Plant Biotechnology and Transgenic Plants

Kirsi-Marja Oksman-Caldentey, Wolfgang H. Barz, Kirsi-Marja Oksman-Caldentey, Wolfgang H. Barz

• Publisher: CRC Press
• Year: 2002
• ISBN:
  9781135562953
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

Contains case studies illustrating the cell culture production of pigments, flavors, and antineoplastic compounds Plant Biotechnology and Transgenic Plants covers topics that range from food to fragrances to fuel. It includes discussions of technologies and research on the engineering, synthesis, utilization, and control of primary and secondary pl

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Plant Biotechnology—An Emerging Field

Wolfgang H.Barz
Institute of Plant Biochemistry and Biotechnology, Westphalian Wilhelm’s University Munich, Munich, Germany

Kirsi-Marja Oksman-Caldentey
VTT Biotechnology, VTT Technical Research Center of Finland, Espoo, Finland

I. INTRODUCTION

Biotechnology is a scientific discipline with focus on the exploitation of metabolic properties of living organisms for the production of valuable products of a very different structural and organizational level for the benefit of men. The products can be the organisms themselves (i.e., biomass or parts of the organismic body), products of cellular or organismic metabolism (i.e., enzymes, metabolites), or products formed from endogenous or exogenous substrates with the help of single enzymes or complex metabolic routes. The organisms under question vary from microbes (bacteria, fungi) to animals and plants. In addition to intact organisms, isolated cells or enzyme preparations are employed in biotechnology. The possibility to submit the producing organisms or the cellular systems to technical and even industrial procedures has led to highly productive processes. The products of biotechnology are of importance for medicine, pharmaceutical sciences, agriculture, food production, chemistry, and numerous other disciplines.

Biotechnology receives the necessary scientific and technical information from a considerable number of disciplines. Cell biology, morphology of the employed organisms, biochemistry, physiology, genetics, and various technical fields are major sources. In the last two decades, molecular biology and gene technology have substantially contributed to the spectrum of scientific disciplines forming biotechnology. As is always true for progress in natural sciences, it is especially true for biotechnology that more rapid development and gain of higher standards depend on the improvement of methods.

In the historical development of biotechnology, microbes have been used preferentially. They still offer an extremely rich potential for biotechnological application. Animal systems and their cells are also valuable systems, especially in view of the very costly products (i.e., antibodies, vaccines). Although much later in the chronological process, plant biotechnology has made an impressive development in gaining basic and applicable knowledge as well as in establishing production processes. It is therefore justified to speak of an emerging field. Major steps will be discussed in this chapter.

II. A LONG HISTORY TO REACH A HIGH STANDARD

In each ecosystem plants and other photosynthetically active organisms are responsible for primary production, which provides the energetic and nutritional basis for all subsequent trophic levels. The extremely high ability of plants to adapt to all kinds of environmental conditions and ecosystems has led to an extremely wide and differentiated spectrum of plants. Since ancient times higher plants have formed the main source of food for men, and therefore, concomitant with early phases of settlements and agriculture, men started to establish and improve crop plants. Archeological evidence has clearly shown how long well-known crop species (i.e., maize, cereals, legumes) have been grown, modified by selection, and thus improved in quality and yield. Plant breeding is indeed an old art that has been continuously developed in efficiency and scope. Quite typical for quality breeding of, for instance, cereals is the long procedure required (sometimes decades) to reach particular genotypes and to cross in specific genes or traits.

An interesting achievement in breeding of wheat is characterized by the term green revolution, in which (around 1950–1960) wheat genotypes from many different countries were used successfully on a very large scale to breed high-yielding and durable lines. For many countries such new varieties were a very great improvement for their agriculture.

Another important goal in breeding improved crop plants is the often achieved adaptation to unfavorable environmental conditions (i.e., heat, drought, salt, and other cues). Although good results have been obtained, such efforts will undoubtedly remain in the focus of future efforts. Better insight into the physiology, biochemistry, and chemical reactions as well as the gene regulation of the endogenous adaptation and defense mechanisms that plants can express will contribute to these objectives. Gene technology will be an essential component in these efforts.

Another characteristic feature of the long-term breeding of cereals, potatoes, or vegetables is the fact that during the long periods the shape and the outer appearance of the plants have changed so much that the original wild types were either lost or no longer easily identified as starting material. A typical example is corn. Modern agricultural crop plants are also bred for very uniform physical appearance, time of flowering, and maturity so that harvest by machines in an industrial manner is possible (examples are cotton, maize, and cereals). It is a feature of our high-yielding agriculture that all possible mechanical techniques are being employed.

Very precious treasures for future agriculture and for plant biotechnology are the gene banks and the International Breeding Centers, where great numbers of genotypes of crop plants are multiplied and carefully preserved for long periods of time. Such “pools of genes” represent the basis for sustainable development and allow future programs for improved adaptation of plants to human needs. Fortunately, the understanding has gained ground in recent years that in addition to crop plants all types of wild plants, in every ecosystem, must be preserved because of the genetic resources to be possibly exploited in the future.

An interesting development in itself, with a long history and remarkable contributions to culture and art, is the numerous and sometimes highly sophisticated ornamental plants produced in many countries. Beauty of color and flower shape were the guidelines in their breeding and selection. Rather early in this development the value of mutagenetic reagents was learned, and these ornamentals also served to shape the term of a mutant. Recent biochemical studies with, for example, snapdragon, tulip, chrysanthemum, or petunia and their flavonoid constituents clearly presented evidence that the various flower colors can contribute to identifing biosynthetic pathways.

In connection with flower pigments, which are secondary metabolites, it should be remembered that numerous other secondary constitutents of very different chemical structures are valuable Pharmaceuticals. In many countries knowledge of plants as sources of drugs has been cherished for long times. Modern pharmacological and chemical studies have helped in the identification of the relevant compounds. Such investigations are still considered important objectives of plant biotechnology. In some cases extensive breeding programs have already achieved the selection and mass cultivation of high-yielding lines. In modern pharmacy, about 25% of drugs still contain active compounds from natural sources, which are primarily isolated from plants.

For a good number of years in the period from 1950 to 1980, plant biochemistry and plant biophysics concentrated on elucidation of the photosynthetic processes. The pathways of CO₂ assimilation as well as structure, energy transfer reactions, and membrane organization of chloroplasts and their thylakoids were objectives of primary interest. Chloroplast organization and molecular function of this organelle can be regarded as well-understood fields in plant biochemistry and physiology.

The last three decades of the 20th century were characterized by very comprehensive molecular analyses of chemical reactions, metabolic pathways, cellular organization, and adaptative responses to unfavorable environmental conditions in numerous plant systems. A very broad set of data has been accumulated so that plant biochemistry and closely related fields can now offer a good understanding of plants as multicellular organisms and highly adaptative systems. From a molecular point of view, the construction and the functioning of the different tissues and organs have become clear. Numerous experimental techniques have contributed to this development and some are typical plant-specific methods (i.e., cell culture techniques) with a very broad scope of application.

A fascinating field of modern plant biochemistry concerns the elucidation of the function and the molecular mechanisms of the various photoreceptor systems of higher plants. Red/far red receptors, blue light-absorbing cryptochromes, and ultraviolet (UV) light photoreceptors are essential components of plant development (1). These systems translate a light signal into physiological responses via gene activation. Quite remarkable, phosphorylated/unphosphorylated proteins are the essential components of the signal transduction system (1,2). Biotechnology will gain from this knowledge, and highly sensitive sensor systems could possibly be constructed.

In the history of plant sciences and biotechnology, the recent development of molecular biology and the introduction of gene technology deserve emphasis. Isolation, characterization, and functional determination of genes have become possible. Many plant genes were rather rapidly identified, and the number is increasing at enormous speed. Promoter analyses and identification of promoter binding proteins have decisively contributed to an understanding of the organization and function of plants as organisms consisting of multiple tissues and different organs. The phenomena of multigenes and multiple enzymes in one protein family were further revealed. Many different techniques in molecular biology and gene technology turned out to be extremely valuable. Recognition of the biology of Agrobacterium tumefaciens and application of its transferred DNA (T-DNA) system represented giant leaps forward. In general, because of these modern gene technological methods, plant biotechnology has grown into a new dimension with putative future possibilities that can hardly be overestimated.

In the following sections of this chapter several recent and future aspects of biotechnological relevance will be discussed.

III. PLANT TISSUE AND CELL CULTURES—A VERY VERSATILE SYSTEM

The present status of plant biotechnology cannot be evaluated without appreciation of the many possibilities and the potential of organ, tissue and cell suspension cultures. Plants of wide taxonomic origin have been subjected to culture under strictly aseptic conditions. Completely chemically defined media supplemented with growth regulators and phytohormones are the basis for the exploitation of this technique. Depending on the explant and the culture conditions cells either preserve their state of biochemical and morphological differentiation or return to a status of embryogenic, undifferentiated cells. The former situation can be used for organ cultures (e.g., pollen, anthers, flower buds, roots), whereas the latter leads to many callus and suspension types of cultures (3). For example, the cell culture technique has opened a facile route to haploid cells and plants, and such systems are of great importance for genetic and breeding studies.

Whenever a heterogeneous group of cells can be turned into a state of practically uniform cells, this much less complicated cellular system can then be exploited to study many problems. This has been performed with plant cell cultures for some 30 years now. Growth of cells in medium-size and large volumes has opened interesting applications for plant biotechnology. Numerous physiological, biochemical, genetic, and morphological results and data on cellular regulation stem from such investigations. Various primary and secondary metabolic routes have been elucidated with the help of cell culture systems. The typical sequence in pathway identification was first product and intermediate characterization, then enzyme studies, and finally isolation of genes. Furthermore, application of gene technology in the field of transgenic plants depends to some extent on the tissue and cell culture techniques (4).

Plants are characterized by totipotency, which means that each cell possesses and can express the total genetic potential to form a fully fertile and complete plant body. This fact, highly remarkable from a cell biological point of view, is the genetic basis for important and widely used applications of the cell culture technique. Differentiation of single cells or small aggregates of cells into embryos, tissues, and even plants allows the selection of interesting genotypes for several different fields of plant application (5).

The well-established procedures for mass regeneration of valuable specimens of ornamental and crop plants constitute an important business section in agriculture and gardening. Endangered plant species can be saved from extinction so that valuable gene pools will not disappear. Remarkable progress has been achieved in mass regeneration of trees from single plants or tissue pieces. This will undoubtedly be of further great benefit for forestry because several problems in tree multiplication can thus be circumvented (6). Furthermore, it should be mentioned that plant cell suspension cultures possess a great potential for biotransformation reactions in which exogenously applied substrates are converted in sometimes high yields. Position and stereospecific hydroxylations, oxidations, reductions, and especially interesting glucosylation of very different substrates have been found (7). The plant cell culture technique has allowed the facile isolation of mutants from many plant species. The overwhelming importance of mutants for biochemical and genetic studies has been known for decades. Over the years, mutations from all areas of cellular metabolism have been selected and characterized. A good deal of our basic knowledge of the functioning and regulation of organisms and cells and their organelles stems from work with mutants. The various techniques of plated, suspended, or feeder cell-supported cell systems and even protoplasts have found wide applications (5). The normal rate of mutation and also increased levels of mutated cells induced by physical (UV light, high-ener...