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Histology of somatic embryogenesis in rice (Oryza sativa cv. 5272)
Rafael Vega1, 4, Nelly Vásquez2, Ana M. Espinoza3, Andrés M. Gatica1 & Marta Valdez-Melara1
1. 2. 3. 4. Escuela de Biología, Universidad de Costa Rica, 11501-2060, San Pedro, Costa Rica. Centro Tropical de Investigación y Enseñanza (CATIE), Turrialba, Costa Rica. Centro de Investigación en Biología Celular y Molecular (CIBCM),Universidad de Costa Rica, 11501-2060, San Pedro, Costa Rica. Dirección actual: Fundación para la Innovación Tecnológica Agropecuaria, El Salvador. Received 11-X-2007. Corrected 29-VIII-2009. Accepted 04-X-2009.

Abstract: Rice (Oryza sativa cv. 5272) embryogenic calli were obtained from mature zygotic embryos culture on Murashige & Skoog (1962) medium supplemented with 2.5 mg/l 2,4-dichlorophenoxyacetic acid. Histological analysis of somatic embryogenesis revealed that after two weeks of culture of explants on the callus induction medium, somatic embryo development began with a cluster of proembryogenic cells in the peripheral region of the calli. The outer cell layer of embryogenic calli consisted of small and isodiametric cells with a dense cytoplasm and a prominent nucleus andnucleolus; whereas the inner cell layer is composed of large cells with small nucleus and large vacuole. These embryogenic cells underwent a series of organized divisions and formed the proembryo with a well-defined protodermis. Rev. Biol. Trop. 57 (Suppl. 1): 141-150. Epub 2009 November 30. Key words: Oryza sativa, rice, in vitro culture, histology, somatic embryogenesis, morphogenesis, Costa Rica.The totipotent character of plant cells allow that any differentiated cells that retains its nucleus has the ability to regenerate an entire new plant by organogenesis or somatic embryogenesis (SE) (Reynolds 1997, Fortes & Pais 2000). SE is the developmental process by which bipolar structures that resemble zygotic embryos are developed from haploid or diploid somatic cell through an orderlyembryological stage without gametes fusion (Jiménez 2001, Jiménez 2005, Quiroz-Figueroa et al. 2006, Namasivayam 2007). Two types of somatic embryogenesis are recognized: direct somatic embryogenesis (DSE) and indirect somatic embryogenesis (ISE). DSE is characterized by the induction of somatic embryos directly from pro-embryogenic cells from leaves, stem, microspores or protoplasts without theproliferation of calli, whereas in ISE somatic embryos are developed from friable embryogenic calli (Jiménez 2001, Molina et al. 2002, Quiroz-

Figueroa et al. 2002b, Quiroz-Figueroa et al. 2006). Somatic embryogenesis is a unique process in plants and it is of remarkable interest for biotechnological applications such as clonal propagation, artificial seeds and genetic engineering (Quiroz-Figueroa etal. 2006, Namasivayam 2007). Precisely, when somatic embryogenesis is integrated with conventional breeding programs and molecular and cell biological techniques, it provides a valuable tool to enhanced genetic improvement of crop species (Quiroz-Figueroa et al. 2006). As rice (Oryza sativa) is the most important staple crop for one third of the world population, there is considerable interest inthe development of new cultivars tolerant to biotic and abiotic stresses (Valdez et al. 1996b). Plant biotechnology represents an alternative to conventional breeding programs; nevertheless, integration of biotechnology into rice

Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 57 (Suppl. 1): 141-150, November 2009

improvement through genetic engineering or mutagenesisrequires a reliable and efficient in vitro culture system. In rice, somatic embryogenesis is the most common regeneration pathway and has been obtained from roots, leaf bases of young seedlings, mature embryos, immature embryos, caryopses, microscopes, cell suspension, protoplast and young inflorescences (Kawata & Ishihara 1968, Inoue & Maeda 1980, Wernicke et al. 1981, Heyser et al. 1983, Abe &...
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