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Because fruit trees such as apple (Malus ×domestica Borkh.) flower and set fruit only after an extended juvenile phase lasting several years, efficient breeding of fruit trees is limited. We previously suggested that MdTFL1 (Malus ×domestica TFL1) functions analogously to TERMINAL FLOWER 1 (TFL1) and that MdTFL1 is involved in the maintenance of the juvenile/vegetative phase in apple. To clarify the function of MdTFL1 in apple, we produced transgenic ʻOrin ̓apple trees expressing MdTFL1 antisense RNA. One of them flowered only 8 months after the transfer to the greenhouse, whereas the nontransformed control plants have not flowered in nearly 6 years. As expected, the expression of endogenous MdTFL1 was suppressed in the transgenic lines that showed precocious flowering. In addition, the expression level of the transgene was correlated with the reduction of the juvenile phase. These findings confirm that MdTFL1 functions like TFL1 and that MdTFL1 maintains the juvenile and vegetative phase in apple. Flower organs of the transgenic apple trees were normal in appearance, and a precocious flowering transgenic line set fruit and seeds. Interestingly, some flowers of the transgenic apple trees developed without undergoing dormancy. The expression of MdTFL1 in apple may affect flower development as well as flower induction. In the development of all woody plants from seed, there is a so-called juvenile phase, lasting up to 30 to 40 years in certain forest trees, during which flowering does not occur and cannot be induced under normal conditions (Hackett, 1985). In the fruit industry, it is important to accelerate flowering by reducing the juvenile or vegetative phase of the trees after planting in order to facilitate the earliest possible production of fruit. Breeding fruit trees such as apple often takes more than 20 years, including periods of cross pollination, seedling selection, and regional trials to produce varieties that meet consumer demands. For example, the ʻFuji ̓apple, one of the most popular apple cultivars in the world (OʼRourke et al., 2003), first set fruit 12 years after being sowed, and it was 23 years before it was released as a cultivar (Sadamori et al., 1963). Generally, the juvenile phase in apple lasts 6 to 12 years (Visser, 1964; Zimmerman, 1972). In fruit breeding, various practical techniques have been considered to accelerate the flowering and fruiting of seedlings. The basic idea is to grow the seedlings rapidly from the germination stage to the transition to flowering (Aldwinckle, 1975; Visser, 1964). In apple, grafting the seedlings onto dwarfing rootstocks such as ʻMalling 9 ̓(ʻM.9ʼ) and ʻMalling 27 ̓(ʻM.27ʼ) will usually bring earlier flowering by 1 or 2 years (Visser, 1964, 1973). Grafting seedlings onto bearing trees (top grafting), ringing, scoring, bark inversion, root pruning, and spraying with growth retardants may stimulate earlier flowering when applied to older seedlings, but in these conditions, the juvenile phase probably has already passed, and the seedlings are in transition (Kender, Received for publication 31 Mar. 2005. Accepted for publication 2 July 2005. We thank Hiroaki Ichikawa, the National Institute of Agrobiological Sciences, for providing the binary vector pSMAK251, and Tomoko Sekita, the National Institute of Fruit Tree Science, for technical assistance. This work was supported by the “Development of innovative plants and animals using transformation and cloning” project of MAFF and the Program for Promotion of Basic Research Activities for Innovative Biosciences (PROBRAIN). This publication constitutes contribution number 1369 of the National Institute of Fruit Tree Science. 1Corresponding author; e-mail address: koto@affrc.go.jp 1974; Zimmerman, 1972). Based on our experience, it would, however, be difficult to reduce the juvenile phase of apple seedlings to <4 years under normal growing conditions even if they are grafted on the dwarfing rootstock. The length of the juvenile phase is influenced more by genetic factors than by environmental factors. Significant correlations have been found between the length of the juvenile phase and parent characteristics such as growth rate and the length of the vegetative phase (Lavi et al., 1992; Visser, 1965). The most striking advances in our understanding of the genetic control of the timing of flowering have come from the study of arabidopsis [Arabidopsis thaliana (L.) Heynh.] (Levy and Dean, 1998; Piñeiro and Coupland, 1998), and several genes such as LEAFY (LFY), APETALA1 (AP1), TERMINAL FLOWER 1 (TFL1), and FLOWERING LOCUS T (FT), which control flowering time, have been isolated from arabidopsis (Bradley et al., 1997; Kardailski et al., 1999; Kobayashi et al., 1999; Mandel et al., 1992; Ohshima et al., 1997; Weigel et al., 1992). Over-expression of LFY, AP1, or FT shortens the vegetative phase, causing early flowering (Kardailski et al., 1999; Kobayashi et al., 1999; Mandel et al., 1995; Weigel and Nilsson, 1995), whereas over-expression of TFL1 causes late flowering in transgenic arabidopsis (Ratcliffe et al., 1998). TFL1 plays a key role in the maintenance of the inflorescence meristem by preventing the expression of LFY and AP1 (Ratcliffe et al., 1999). Apple orthologues of several of these genes such as AFL1, AFL2, MdAP1 (MdMADS5), and MdTFL1 have been isolated and characterized (Kotoda et al., 2000, 2002; Kotoda and Wada, 2005; Sung et al., 1999; Wada et al., 2002; Yao et al., 1999). With the goal of elucidating the long juvenility of fruit trees, we are interested in regulatory genes, such as arabidopsis TFL1, snapdragon (Antirrhinum majus L.) CENTRORADIALIS (CEN), and tomato (Lycopersicum esculentum Mill.) SELF-PRUNING (SP) genes, which maintain the vegetative phase and promote the emergence of shoots rather than flowers (Bradley et al., 1996; Pnueli et al., 1998), as well as genes such as LFY and AP1, which promote flowering. Therefore, we isolated and characterized

Antisense Expression of MdTFL1, a TFL1-like Gene, Reduces the Juvenile Phase in Apple