Breeders of woody plants, including ornamentals, fruit trees, and forest trees, are especially inclined to adopt micropropagation in their breeding programs. Clearly, this technique has reached practical application and can in some cases considerably shorten the tail end of a breeding program: the production of material used in cultivation. In some of the most drastic cases, mutations found in natural populations can be directly micropropagated for usage, e.g., in propagating ornamental trees of special form or color. In Finland this technique has found direct application in propagating various birch clones (curly birch, laciniate forms, high-yielding single pair matings) and hybrid aspen clones. However, the advantages of micropropagation must be evaluated economically in comparison with other cloning techniques, such as root and shoot explants or direct rooting of cuttings. It appears in many cases that the more conventional rooting of cuttings is simply less expensive and therefore applicable to forest trees, where the magnitude of cloning is frequently in the million ramets range.
Economic comparisons of cloning techniques must be viewed in the light of final plant prices. For example, plants produced for forestry plantations must sell for a maximum $1 apiece as the landowner needs to plant stands of approximately 2000 plants per hectare, whereas ornamental rhododendron plants may sell for $20 apiece as the landowner considers only single plants and not stands. Thus, the extra price added through micropropagation is essential in case of a forestry plant but futile in the case of an expensive ornamental. The final method of cloning plants depends on the numbers required and unit prices.
The dream of every plant and tree breeder is the application of somatic embryogenesis to a breeding program. First, in this way one could avoid the tissue and plant aging phenomenon, which causes many problems particularly in woody species, including trees. It may cause plagiotrophic growth of tissues taken from mature trees as the basically totipotent cells are genetically fixed in developmental stages. Second, it may, in combination with cryopreservation, be used for keeping cell lines or tissues embryogenic and ready for multiplication once the same material has been through comparative field testing for yield or other special values for a number of years. Third, a somatically induced embryonic cell line or tissue can eventually be handled for the production of "artificial seed." This would be not only a seed but also a "clonal seed" that could be multiplied in great numbers. If the somatic tissue in addition has a desirable transgene, it would open great vistas for fast adoption of transgenic plants. Fourth, somatic embryogenesis could be used for "fixing heterosis" in clones. In this way one could circumvent the tedious construction of male sterility systems to produce hybrid seed. Basically, one would need only one heterotic plant for manufacturing large numbers of hybrid clonal seeds. There are several examples of vigorous growth in interspecific hybrids of trees. However, it has often been impossible in practice to utilize hybrid vigor because of difficulties in producing hybrid seed. This is the case in larch hybrids, such as Larix decidua X L. gmelini japonica (Fig. 1). To have practical importance, hybrid seed must be produced in hundreds of kilograms. This has not been possible by the seed orchard mode of breeding because of species differences in timing of flowering. Selection of superior larch hybrids, already present as mature trees, and the conversion of their somatic cells to an embryonic state would solve many present problems for practical forestry. Alternatively, cell lines
Figure 1 Shoot proliferation of larch hybrid (Larix decidua x L. gmelini japónica) through organogenesis in tissue culture. The culture was started from lateral buds of a 60-year-old tree in 1996. (Photo by A.-M. Niskanen 1997.)
or tissues could be kept cryopreserved until field testing indicates which hybrid genotypes to clone.
However, this section has to be ended with the statement that there is still a long way to go before things work on a practical scale. What you can manipulate in the research laboratory is a long way from practical application in a plant or tree breeding program. Eventually, however, with the large concentration on basic research at the moment, new useful applications will be adopted.
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