Abstract

Somatic embryogenesis is a useful tool for Theobroma cacao improvement and propagation. Depending on culture medium composition, different morphogenetic structures (including somatic embryo) occur in response to alteration of genes expression patterns and biochemical changes. The effect of SO₄^2- ion deficiency in culture media on somatic embryogenesis was studied through sequential replacement of MgSO₄ and K₂SO₄ by MgCl₂ and KCl, respectively, at different steps of somatic embryogenesis. It appears that explants gradually lost their embryogenic competence as the period of exposition to sulphur free medium increases. These results suggest that, sulphur availability and the duration to sulphur exposition might modulate the expression of genes involved in somatic embryo differentiation in T. cacao. Cysteine, glutathione, reducing sugars, cysteine synthase and cysteine desulfurase activities were analysed in different morphogenetic structures obtained in vitro. Cysteine and reducing sugars contents appeared to be higher in embryogenic calli than their non-embryogenic homologues, whereas glutathione content appears to be lower in embryogenic calli. Cysteine synthase activities also discriminate the embryogenic calli from non embryogenic calli. In the embryogenic calli, the ratio cysteine synthase/cysteine desulfurase activities were above unit. The assimilation of exogenous sulphur (sulphate) for the synthesis of cysteine might hence be crucial for somatic embryogenesis in T. cacao. This explains the reduction and the absence of somatic embryo response observed during sulphur depletion in culture media. Sulphur nutrition is therefore critical in cacao somatic embryogenesis.

Keys words: Cacao, embryo, sulphate, cysteine synthase, glutathione, deficiency.