Recherches récentes et biotechnologies de la multiplication végétative

Plants have two natural modes of multiplication, i.e. sexual and vegetative as a consequence of propagation via the production of axillary buds, which are copies of the shoot meristem. These buds, after isolation from the mother plant, produce new shoots and plants with the formation of adventitious roots. This type of multiplication occurs irrespective of flowering and sexual reproduction, and results in clonal propagation. Clonal propagation has long been used by man, mainly for multiplying most tuber-bearing plants. Moreover, fruit trees, ornamental plants and many flowers are propagated from cuttings or scions. Although clonal propagation is traditionally used for plant multiplication, new techniques are now available (including tissue culture) that take recent findings on fundamental mechanisms involved in plant embryogenesis and organogenesis into account. The aim of this paper is to summarize the state of the art and review recent progress and prospects concerning clonal propagation. Significance and uses of clonal propagation : Repeated production of new axillary buds is a prerequisite for efficient clonal propagation, i.e. an alternative strategy that enables plants to survive and/or conquer new territories. Concerning agricultural applications, clonal propogation is often used for the production of tuberous food crops or trees. What's new about clonal propagation : Information on meristem origin and function is now available. The shoot meristem is initiated early during embryogenesis. Its size and organisation are under strict genetic regulations. Moreover, axillary meristems differ physiologically according to the plant age and their position on the plant. These two main points are important in the choice of buds for successful clonal propagation or regenerating new shoot meristems from tissue culture. For root meristems, the induction, activation and function of adventitious root meristems, which are also under strict genetic control, have to be taken into consideration. Progress and prospects in clonal propagation : Clonal propagation has been greatly improved through in vitro culture, which allows massive multiplication of plants and eradication of viruses by meristem culture. Moreover, somatic embryogenesis can be induced for rapid multiplication of many species. Work is under way to identify true embryogenic cells and mechanisms involved in cell polarity and asymetric mitosis, which seem necessary for cells to adopt an embryogenic pattern of development. The germinative potential of somatic embryos can be improved by inducing embryo maturation through an exogenous supply of abscissic acid (ABA) and nutrients, which may enhance the production of storage proteins. It is now possible to manage different plant multiplication strategies through sexual or vegetative reproduction. Clonal propagation can thus be obtained by the production of artificial or natural seeds through somatic embryogenesis and apomixis, respectively. These tools offer new opportunities for improving crop plants and accelerating the release and dissemination of improved genotypes.