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117 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 118 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. 119 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.