transgenic plants. Walmsley and Arntzen
(pp 126–129) give an update on recent progress in this
field. Vaccinogens can be expressed either from plant viral
vectors or as transgenes. In the former case, the vaccinogens need to be purified from the virus-infected plants,
whereas in the latter case, transgenic plant organs expressing vaccinogens can be consumed directly with the hope
that they will act as oral vaccines to confer immunity. So
far, at least seven antigens from various animal and human
pathogens have been successfully expressed in plants and,
in several cases, initial results indicated that they are able
to elicit mucosal immune responses. Future challenges
include the design of vaccinogens so that they can be protected from digestive enzymes in the gut to be more
effective in providing immunity.
As hormones regulate the growth and development of
plants, it is not surprising that changes in hormone levels
through manipulation of biosynthetic genes are expected
to alter their developmental and environmental responses. One of the first examples came from studies of the
mechanisms of crown gall disease, which we now know
is due to the overproduction of both auxins and
cytokinins in transformed cells. Since then, a number of
hormone biosynthetic genes, initially derived from bacteria, and later from plants, have been used to
manipulate endogenous plant hormone levels with dramatic developmental consequences. Hedden and
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Plant biotechnology
Phillips (pp 130–137) summarize recent results on the
characterization of plant hormone biosynthetic genes
and discuss their biotechnological applications. It should
be noted that in addition to hormone biosynthetic genes,
genes in the hormone response pathway can also be used
for similar manipulations. Indeed, the green revolution
was in part fuelled by the discovery of dominant mutations that result in high yielding dwarf varieties of wheat,
and Peng et al. [2] have recently shown that the mutated
gene in wheat in fact encodes a component of the gibberellin response pathway.
Because hormones modulate plant growth, development,
and differentiation, it is reasonable to expect that they execute their effects in part through control of the cell cycle.
Although genes encoding many major components of the
cell cycle have been characterized, the precise mechanism
by which hormones interact with cell-cycle components
are still largely unknown, as are the precise mechanisms
that determine cell size and cell division rate. In their article, den Boer and Murray (pp 138–145) review our present
knowledge of plant cyclins and cyclin-dependent kinases,
and discuss various strategies to manipulate the plant cell
cycle through changes in activities of its regulatory components. From the results discussed in this review, it is clear
that it should be possible in the future to modify plant
growth, architecture, and even yields, through modulations
of the activity of the plant cell-cycle machinery.
Chemical-inducible expression systems are an important
tool for plant research as they can allow controlled
expression of input and output traits in the desired plant
tissues and at the right time. Currently, several chemicalinducible systems are available, and in the article on this
subject, Zuo and Chua (pp 146–151) review their mode of
action and compare their relative merits and utility. Such
inducible systems have obvious commercial applications.
The value of transgenes encoding input or output traits
can only be protected if the genes are incorporated into
hybrid seeds. For non-hybrid crops (e.g. cotton), transgene value may be eroded with time after the initial seed
purchase because of possible seed saving by growers.
The recurring capture of value can be ensured, however,
if the transgene is placed under chemical protection.
Expression of the encoded trait can then be triggered by
a proprietary chemical, which is sold to the growers. This
system would also offer flexibility and cost savings to the
growers as they can apply different amounts of chemicals
depending on the prevailing field conditions. In this
regard, Zuo and Chua also discuss various strategies for
future development of chemical-inducible systems, with
emphasis on registered agrochemicals that can be applied
to field crops.
The ability to reduce or abolish the expression of a cloned
gene is important both for elucidating its function and for
manipulating gene expression in transgenic plants. It has
been known for several years that this can be accomplished
by exploiting a fascinating phenomenon called post-transcriptional gene silencing, which was originally discovered
in plants. Similar silencing phenomena have been subsequently described in fungi and in animals as well. In his
review, Ding (pp 152–156) discusses several recent breakthroughs in understanding the underlying mechanism of
this phenomenon, and summarizes results from all three
kingdoms. The silencing mechanism, which targets highly
expressed RNAs, might have evolved as a host defense
mechanism; in the case of plants, its primary role might be
to defend against viral pathogens. As there are no currently available methods for efficient targeted gene
inactivation through recombination in plants, the application of RNA silencing on a wider scale could become an
important tool for functional genomics.
The complete genome sequence of Arabidopsis thaliana
will become available in mid 2000, and with it the
sequences of all the genes that are required to make a
flowering plant. Thus the focus of research on this model
plant will shift from forward genetics to reverse genetics.
Parinov and Sundaresan (pp 157–161) review the largescale insertional mutagenesis efforts that have made it
possible to generate knockouts of a significant fraction of
the estimated 25,000 genes in Arabidopsis. Different strategies to identify and characterize knockouts in specific
genes are discussed and evaluated. As many potentially
useful genes in crop plants can be cloned using orthologs
isolated initially from Arabidopsis, these studies will have a
direct impact on genetic manipulation of crop plants [3]. In
addition, reverse genetics strategies that have been refined
using Arabidopsis are likely to prove useful in functional
genomics studies of crop plants as well.
In the final chapter, Schaffer et al. (pp 162–167) review
novel technologies, such as microarrays and DNA chips,
that can be used for the analysis of global gene expression
patterns. The power of these technologies has been
demonstrated by detailed studies in the budding yeast
Saccharomyces cerevesiae. Currently, these technologies have
not been extensively deployed in studies on plants, but
Schaffer et al. discuss the impact that they are likely to
have on plant genomics in the future, especially in those
crop plants where other methods for functional genomics,
such as insertional mutagenesis, may not be feasible.
It is evident that several exciting technologies are being
developed that will accelerate the already dizzying pace of
progress in this field. The advances in plant genomics
research in particular are resulting in an explosion of information that will yield new knowledge about plant biology
at a rate that was unimaginable a decade ago. The challenge now is for plant biotechnologists to translate this
knowledge into practical applications, especially those that
directly benefit the ultimate end users, that is, the consumers. If this can be successfully accomplished then the
continued rapid growth of plant biotechnology and of its
revolutionary contributions to agriculture will be assured.
Editorial overview Chua and Sundaresan
References
1.
Ye X, Al-Babili S, Kloti A, Zhang J, Lucca P, Beyer P, Potrykus I:
Engineering the provitamin A (beta-carotene) biosynthetic
pathway into (carotenoid-free) rice endosperm. Science 2000,
287:303-305.
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2.
Peng J, Richards DE, Hartley NM, Murphy GP, Devos KM, Flintham JE,
Beales J, Fish LJ, Worland AJ, Pelica F: ‘Green revolution’ genes encode
mutant gibberellin response modulators. Nature 1999, 400:256-261.
3.
Somerville C: The twentieth century trajectory of plant biology. Cell
2000, 100:13-15.