Tissue culture involves the use of small pieces of plant tissue (explants) which are cultured in a nutrient medium under sterile conditions. Using the appropriate growing conditions for each explant type, plants can be induced to rapidly produce new shoots, and, with the addition of suitable hormones new roots. Alstroemeria explants used in tissue culture consist of a
rhizome and several aerial shoots that have been excised for the larger part (see in Fig. 1 for the type of explant used in the experiments; the two aerial shoots have been cut off and two small sections of shoot-stem remain). Outgrowth of axillary buds from the remaining sections of the aerial shoots is a main point for micro-propagation in Alstroemeria. However,
Alstroemeria plants are characterized by strong apical
dominance which the growing main shoot apex and the rhizome tip inhibit the outgrowth of axillary buds.
For the mechanism of apical dominance, there is a classical model, which states that auxin synthesized by the apex is transported downwards in the stem, and inhibits the growth of axillary bud. Cytokinin is synthesized in the root (and, but at a smaller rate, also in the stem) and is transported upwards in the stem and promotes the growth of inhibited axillary bud. On the other hand, a carotenoid derived hormone that has been identified, which reduce auxin transport capacity so that the axillary bud cannot export its auxin and is thereby inhibited.
Thus, to promote shoot branching by forced outgrowth of second axillary bud and increase the multiplication rate, gibberellins biosynthesis inhibitors are added in the medium for tissue culture of Alstroemeria and...