Micropropagation à la carte

Abstract

Tissues from woody plants have been cultured since the 1930s [15, 16]. Over the next 40 years callus cultures and organ cultures became established in woody plants. A degree of organogenesis was observed, but apparently shoots and roots developed separately on these cultures, or occasionally they may have given the appearance of plantlets [19]. It was not until 1968 that Winton [24] produced the first plantlets in callus cultures of triploid aspen, Populus tremuloides. Subsequently, it has been demonstrated that tissue explants from a number of tree species can be grown, and induced to undergo organogenesis and plantlet regeneration in vitro [3,5, 12–14, 20, 23, 25]. That is not to say that tissues from all woody plants, including forest trees, can be induced to grow and differentiate in vitro. Some do, while others are recalcitrant. In addition to the genotypic control of organogenesis, maturation also restricts in vitro differentiation. In general juvenile explants, such as embryos, cotyledons, hypocotyls, or bud explants from seedlings are more responsive to in vitro regeneration than are tissues, for example bud explants, from mature trees. For this reason, juvenile explants have been extensively employed for the clonal propagation of woody plants. Micropropagation from juvenile explants, although useful for differentiation studies, has a disadvantage in that it is rather difficult to predict how a seedling will perform upon reaching maturity. Progenies from selected trees might provide juvenile material that could be employed in clonal forestry programs. However, this approach has also limitations in that all the progeny will not be superior; there will be gentic variation within families. The next best possibility would be to have biochemical (isozymes) or molecular (restriction fragment length polymorphisms, RFLPs) markers in the juvenile phase that are correlated to the adult yield traits. If availabel, these markers would make juvenile micropropagation more acceptible.