Isolation of Agronomically Useful Mutants from Plant Cell Cultures

Herbicide-resistant mutants from tobacco cell cultures

Several mutants resistant to the herbicides chlorsulfuron and sulfometuron methyl were isolated form cultured cells of Nicotiana tabacum. Resistance was inherited as a single dominant or semidominant mutation in all cases. Linkage analysis of six mutants identified two unlinked genetic loci. Studies of plants homozygous for one mutation showed the mutant plants to be completely resistant to treatment with a concentration of chlorsulfuron 100 times higher than that which produces symptoms of phytotoxicity on normal plants.

Acetolactate synthase is the site of action of two sulfonylurea herbicides in higher plants

Biochemical and genetic studies of a tobacco mutant resistant to the herbicides chlorsulfuron and sulfometuron methyl have demonstrated that these sulfonylurea herbicides inhibit acetolactate synthase, the first enzyme specific to the branched chain amino acid biosynthetic pathway. Resistance of this mutant is accomplished by production of a form of the enzyme that is insensitive to inhibition by the two herbicides.

Plant biotechnology for crop improvement

The typical crop improvement cycle takes 10-15 years to complete and includes germplasm manipulations, genotype selection and stabilization, variety testing, variety increase, proprietary protection and crop production stages. Plant tissue culture and genetic engineering procedures that form the basis of plant biotechnology can contribute to most of these crop improvement stages. This review provides an overview of the opportunities presented by the integration of plant biotechnology into plant improvement efforts and raises some of the societal issues that need to be considered in their application.

The development of sulfonylurea herbicide-resistant birdsfoot trefoil (Lotus corniculatus) plants from in vitro selection

Herbicide-resistant lines of birdsfoot trefoil (Lotus corniculatus L. cv 'Leo') were isolated after sequential selection at the callus, shoot, and whole plant levels to the sulfonylurea (SU) herbicide Harmony {DPX-M6316; 3-[[[(4-methoxy-6methyl-1,3,5, triazine-2-yl) amino] carbonyl] amino] sulfonyl-2-thiophenecarboxylate}. In field and growth chamber tests the Harmony regenerant lines displayed an increased tolerance as compared to control plants from tissue culture and controls grown from seed. Results of evaluation of callus cultures of regenerated mutant lines signify stability of the resistance. Outcrossed seeds collected from field trials, and tested in vitro for herbicide resistance, indicate that the trait is heritable and that resistance may be due to reduced sensitivity of acetolactate synthase to SU inhibition. Genetically stable herbicide-resistant lines of birdsfoot trefoil were successfully isolated using in vitro selection.

Mutations in corn (Zea mays L.) conferring resistance to imidazolinone herbicides

Three corn (Zea mays L.) lines resistant to imidazolinone herbicides were developed by in vitro selection and plant regeneration. For all three lines, resistance is inherited as a single semidominant allele. The resistance alleles from resistant lines XA17, XI12, and QJ22 have been crossed into the inbred line B73, and in each case homozygotes are tolerant of commercial use rates of imidazolinone herbicides. All resistant selections have herbicide-resistant forms of acetohydroxyacid synthase (AHAS), the known site of action of imidazolinone herbicides. The herbicide-resistant phenotypes displayed at the whole plant level correlate directly with herbicide insensitivity of the AHAS activities of the selections. The AHAS activities from all three selections have normal feedback regulation by valine and leucine, and plants containing the mutations display a normal phenotype.

Somaclonal variation--genetic basis and breeding applications

Somaclonal variation, the recovery of genetic changes in plants regenerated from tissue culture, offers an opportunity to uncover natural variability and to use this variability for the development of new varieties. This review focuses on the unique variation generated by this technique and the current use of somaclonal variation to develop new plant varieties.

Somaclonal variation in some agronomic and quality characters in wheat

A total of 256 selected lines derived from tissue culture of three hexaploid wheat cultivars were grown in a replicated hill plot experiment to examine somaclonal variation in agronomic characters. The lines were derived by single plant selection for various characters from a total of 100 regenerants, and were either SC3 or SC4 generation in the test. Significant variation was found in all the characters measured: height, grain number per spike, kernel weight, yield, total dry weight and harvest index. In most cases, variation could be identified which was both less than and greater than the parental controls. However, there was also an apparent effect of the parent cultivar on the total amount and direction of the variation. For two cultivars, lines could be traced back through the culture phase to individual explant embryos. Many of the original embryos contributed significant variation, and most characters showed significant variation arising from more than one embryo. In the following year, 32 lines selected from the hill plot experiment were grown in larger replicated plots and yield, harvest index and a number of grain and baking quality characters were measured. No lines selected for high yield or harvest index maintained significant improvements over their parental controls. However, significant variation was displayed for many of the quality characters examined. Significant increases in kernel weight, hardness and protein content, and a significant reduction in yellow pigmentation represented potentially useful improvements. Only unfavourable variation was seen in flour yield and in mixograph height, time and breakdown.

The biotechnology of Bacillus thuringiensis

One of the challenges in the application of biotechnology to pest control is the identification of agents found in nature which can be used effectively. Biotechnology offers the potential of developing pesticides based on such agents which will provide environmentally sound and economically feasible insect control alternatives. Such an agent, the insect pathogen Bacillus thuringiensis, is the subject of intense investigations in several laboratories. Insecticides which use the entomocidal properties of B. thuringiensis are currently produced and sold worldwide; new products are currently in the development stage. Herein, the biology and genetics of B. thuringiensis and the problems associated with current products are critically reviewed with respect to biotechnology. Moreover, the economic and regulatory implications of technologically advanced products are evaluated.

An in vitro whole plant selection system: paraquat tolerant mutants in the fern Ceratopteris

A whole plant selection system using the haploid gametophyte generation of the fern Ceratopteris richardii has been developed to select for mutations that confer resistance or tolerance to various selection pressures. The expression of the mutations can be analyzed and characterized in both the haploid gametophyte and diploid sporophyte generations. Genetic analyses are facilitated by the fern's rapid life cycle and the ease of manipulating the gametophyte generation. Selection for tolerance to the herbicide paraquat has yielded two mutants which have an increased tolerance to the herbicide in both the gametophyte and sporophyte generations. Both mutants exhibit single nuclear gene inheritance patterns and appear to be closely linked or allelic.

An unstable anthocyanin mutation recovered from tissue culture of alfalfa (Medicago sativa) : 1. High frequency of reversion upon reculture

A white-flowered mutant ("WFM") was regenerated from tissue culture of a purple-flowered plant of tetraploid alfalfa (Medicago sativa L.). When WFM was recultured, many regenerated plants (>20%) were purple-flowered. Genetic analysis established that a functional allele, C2, of a locus required for anthocyanin pigmentation was in the simplex condition (C2c2c2c2) in the donor genotype when it mutated to an unstable recessive ("mutable") allele, c2-m4, which is carried by WFM. Tissue culture experiments demonstrated that c2-m4 reverts to function at a high frequency in vitro. Results indicate that reversion occurs early in culture and may be the result of a genome shock associated with callus formation. Reversion also occurs in planta, but at a much lower frequency than in vitro. The c2-m4 allele is transmitted to progeny which revert in tissue culture. Revertant alleles, like the progenitor allele, are stable and are sexually transmitted. The action of a transposable element which is especially active in vitro is suggested.

Variation from plant tissue cultures: Biotechnological application to improving salinity tolerance

Breeding for salt tolerance in crop plants is envisaged as one way to combat a worldwide problem of increasing soil salinity in agricultural land. Tissue culture techniques may prove valuable as a means of achieving this goal. In this review, reports of the selection and characterization of plant cell cultures tolerant to excess salt are assessed, in the context of variability from tissue culture and the significance of cellular physiological adaptation to salinity. The examples of plant regeneration from salt grown cell cultures are also outlined, with emphasis on correlation to the effect of salt on cell cultures, genetic variability for salt tolerance in vitro, and the value of regenerates in the development of salt tolerant plants.

Large scale selection of aluminum-resistant mutants from plant cell culture: expression and inheritance in seedlings

A large number of aluminum-resistant variants, selected from non-mutagenized homozygous diploid cell cultures of Nicotiana plumbaginifolia Viv., are characterized. Of 115 variants cloned and reselected from single cells, 67 retained Al resistance in callus cultures after 6-9 months of growth in the absence of Al. There was no association between Al resistance and callus growth in the absence of Al, suggesting that the Al-resistant phenotype is not detrimental in the absence of Al challenge and that Al resistance is not the result of increased vigor. Plants regenerated from initially resistant callus lines that subsequently lost their resistance failed, with one exception, to transmit resistance to their seedling progeny. Fertile plants were regenerated from 40 of the 67 variants that retained stable Al resistance in callus culture. All 40 transmitted Al resistance to their seedling progeny (selfed and backcrossed) in segregation ratios expected for a single dominant mutation. The selfed progeny of many variants also segregated for recessive lethal mutations which were attributed to independent mutations that occurred during cell culture.

Biotechnology in Food Production and Processing

The food processing industry is the oldest and largest industry using biotechnological processes. Further development of food products and processes based on biotechnology depends upon the improvement of existing processes, such as fermentation, immobilized biocatalyst technology, and production of additives and processing aids, as well as the development of new opportunities for food biotechnology. Improvements are needed in the characterization, safety, and quality control of food materials, in processing methods, in waste conversion and utilization processes, and in currently used food microorganism and tissue culture systems. Also needed are fundamental studies of the structure-function relationship of food materials and of the cell physiology and biochemistry of raw materials.

Natural plant chemicals: sources of industrial and medicinal materials

Many higher plants produce economically important organic compounds such as oils, resins, tannins, natural rubber, gums, waxes, dyes, flavors and fragrances, pharmaceuticals, and pesticides. However, most species of higher plants have never been described, much less surveyed for chemical or biologically active constituents, and new sources of commercially valuable materials remain to be discovered. Advances in biotechnology, particularly methods for culturing plant cells and tissues, should provide new means for the commercial processing of even rare plants and the chemicals they produce. These new technologies will extend and enhance the usefulness of plants as renewable resources of valuable chemicals. In the future, biologically active plant-derived chemicals can be expected to play an increasingly significant role in the commercial development of new products for regulating plant growth and for insect and weed control.

Progress in cellular engineering of plants: biochemical and genetic assessment of selectable markers from cultured cells

Recent availability of stable and well characterized selectable markers and ability to combine alien genomes parasexually have contributed to the development of molecular biology in higher plants, including gene expression and genetic manipulation.Several types of biochemical mutants (resistant to inhibitory concentrations of aminoacid(s) or aminoacid analogs as well as deficient for enzyme activity) have recently been isolated and characterized biochemically and genetically. Among them, mutants with alterations in the nitrogen and aminoacid metabolism, or in the activity of alcohol dehydrogenases are being used in the development of more efficient techniques of gene transfer.The manipulation of whole genomes by sexual or somatic cell fusion offers new potential in this field, but refinement of transfer techniques is desirable. The new set of selectable markers obtained through advanced cellular technology, as well as our ability to regenerate plants from manipulated cell lines are expected to play a major role in cellular engineering.

Advances in somatic cell genetics of higher plants - the protoplast approach in basic studies on mutagenesis and isolation of biochemical mutants

Selection strategies developed in microbial genetics were successfully extrapolated to in vitro cell culture systems of higher plants and are having a major impact in the elucidation of regulatory mechanisms of basic cellular processes in eukaryotes. Although an increasing number and wide spectrum of biochemical variants have been isolated in such cell culture systems, their routine selection, characterization, and manipulation have not yet been achieved. Methodological limitations are considered to be one of the major reasons. Suspension or callus cultures, so extensively employed during the last decade in mutation-selection experiments and so useful in demonstrating the potentialities of in vitro screening techniques in obtaining various biochemical markers, have inherent drawbacks which limit in our opinion their further contribution in this field. Protoplast cultures represent an ideal tool for mutation and selection experiments. It is the purpose of this review to show how, due to recent methodological advances in the manipulation of some model protoplast culture systems, essential aspects of mutagenesis and selection of biochemical mutants can be reconsidered. These systems are simple and efficient, and lend themselves to statistical interpretation. Genetic analysis of selected variants should help us to understand and define better the new set of problems and concepts revealed by the somatic cell genetics of higher plants; combined with biochemical analyses it should elucidate the basic relationship between control of biological processes at cellular and whole organism level.

Genetic manipulation in plant breeding: somatic versus generative

A comparison is made between molecular/ in vitro/somatic and plant-level/generative approaches in the reconstruction of genotypes and reproductive systems. Although classical methods will remain the basis of plant breeding, a number of new somatic as well as generative genetic manipulation techniques are definitely applicable in several special situations. The first are technically more demanding, the latter are often conceptually more difficult, and both are laborious. Choice of approach is determined by the plant species, the stage of development of the techniques, the amount of background genetic information and the genetic diversity available, and the capacity of the institution involved. In the final stages of the program traditional selection and testing procedures remain indispensable. Whether any particular breeding program will profit from the incorporation of sophisticated genetic manipulation techniques must be carefully analysed. This discussion is intended to provide a basis for this analysis.

The detection of cells with a laticifer-like metabolism in Asclepias syriaca L. suspension cultures

Plant cell suspension cultures of Asclepias syriaca L. are morphologically homogeneous, that is, most cells in a suspension population are thin-walled, parenchyma-like cells. However, metabolic specialization of certain cells may occur in culture without morphological specialization. This study shows that a small number of suspension cells may synthesize secondary metabolites normally synthesized by a secretory cell, the non-articulated laticifer. The laticifer cell is an elongate, non-septate cell that is present in most intact plant tissues of Asclepias syriaca but which has not been observed to develop in vitro.

Prospects in plant genetic engineering

The functional expression of a novel gene in a genetically engineered plant has not yet been reported. One major barrier in movement toward this goal is our limited understanding of the molecular bases of gene expression. Attempts to establish genetic engineering as a practical facet of plant breeding are also complicated by the fact that genes for most important plant characteristics have not yet been identified. However, the benefits to be gained from all aspects of plant improvement are stimulating research into both the development of plant transformation technology and the isolation and characterization of genes responsible for valuable traits. As scientists develop greater knowledge of plant molecular genetics, we can expect to see practical applications in such diverse areas as improvement of plant nutritional quality, decreases in fertilization requirements, and increases in resistance to environmental stresses and pathogens.

Development of DNA-mediated transformation systems for plants

The genetic engineering of plants by DNA-mediated gene transfer requires that efficient transformation systems be developed. Considerable progress has been made in manipulating the Ti plasmid of Agrobacterium tumefaciens as a vehicle for delivery of foreign genes into protoplasts of dicotyle-donous plants. Part of the Ti plasmid, the T-DNA, can be incorporated into the genome of the host cell; the T-DNA can carry a foreign DNA sequence which co-integrates with it; under normal conditions, the tumorigenic-causing portion of the T-DNA can be inactivated so that transformed protoplasts can be regenerated and T-DNA with an inserted foreign gene can be stably maintained during regeneration, meiosis and gamete formation. A foreign gene has yet to be expressed in regenerated plants although a T-DNA gene for opine synthesis can function in regenerates. Developing a more ubiquitous transformation system for monocotyledons is further from fruition. Based on transformation systems for simple eukaryotic organisms, it is reasonable to expect that a DNA vector which is capable of amplifying a novel plant gene and which contains both a drug resistance marker to facilitate the selection of transformed plant protoplasts and a species-specific autonomously replicating sequence to ensure the stable maintenance of the input gene in the recipient cell can be constructed.