Mobilização de xiloglucano de reserva em sementes de Hymenaea courbaril L

AUTOR(ES)
DATA DE PUBLICAÇÃO

1997

RESUMO

Some species of Leguminosae accumulate high amounts of polysaccharides in the seed. These polysaccharides are translocated to the embryo after the germination, being an important source of carbon and energy to the developing plantlet. Seeds of Hymenaea courbaril L. store up to 40% of the seed mass in the form of xyloglucan deposited in the walls of parenchyma cells. In this work, the mobilization of the storage xyloglucan in the seeds of Hymenaea courbaril L. was followed citochemically and biochemically. After planting, 20 seeds were collected every 5 days until the fall of the cotyledons: 15 to follow ftesh and dry weight; 2 to follow cytological alterations and; 3 were collected for biochemical analysis (of the carbohydrates present and the enzymes involved in the degradation of xyloglucan). At least 3 other types of reserves other then xyloglucan were detected: oligosaccharides of the rafinose family (metabolised during the first 15 days after planting), protein bodies (mobilized unti135 days after planting) and granulation in the protein bodies (probably fitin, mobilized within the first 10 days after planting). The storage xyloglucan was mobilised between 35 to 50 days after planting. This observation can be confirmed as follows: (1) in this period the walls of the cotiledonary cells became thinner; (2) the amounts of water and alcali soluble sugar decrease; (3) there is an increase in the activity of the enzymes involved in the degradation of xyloglucan and; (4) there is accumulation of transitory starch inside the cells, suggesting that the monosaccharides derived ftom xyloglucan breakdown are entering in the cell. The rise of the activity of the enzymes was not simultaneous. The first enzyme to increase its activity was a-xylosidase ( around 15 days after planting), followed by p­galactosidase and p-glucosidase (whose activity rise 35 days after planting, although there is activity since the beginning of inbibition) and xyloglucan-endo-transglycolase (XET, whose activity was detected only after 55 days of planting). The study of xyloglucan fine structure showed that chances in the proportion between the oligosaccharides occurred before the mobilisation period. This observation indicates that the activities of p-galactosidase and P-glucosidase detected prior to the mobilisation are not an artifact caused by the lost of compartimentalization of the cell, but a real activity upon the polysaccharide in vivo. Only sucrose and monosaccharides (specially glucose and fructose) were detected in the fraction of the alcohol soluble carbohydrates during the mobilisation of xyloglucan. On the basis of these results, this seems to be the way the carbohydrates are transported to the embryo. The small amount of other monosaccharides (specially xylose and galactose, which are constituents of xyloglucan) strengthen the hypothesis that the sugars coming from the wall enter in the cell. Once inside it, the monosaccharides would be converted to glucose and fructose and could be used in the following ways: (1) biosynthesis of sucrose to be translocated to the embryo; (2) use in the cell metabolism (respiration, protein synthesis, etc.); (3) biosynthesis of starch, draining the excess of free carbohydrates ITom the cytoplasm, but keeping it available for further transport to the embryo and; (4) biosynthesis of xyloglucan oligosaccharides. The biosynthesis of xyloglucan could be a way to control the degradation of cell wall polysaccharide. Since the degradation mechanism does not involve a real endo­ J31,4-glucanase, but axyloglucan-endo-transglycolase (XET), xyloglucan oligosaccharides ITom an external source would be required to start the degradation of xyloglucan

ASSUNTO(S)

polissacarideos semente

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