Meristem-tip culture.

The essence of meristem-tip culture is the excision of the organized apex of the shoot from a selected donor plant for subsequent in vitro culture. The conditions of culture are regulated to allow only for organized outgrowth of the apex directly into a shoot, without the intervention of any adventitious organs (1-3). The excised meristem-tip is typically small (often less than 1 mm in length) and removed by sterile dissection under the microscope (Fig. 1). It comprises the apical dome and a limited number of the youngest leaf primordia, and excludes any differentiated provascular or vascular tissues. A major advantage of working with such a small explant is the potential that this holds for excluding pathogenic organisms present in the donor plant from the in vitro culture (see below). A second advantage is the genetic stability inherent in the technique, since plantlet production from adventitious organs can be avoided (3-7). However, if there is no requirement for such benefits, then the related technique of shoot-tip culture (see Chapter 8 , this vol.) may be more expedient for plant propagation. In this procedure, the explant is still a dissected shoot apex, but a much larger one, containing a relatively large number of developing leaf primordia. Typically, the explant is between 3-4 mm and 2 cm in length, and development in vitro is still regulated so as to be confined to outgrowth of an organized shoot, without adventitious propagation.