Xyloglucan mobilisation in cotyledons of developing plantlets of Hymenaea courbaril L. (Leguminosae-Caesalpinoideae)

Many seeds contain storage compounds that are used by the embryo/plantlet a s a source of nutrients after germination. In seeds of Hymenaea courbaril, a leguminous tree, the main reserve consists of a structurally unusual xylo glucan stored in thickened walls of the cotyledon cells. The present work a imed to study H. courbaril xyloglucan metabolism during and after germinati on in order to compare its degrading system with the other known xyloglucan containing seeds. Polysaccharide degradation occurred after germination be tween 35 and 55 days after planting. The activities of alpha-xylosidase, be ta-glucosidase, beta-galactosidase and XET rose during the period of xylogl ucan disassembling but a low level of endo-beta-glucanase activity was dete cted, suggesting that this XET has high affinity for the oligosaccharides. The pH optimum of beta-galactosidase was different from the alpha-xylosidas e, beta-glucosidase and XET optima suggesting that the former may be import ant in the control of the mobilisation process. A tentative model for xylog lucan disassembling in vivo is proposed, where beta-galactosidase allows th e free oligosaccharides to bypass a transglycosylation cycle and be disasse mbled by the other exo-enzymes. Some ecophysiological comparisons among H. courbaril and other xyloglucan storing seeds are discussed. (C) 2000 Publis hed by Elsevier Science Ireland Ltd. All rights reserved.