53 Propagation of Ornamental Plants Vol. 11, № 2, 2011: 53-62 Received: April 5, 2011 Accepted: May 20, 2011 INTRODUCTION Ornamental plants, including herbs shrubs and trees, are produced mainly for aesthetic and decorative value and their commercial production and distribution have grown tremendously over the last few decades. Cut lowers represent the largest segment of the ornamental industry followed by lowering pot plants, tree and nurs- ery crops, lower bulbs, and other propagation material (Lawson 1996). The ornamental cut lower market in Western Europe surpasses $12 billion in sales. Japan is the next with $7.8 billion and the United States of America follow with $6.9 billion in sales. The Nether- lands contributes nearly 60% of world lower exports followed by Colombia, Italy, and Israel as the major four exporters (Malter 1995). Therefore, ornamental plants are in high demand for commercial production, landscaping and gardens. To date about 156 genera are produced via tissue culture and this number is projected to increase exponentially over the next few years. A list of sample species is compiled in Table 1. Large scale propagation of ornamental plants has relied heavily on the utilization of in vitro techniques, which can be employed not only for mass propagation purposes, but also for gene pool conservation, trans- formation, and tissue preservation. The concept of in vitro culture is based upon the basic notions of totipo- tency and plasticity. Cell totipotency can be regarded as the capacity of single cells to perform all functions of development and differentiate into unlike cells thereby producing tissues, organs and ultimately whole organisms. The irst hint of totipotency is ascribed to Haberlandt (1902) who was able to culture isolated mesophyll cells in in Knop’s salt solution enriched with sucrose. Subsequent studies showed that functional plantlets could be regenerated from single cells (Reinert 1958). Cell plasticity relects the ability of plant cells to modify their developmental fate in response to ex- ternal stimuli, including abiotic and biotic factors. This capacity, which is a unique prerogative of plant cells developed as a survival mechanism, can be exploited in culture to re-address cell fate and behavior towards speciic morphogenic pathways culminating to the re- generation of a functional plant. Both totipotency and plasticity are exempliied in vitro by the processes of somatic embryogenesis, the formation of bipolar struc- INSIGHTS ON THE REGULATION OF THE SHOOT APICAL MERISTEM AND APPLICATIONS FOR ENHANCING PROPAGATION SYSTEMS Mohamed Elhiti 1 and Claudio Stasolla* Department of Plant Science, University of Manitoba, Winnipeg, R3T 2N2, Manitoba, Canada, *Fax. + 1 204-474-7528, *E-mail: stasolla@ms.umanitoba.ca, 1 Current address: Department of Botany, Faculty of Science, Tanta University, Tanta, Egypt, 31527 Abstract The success of many tissue culture techniques, including organogenesis and somatic embryogenesis depends upon the proper formation of de novo meristems, the function of which affects the ultimate ability to regener- ate viable plants. Therefore studies on meristem formation and maintenance are crucial for improving in vitro techniques. Given the economic importance of ornamental plants, it surprising that very few species are com- mercially mass propagated via somatic embryogenesis. This limitation is mainly due to the lack of optimization of protocols and/or speciic culture conditions. This review examines the physiological and molecular events occurring during shoot apical meristem (SAM) formation in vivo and outlines differences in the ontogeny of the SAM produced in vivo and in vitro. Evidence is also provided that the genetic network regulating the func- tion of the SAM in vivo might also operate in culture during the formation of embryogenic and/or organogenic cells. Recent experiments reveal that altered expression of SAM marker genes affects the organogenic and somatic embryogenic processes in vitro with the potential to improve tissue culture in ornamental species. Key words: meristem, organogenesis, regeneration, somatic embryogenesis