INVITED REVIEW: SHOOT APICAL MERISTEM: A SUSTAINABLE EXPLANT FOR GENETIC TRANSFORMATION OF CEREAL CROPS MARIAM B. STICKLEN* AND HESHAM F. ORABY 362 Plant and Soil Science Building, Department of Crop and Soil Sciences, Michigan State University, East Lansing, MI 48824 (Received 17 May 2004; accepted 29 October 2004; editor D. D. Songstad) Summary Immature zygotic embryo has been the widely used explant source to develop embryogenic callus lines, cell suspensions and protoplasts for transformation of cereal crops including maize, wheat, rice, oat, barley, sorghum, and millet. However, the lack of competence of immature embryos in certain elite lines is still a barrier to routine production of transgenic cereal crops in certain commercial cultivars. In addition, a great deal of effort is required to produce immature embryos, manipulate cultures of immature embryos or their cell suspensions, and cryopreserve cultures for further use. In addition, undifferentiated cells may have reduced regenerability after a few months of in vitro culture. Alternative explants and regeneration systems for efficient transformation of cereal crops are needed to avoid or reduce the above limitations. During the past decade, scientists have successfully manipulated the shoot apical meristems from seedlings of maize, oat, sorghum, millet, wheat, and barley in an effort to develop a less genotype-dependent and efficient cereal regeneration system that can be maintained in vitro for long periods of time without the need for cryopreservation. Furthermore, apical meristem regeneration systems were used to stably transform maize, wheat, rice, oat, barley, sorghum, and millet. Key words: cereals and genetic engineering; shoot meristem; transformation. Introduction Theoretically, there are two possibilities for recovering transgenic plants via transfer of DNA into the shoot apical meristem. One possibility is that transgenic progeny may be directly produced via transformation of the single shoot apical meristem subepidermal cells and/or the germline cells which participate in floral formation, followed by the development of a partially transgenic reproductive organ. Due to the nature of this transformation method, the primary transformants will always be chimeric. In this system, it is very difficult to use selectable markers, such as an antibiotic or herbicide resistance, to improve transformation frequency and to recover transgenic progenitors. A reporter marker, such as b-glucuronidase (gus), green fluorescent protein (GFP), or anthocyanin biosynthesis may be useful for screening the original transformants. Genetically transforming a single apical meristem can become more efficient when one removes the chimerism by producing and multiplying shoots for 4 – 8 wk before the transfer into selection media. Using this method and the reporter marker, gus, several successful transformations at low frequencies have been made in dicotyledo- nous species, including soybean (McCabe et al., 1988; Christou et al., 1990), cotton (McCabe and Martinell, 1993), peanut (Brar et al., 1994), and sunflower (Bidney et al., 1992), via particle bombardment of single shoot meristems alone or in combination with Agrobacterium-mediated transformation, which resulted in the transformation of multiple genotypes of soybean (Christou et al., 1990) and peanut (Brar et al., 1994). The second possibility of a shoot meristem-based transformation system is to multiply transgenic shoot apical meristem cells and/or germline cells in vitro, which can be reprogrammed in the developmental direction under such in vitro conditions. Transgenic plants can be regenerated from these cells with selection or, if multiplied sufficiently, without selection. Transient and/or stable gene expression in cereals has been reported after delivery of DNA into cells via Agrobacterium- mediated transformation, microinjection, electroporation, and/or polyethylene glycol-mediated protoplast transformation, pollen tube pathway, ultrasonication-mediated DNA transfer, and whiskers- mediated DNA transfer (Tables 2–8). Previous research on morphogenesis of maize (Lowe et al., 1995; Zhong et al., 1992a, b), oat (Zhang et al., 1996b), sorghum (Zhong et al., 1998), millet (Devi et al., 2000), and wheat (Ahmad et al., 2002b) demonstrated that cereal meristems are morphogenetically plastic and can be manipulated to produce multiple shoots or somatic embryos. In the case of maize (Zhong et al., 1992b) and millet (Devi et al., 2000), shoot apical meristems produced floral parts by simple variation of in vitro culture conditions. Based on the plasticity and manipulation ability of shoot apical meristems and their in vitro multiplication ability, maize (Zhang et al., 1996a; Zhong et al., 1996a, b, 2003), oat (Maqbool et al., 2002), and millet (Devi and Sticklen, 2002) were stably transformed via microprojectile bombardment with a series of chimeric genes including the bacterial gus and bar (phosphinothricin acetyltransferase) genes in the case of maize, and *Author to whom correspondence should be addressed: Email stickle1@msu.edu In Vitro Cell. Dev. Biol.—Plant 41:187–200, May–June 2005 DOI: 10.1079/IVP2004616 q 2005 Society for In Vitro Biology 1054-5476/05 $18.00+0.00 187