The genus Dracaena Vand. Ex L. encompasses 60 species of glabrous, herbaceous, woody shrubs or trees that are largely indigenous to tropical Africa and Asia (Hutchinson, 1986). As a result of their distinct foliage variegation and tolerance of low light levels, several species, including D. deremensis Engl., D. draco (L.) L., D. fragrans, (L.) Ker-Gawl., D. marginata Lam., D. reflexa (Decne) Lam., D. sanderiana hort. Sander ex Mast., and D. surculosa Lindl., are important ornamental plants (Henny and Chen, 2003). Dracaenas rank second in Europe (Vonk Noordegraaf, 1998) and third in the United States (U.S. Department of Agriculture, 1999) as popular foliage plants used for interiorscaping. Dracaenas are also rich in steroidal sapogenins and saponins (Mimaki et al., 1998, 1999; Yokoduk et al., 2000), some of which have cytotoxic activities against cultural tumor cells (Mimaki et al., 1999), making them an important group of plants for pharmacognosy research.
Dracaenas as ornamental plants are propagated through cuttings, which are predominantly imported from Central America. Imported cuttings may carry and spread pathogens and pests (Palm and Rossman, 2003; Prado et al., 2008). For example, an invasive pathotype of Ralstonia solanacearum race 1 was identified from eye cuttings of golden pothos imported from Costa Rica to Florida (Norman and Yuen, 1998). In 2003, Childers and Rodrigues (2005) sampled 24 plant shipments, including cuttings of D. surculosa, entering the United States from Costa Rica, Honduras, and Guatemala and found half of the shipments infested with mites. A total of 81 mite species from 11 families was detected. The mites not only affect host plant growth, but also are vectors of viruses, including citrus leprosies virus, orchid fleck virus, and coffee ringspot virus (Miranda et al., 2007).
Micropropagation has become an important method of generating disease-free propagules of ornamental foliage plants (Chen and Henny, 2008). Reports of tissue culture of dracaenas date back to the 1970s when Miller and Murashige (1976) propagated D. surculosa using shoot tips. Subsequent studies included in vitro culture of axillary buds of D. deremensis (Badawy et al., 2005; Blanco et al., 2004; Debergh, 1976) and D. fragrans (Deberghs, 1975, 1976; Debergh and Maene, 1981) and regeneration of stem explants of D. marginata (Chua et al., 1981) and D. fragrans (Lu, 2003; Vinterhalter and Vinterhalter, 1997; Vinterhalter, 1989). However, in commercial practice, dracaena propagation has still relied on imported cuttings.
The use of cuttings may be justifiable if cane or stem length is an important component of the aesthetic value of finished plants such as D. fragrans and D. marginata. However, if plants are prized largely for their attractive foliage such as D. draco, D. surculosa, D. derenmesis, and D. sanderiana, micropropagated liners offer a better alternative to imported cuttings. However, micropropagation of dracaenas has been hampered by low multiplication rates and high occurrence of somaclonal variation. The numbers of shoot tips or axillary buds are often limited in dracaenas, and their use as explants produced limited numbers of shoots (Miller and Murashige, 1976). The occurrence of somaclonal variants or off types was high if plantlets were regenerated from variegated cultivars through indirect shoot organogenesis (Debergh, 1975; Vinterhalter and Vinterhalter, 1997). Thus far, a method for regenerating genetically stable plants from variegated dracaenas has not been well developed.
The objective of this study was to develop a reliable regeneration method for efficient propagation of genetically stable dracaenas using D. surculosa, commonly known as gold dust dracaena or spotted dracaena, as a model plant. Gold dust dracaena is a woody shrub, branching in an erect or arching habit. Leaves are elliptic, dark green, and variegated with irregular small, white or yellow round dots. In addition to the common steroidal saponins identified in dracaenas, four new 3,5-cyclosteroidal saponins were recently found in this species (Yokoduk et al., 2002). The most popular cultivar of D. surculosa is Florida Beauty, which is widely used in dish gardens and terrariums as well as for landscape in tropical and subtropical regions. The establishment of a reliable regeneration method for its propagation could reduce importation of cuttings and also provide uniform raw materials for future pharmaceutical compound extraction (Rout et al., 2000).
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