Why and How Using Micropropagated Trees rather than Germinations for Controlled Synthesis of Ectomycorrhizal Associations?

Synthesis of ectomycorrhiza (EM) on genetically uniform plant material in a system aimed at rapid production is a difficult undertaking, as EM almost exclusively form on trees.

To overcome difficulties of genetic heterogeneity, seasonal fluctuations in plant growth and problems with maintenance of aseptic conditions of seedlings, in vitro propagation techniques for trees were proposed in the early 1980s. For micropropagation of trees, differences between episodically and non-episodically growing species have to be considered. Episodically growing trees are much more recalcitrant and often require rejuvenation of the stock plant. Plant morphogenetic and physiological traits and genetic origin play important roles for successful in vitro propagation and rooting. Mycorrhization of micropropagated trees was first established in 1984. Nevertheless, the use of microcuttings for controlled symbiosis has not prevailed and in laboratory studies the work with young seedlings dominates. Since 1985, a micropropagated Quercus robur L. clone DF159 has been used for the establishment of the EM symbiosis. A detailed micropropagation procedure is described for this episodically growing model tree. Difficulties to overcome during the four successive steps entailing primary culture, stabilization and long-term culture, rooting and acclimatization are illustrated and solutions for an optimization of the microcutting production of trees are detailed.