Influence of Genotype, Explant Source, and Gelling Agent on in Vitro Shoot Regeneration of Chrysanthemum Ki-Byung Lim 1 , Soo Jin Kwon 2 , Soo In Lee 2 , Yoon-Jung Hwang 1 , and Aung Htay Naing 1* 1 Department of Horticultural Science, Kyungpook National University, Daegu 702-701, Korea 2 Department of Agricultural Biotechnology, National Academy of Agricultural Science, Rural Development Administration, Suwon 441-707, Korea *Corresponding author: aunghtaynaing2005@gmail.com Received May 15, 2012 / Revised August 5, 2012 / Accepted August 17, 2012 Korean Society for Horticultural Science and Springer 2012 Abstract. The capacity for shoot regeneration of leaf, petiole, and stem explants of eleven chrysanthemum cultivars was examined on the MS medium containing 1 M naphthaleneacetic acid and 10 M 6-benzyladenine solidified with 0.8% Agar, 0.4% Agarose, or 0.25% Gelrite. Significant differences in frequency of callus formation and regeneration from the different explants were observed among the different cultivars when grown on the media solidified with the different gelling agents. Gelrite was found to be the most effective gelling agent in promoting of the shoot. Of the different explants used, in general, stem exhibited the highest frequencies of shoot organogenesis and mean number of shoots per explant regardless of cultivar and gelling agent. However, the highest frequency of regeneration (11.67 shoots per explant) was noted from leaf explants of cv. Borami followed by (4.33 shoots per explant) from stem explants of cv. Yes Nuri. Shoots were directly developed from the surface of explants, not through callus formation. Low frequencies of shoot organogenesis were observed for the remaining cultivars except for cvs. Yes Time and Yes Star, which exhibits no shoot formation at all. In this study, we have developed an efficient in vitro protocol for cvs. Borami and Yes Nuri from suitable explant. Additional key words: 6-benzyladenine, adventitious shoots formation, in vitro propagation, naphthaleneacetic acid, ornamental plant Hort. Environ. Biotechnol. 53(4):329-335. 2012. DOI 10.1007/s13580-012-0063-x Research Report Introduction Chrysanthemum is one of the most important cut flowers, pot plants and herbaceous landscape plants grown worldwide. In breeding programs, desirable traits have been introduced by conventional breeding; however, there are limitations to the techniques due to a limited gene pool and cross incom- patibility (Rout et al., 1996). As alternative approaches to conventional breeding attempts are being made to develop transformation system using agrobacterium-based gene vectors. Currently, there are several reports on transgenic chrysanthemum carrying genes for novel flower color, resistance against gray mold, and insect resistance (Soh et al., 2009; Teixeira da Silva et al., 2004). Adventitious in vitro shoot regeneration in chrysanthemum is important mostly in creating new cultivars, both in mutation breeding (Zalewska and Jerzy, 1997) and in genetic trans- formation (Nahid et al., 2007). However, in vitro propagation of chrysanthemum is affected by many factors: the com- position of the medium applied, growth regulators interaction, the kind of explant, plant genotype, and the developmental stage of the stem explant (Himstedt et al., 2001; Lee et al., 1997; Lu et al., 1990; Nahid et al., 2007). The choice of gelling agent is also very important for in vitro plant regeneration (Debergh, 1983; Pochet et al., 1991; Singha, 1984). The medium must be firm enough to support explants, but if the rigidity is too high it may prevent adequate contact between the medium and the explants. In addition, the phenomenon known as vitrification is considered as a striking agar-related problem (Debergh et al., 1992). Thus, protocols that are efficient for one cultivar are not easily adapted to other cultivars. A challenge for chrysan- themum breeders is to quickly create several new and attractive cultivars yearly for current market trends. Therefore, it is important to develop efficient regeneration protocols for crop improvement of chrysanthemum cultivars. In this study, shoot regeneration from different explants, which were col- lected from 11 commercial cultivars of chrysanthemum, was