Mobilisation of the raffinose family oligosaccharides and galactomannan in germinating seeds of Sesbania marginata Benth (Leguminosae-Faboideae)

Seeds of the tropical species Sesbania marginata present a living endosperm whose cells possess thickened walls which contain galactomannan. Oligosaccharides of the raffinose family and protein bodies are stored intracellularly. Between the seed coat and the endosperm an aleurone layer is present. The anatomy of the endosperm of Sesbania marginata is intermediate amongst other galactomannan-containing seeds that have been anatomically examined (Trigonella foenum-graecum and Ceratonia siliqua). Before radicle protrusion, oligosaccharides from the raffinose family are broken down concomitantly in the endosperm and embryo whereas galactomannan (apparently restricted to the endosperm) is degraded after germination. Three galactomannan hydrolases: alpha-galactosidase, e ndo-beta-mannanase and beta-mannosidase were detected in isolated endosperms and their activities peaked during the exponential phase of galactomannan degradation, suggesting their close correlation with the breakdown of this cell wall polysaccharide following germination. Our results indicate that unlike Trigonella foenum-graecum enzymes, the hydrolases of seeds of Sesbania marginata seem not to work in a concerted fashion, since the mannose:galactose ratio of the polysaccharide increased significantly at the end of the degradation process, suggesting that alpha-galactosidase attacks the polymer before the other hydrolases. The products of galactomannan degradation (galactose and mannose) do not accumulate either in the endosperm or in the embryo, where they are probably metabolised and used as a source of energy for the early growth of the plantlet.