Import of Sucrose and its Partitioning in the Synthesis of Galactomannan and Raffinose-oligosaccharides in the Developing Guar (Cyamopsis tetragonolobus) Seed

Abstract

Import of sucrose and its transformation to galactomannan and raffinose-oligosaccharides have been studied in the developing guar seed. The amount of galactomannan gradually increased with the ageing of the seed. During the entire period of pod development, sucrose constituted the major portion of the free sugars in the seed (both endosperm and cotyledons) as well as in the pod wall. Besides myo-inositol, the free sugars detected in the developing endosperm and cotyledons were glucose, fructose, raffinose and stachyose. Some compounds, possibly glycosides (RG values higher than that of fructose), were also detected in the endosperm. In the later stages of seed development, the relative proportion of raffinose in the free sugars increased, reaching 50% of the total free sugars in 77-d-old cotyledons. With pod maturity, the activities of soluble acid and bound acid invertases in the pod wall increased manifold with a concomitant decline in the non-reducing sugar content. These enzymes seem to be involved in the mobilization of sucrose from this fruiting structure into the seed. An increased synthesis of raffinose-oligosaccharides both in the endosperm and cotyledons was associated with high activities of soluble acid invertase (pH 4-8) and sucrose-UDP glucosyl transferase in these tissues. Feeding uniformly labelled 14C-sugars to the detached intact pods as well as to the isolated endosperm and cotyledons resulted in labelling of all endogenous free sugars and galactomannan. The uptake and incorporation into galactomannan of 14C was stimulated by Co2+, Mn2+ and Mg2 + . Except for mannose, a major proportion of the 14C from glucose, fructose and sucrose appeared in sucrose in both endosperm and cotyledons indicating a fast reconstitution of sucrose in situ. Based on the present results, a possible mode of transformation of sucrose to galactomannan and raffinose-oligosaccharides has been proposed.