3.2.1. Direct adventitious shoot initiation
In certain species, adventitious shoots which arise directly from the tissues of the explant (and not within previously-formed callus) can provide a reliable method for micropropagation. However, the induction of direct shoot regeneration depends on the nature of the plant organ from which the explant was derived, and is highly dependent on plant genotype. In responsive plants, adventitious shoots can be formed in vitro on pieces of tissue derived from various organs (e.g. leaves, stems, flower petals or roots); in others species, they occur on only a limited range of tissues such as bulb scales, seed embryos or seedling tissues. Direct morphogenesis is observed rarely, or is unknown, in many plant genera.
Direct shoot formation is sometimes accompanied by proliferation of unorganised cells, and a regenerative tissue that could be classed as callus, may ultimately appear. Its formation can usually be reduced by adjustment of the growth regulators in the medium. Because there is a risk of regenerating plants with a different genetic identity (see Vol. 2), use of the callus for further propagation is not recommended unless it has a highly organised nature (see later). In some instances, the growth regulators used to initiate shoot buds directly on explants may not be conducive to continued bud growth. A closely packed mass of shoot primordia may then be mistaken for organised callus.
In those species where adult tissues have a high regenerative capacity, the main advantages of micropropagation by direct adventitious shoot regeneration are that:
• Initiation of Stage I cultures and Stage II shoot multiplication, are more easily achieved than by shoot culture. It is, for example, simpler to transfer aseptically several pieces of Saintpaulia leaf petiole to culture medium, than to isolate an equivalent number of shoot meristems.
• Rates of propagation can be high, particularly if numerous small shoots arise rapidly from each explant.
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