Protocol for Somatic Embryogenesis in Woody Plants

Abstract

Theobroma cacao L. is a tropical tree, with origins in the Amazon basin, currently cultivated throughout the tropics to supply the global demand for cocoa, the main ingredient for the manufacture of chocolate. A high degree of yield variation is a general characteristic of cacao plantations worldwide, due in part to the predominant use of seed propagation, in this mostly self-incompatible and highly heterozygous tree crop (Figueira and Janick, 1995). A recent study of the early yield of five high producing cacao families grown in full sun in Puerto Rico indicated that 2 to 3% of the trees in a population accounted for more than 60% of the yield (Irizarry and Rivera, 1999). Improvements made by plant breeders are often rapidly lost, as farmers propagate plants through seeds, and segregation results in a highly heterozygous population of plants. Vegetative propagation systems provides a means to capture such additive genetic gain, and has been used to some degree with cocoa with the development of grafting, and rooted cutting techniques. Modern biotechnology offers a suite of new approaches to speed up the development and deployment of genetically improved genotypes. Recently, research conducted at Plant DNA Technology, (Sondahl et al., 1994) CIRAD, Montpellier, France (Alemanno et al., 1996a, b, c; Alemanno et al., 1997); Nestles Inc., Tours, France; Almirante Cacao Research Station, Bahia, Brazil (Lambert et al., 2000); and at The Pennsylvania State University (Antúnez de Mayolo et al., 2003; Li et al., 1998; Maximova et al., 2002; Traore et al., 2003), has led to the development of efficient methods for somatic embryogenesis of cacao. The main advantages of tissue culture methods include the possibility of rapidly generating asexually propagated, uniform plants with highly valued genetic traits. Additionally, for cacao, somatic embryogenesis offers a system for clonal production of orthotropic plants with normal dimorphic architecture and taproot formation. The production and testing of disease free materials and germplasm conservation via cryo-preservation are other important potential contributions of plant tissue culture to the improvement, preservation and distribution of cacao germplasm, which is currently preserved only in living collections in the tropics.