Abstract
The first objective of this study was to assess and optimize somatic embryo production in a genetically diverse range of cacao genotypes. The primary and secondary somatic embryogenesis response of eight promising cacao clones and a positive control was evaluated using modified versions of standard protocols. The second objective was to optimize the efficiency of primary somatic embryogenesis for a commercially important cacao clone, CCN 51, which has proven to be quite recalcitrant to standard protocols, relative to CCN 10, a clone also included in our analysis. The efficiency of the overall process was assessed by determining the number of somatic embryos produced per starting somatic tissue explant, as well as the quality of embryos (normal vs. abnormal) produced. Donor floral explants were subjected to five tissue culture steps, each 15-25 days in duration. Although all studied genotypes produced primary somatic embryos, most of them originated only from brown or brown-white callus. Overall, flower petals performed better than staminodes, and our best performing genotype yielded an average of 7-10 embryos produced in brown callus explants with embryogenic response during primary somatic embryogenesis procedures. In conclusions our analysis from a pilot in a small-scale are: 1) it is possible to achieve a high production of plants by somatic embryogenesis, although the efficiency is highly genotype-dependent; it is therefore necessary to optimize hormone balance and hormone type, as well as the explant type for each genotype, 2) through the use of secondary somatic embryogenesis, it is possible to increase somatic embryogenesis production at least ten-fold, and 3) the observed response variation between genotypes may reflect differences in endogenous and exogenously-supplied hormones. The importance of adapting the tissue culture protocol to the genotype is discussed. Key words: Somatic embryogenesis, 2,4-D, cellular competence, propagation, abnormalities, ethylene, cacao.
Highlights
Theobroma cacao L. is a tropical understory tree originating from the Amazon and Orinoco valleys (Wood and Lass, 2008; Motamayor et al, 2008)
The second objective was to optimize the efficiency of primary somatic embryogenesis for a commercially important cacao clone, CCN 51, which has proven to be quite recalcitrant to standard protocols, relative to CCN 10, a clone included in our analysis
Petals had the best overall performance compared with staminodes; the average of number of callus exhibiting and embryogenic response ranging from 20-70%
Summary
Theobroma cacao L. is a tropical understory tree originating from the Amazon and Orinoco valleys (Wood and Lass, 2008; Motamayor et al, 2008). By the late 1980s, annual Brazilian production decreased to 291,868 tons (IBGE, 2015), primarily due to outbreaks of devastating cacao diseases, in particular witches' broom (Moniliophthora perniciosa), which caused massive economic loss and much social distress (Marelli, 2008). In response to this crisis, numerous government agencies and universities were enlisted to develop disease-resistant, high-yielding cacao clones (CEPLAC, 2009). More numerous, plagiotropic grafts produce trees of undesirable architecture that require long time and labor investments (Miller, 2009)
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