Vegetative Propagation by Multiplication of Protocorm-Like Bodies
Vegetative propagation describes the asexual reproduction of a plant which results in offspring genetically identical to the parent. MOREL (1960) revolutionized orchid propagation by developing a method called meristem culture to gain virus-free Cymbidium plants. He used shoot tips in vitro from which protocorm-like bodies (PLB) developed similar to seed germination. Nowadays, micropropagation by meristem culture is a standard method for a fast high scale mass propagation of most commercially important orchids, including Phalaenopsis species.
For vegetative propagation by protocorm-like bodies meristems or small shoot tips (meristem with leaf primordia) are originally used. A benefit of this explant type is the production of virus-free plants; the lacking of the vascular tissue, high auxin concentration and a high cell division rate contribute to the inhibition of virus spreading. However, also protocorms (TENG ET AL. 2004), leaves segments (TANAKA & SAKANISHI1977), root tips (PARK ET AL. 2003) and flower stalks (TANAKA & SAKANISHI 1978) are suitable. The choice of the explant is depending on the orchid species and regeneration protocol. Explants from ex vitro plants need to be sterilized before putting onto culture medium commonly by dipping in a hypochlorite solution.
Formation and Development of PLBs
Formation of PLBs is comparable to somatic embryogenesis (LEE ET AL. 2013) and could be classified into two types: the direct regeneration of PLBs from cut surfaces of the explant and the indirect formation of PLBs from callus in solid and liquid cultures. Synthetic auxins and cytokinins are usually playing a crucial role in the induction of PLBs and callus. However the type of plant growth regulator and concentration is highly dependent upon orchid species. Globular cell complexes arise from somatic cells which later develop a bipolar structure with a shoot and root meristem (primary PLBs) and which convert into plantlets. One PLB could be cut into pieces which will result in the formation of 6-8 adventitious PLBs (secondary PLBs). The advantage of this procedure is the regeneration of thousands of plants from only a single shoot tip or explant. Subculturing is particularly important for endangered orchid species.
Lee Y. I., Hsu S. T. & Yeung E. C. (2013): Orchid protocorm-like bodies are somatic embryos. American journal of botany, 100(11): 2121-2131
Morel G. (1960): Producing virus-free Cymbidiums. American Orchid Society Bulletin 29 (1): 495-497
Park S. Y., Murthy H. N. & Paek K. Y. (2003): Protocorm-like body induction and subsequent plant regeneration from root tip culture of Doritaenopsis. Plant Science 164 (6): 919-923
Tanaka M. & Sakanishi Y. (1977): Clonal propagation of Phalaenopsis by leaf tissue culture. American Orchid Society Bulletin 46 (1): 733–737
Tanaka M. & Sakanishi Y. (1978): Factors affecting the growth of in vitro cultured lateral buds from Phalaenopsis flower stalks. Scientia Horticulturae 8(2):169-178
Teng W. L., Nicholson L. & Teng M. C. (2004): Micropropagation of Spathoglottis plicata. Plant Cell Reports 16 (12): 831-835