RESCUE OF GENETICALLY TRANSFORMED AVOCADO BY MICROGRAFTING Simon Raharjo and Richard E. Litz Tropical Research and Education Center, University of Florida, 18905 SW 280 St., Homestead FL 33031-3314 USA ABSTRACT Development of avocado somatic embryos to maturity appears to be normal; however, the majority of somatic embryos lack bipolarity, often lacking a shoot apex. Developing shoots generally beco- me necrotic in vitro. The germination and conversion rate of somatic embryos has therefore been low under optimal in vitro conditions. Avocado embryogenic cultures have been transformed with several gene constructs. In order to increase the plant recovery rate, shoots (5-10 mm long) that develop from somatic embryos have been grafted onto 3 to 4-week-old ‘Booth’ and ‘Lula’ in vitro-ger- minated seedling rootstocks with a success rate that is ca. 70%. The first nursery trials of trans- genic avocado plants have been entirely derived from micrografted somatic embryo shoots. Key Words: somatic embryo, avocado, micrografting INTRODUCTION Somatic embryogenesis of avocado has been described from zygotic embryos (Mooney and Van Staden, 1987; Pliego-Alfaro and Murashige, 1988; Raviv et al., 1998; Witjaksono and Litz, 1999a, b) and from the nucellus (Witjaksono et al., 1999a). Although somatic embryos can be recovered efficiently from embryogenic cultures, most of them are developmentally abnormal, and generally lack either a root or a shoot meristem (Pliego-Alfaro and Murashige, 1988; Witjaksono et al., 1999a; Efendi, 2003). The frequency of recovery of bipolar somatic embryos has been reported to range from 0.002% to 5-6% (Pliego-Alfaro and Murashige, 1988; Raviv et al., 1998; Witjaksono, 1997; Witjaksono and Litz, 1999a, b; Witjaksono et al., 1999a; Efendi, 2003), and is genotype- dependent. Protoplast isolation and culture from embryogenic tissues of avocado has been reported (Witjak- sono et al., 1998) together with limited somatic hybridization (Witjaksono, 1997). Embryogenic 119 Proceedings V World Avocado Congress (Actas V Congreso Mundial del Aguacate) 2003. pp. 119-122.