Traditional vegetative propagation methods such as cuttings and grafting are important for propagation of plant cultivars heterozygous for some loci, because those loci segregate in selfed progeny. In tomato (Solanum lycopersicum), although cultivars are usually propagated from seeds, several cultivars propagated by vegetative methods have been developed recently in Japan, i.e., ‘Koshi-no-Ruby’ developed at Fukui Prefectural University and ‘Koshi-no-Ruby Sayaka’ and ‘Koshi-no-Ruby Urara’ developed in Fukui Prefecture (Sato et al., 2009). However, the efficiency of vegetative propagation by traditional methods is insufficient for mass propagation of plantlets.
Adventitious shoots can be regenerated from cut surfaces of hypocotyls or stems in decapitated plants in vivo. Such a phenomenon has been observed in many plant species, including tomato (Harada et al., 2005; Steinitz et al., 2006), Cucurbita pepo (Amutha et al., 2009), and poinsettia (Nielsen et al., 2003). Taking advantage of the inherent regenerative ability of plants, a new method, termed the complete decapitation method (CDM) by Johkan et al. (2008c), was developed for mass propagation in tomato and poinsettia (Harada et al., 2005; Nielsen et al., 2003). This method enables in vivo adventitious shoot regeneration from stumps after decapitation of the primary shoot and all lateral branches. Harada et al. (2005) reported that 79 shoots were regenerated from the cut surface of primary shoots and lateral branches 36 d after CDM treatment without exogenous plant growth regulator (PGR) application in ‘Petit’ tomato.
Increasing the efficiency of adventitious shoot regeneration is important if CDM is to be applied to plantlet production in tomatoes. Etiolation of stems by covering the cut surface with an aluminum cap increased the number of regenerated shoots (Johkan et al., 2008a, 2008c, 2011). Foliar application of ascorbic acid after decapitation also promoted adventitious shoot formation (Johkan et al., 2008b, 2011). However, efficiency of shoot regeneration might be further improved by incorporation of other techniques such as treatment with PGRs. In plant tissue culture, organ differentiation is influenced by the relative concentration of PGRs in the culture medium; relatively high levels of cytokinin promote shoot formation, whereas high levels of auxin promote rooting, and intermediate levels induce callus formation (Thorpe, 2007). In CDM, adventitious shoots are likely to be regenerated from calli formed on the cut surface of primary shoots and lateral branches (Harada et al., 2005), which indicates there are similarities between shoot regeneration by CDM and tissue culture methods. Thus, PGRs might influence in vivo adventitious shoot formation from decapitated tomato plants.
In the present study, the number of adventitious shoots regenerated after decapitation was investigated using four cultivars of tomato. Histological observations were conducted to determine whether the shoots originated from calli formed on the cut surface of stems. It is well known that auxin is synthesized in the shoot apex (Leyser, 2005). Therefore, we investigated the effects of the presence of lateral branches on shoot regeneration. The PGRs 1-naphthaleneacetic acid (NAA) and 6-benzyladenine (BA), which are commonly used in tissue culture, were applied to decapitated plants as foliar sprays to understand the physiology of shoot regeneration by CDM and to improve the efficiency of shoot regeneration.
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