Evidence That Sucrose Loaded into the Phloem of a Poplar Leaf Is Used Directly by Sucrose Synthase Associated with Various β-Glucan Synthases in the Stem

Abstract

Sucrose (Suc) synthase (SuSy) is believed to function in channeling UDP-Glc from Suc to various β-glucan synthases. We produced transgenic poplars (Populus alba) overexpressing a mutant form (S11E) of mung bean (Vigna radiata) SuSy, which appeared in part in the microsomal membranes of the stems. Expression of SuSy in these membranes enhanced the incorporation of radioactive Suc into cellulose, together with the metabolic recycling of fructose (Fru), when dual-labeled Suc was fed directly into the phloem of the leaf. This overexpression also enhanced the direct incorporation of the glucosyl moiety of Suc into the glucan backbone of xyloglucan and increased recycling of Fru, although the Fru recycling system for cellulose synthesis at the plasma membrane might differ from that for xyloglucan synthesis in the Golgi network. These findings suggest that some of the Suc loaded into the phloem of a poplar leaf is used directly by SuSys associated with xyloglucan and cellulose synthases in the stem. This may be a key function of SuSy because the high-energy bond between the Glc and Fru moieties of Suc is conserved and used for polysaccharide syntheses in this sink tissue.

Lignin and Biomass: A Negative Correlation for Wood Formation and Lignin Content in Trees

“…Suc and amino acids, including Phe, are transported in the phloem and unloaded into wood-forming cells through specific transporters (Couturier et al, 2010;Merchant et al, 2010). Suc imported from the phloem can either be directly converted to cellulose or other cell wall carbohydrates through the activity of Suc synthase (Konishi et al, 2004). Suc can also be stored in the vacuole or be cleaved into Glc and Fru by invertases, principally for glycolysis (Trethewey et al, 1998).…”

Sucrose synthase expression pattern in the rhythmically growing shoot of common oak (Quercus robur L.)

“…This localization in common oak leads also to suggest an involvement of SuSy in controlling sugar fluxes in the vascular system. Indeed, it has been suggested that the hydrolysis of sucrose by SuSy in phloem cells could either constitute a source of energy for loading/unloading of the sucrose [27] or for the provision of UDP-glucose for callose synthesis in the sieve plates [14]. In common oak, the leaves of the first flush are source organs for young leaves of the second flush [1].…”

Caracterização de perfis transcricionais de folhas e da região cambial de Eucalyptus grandis usando o SAGE

“…de acetaldeído e etanol, em condições de baixa pressão de oxigênio, tem sido freqüentemente associada com a adaptação a condições de hipoxia (KREUZWIESER et al, 2004).…”

“…Grande parte dos açúcares sintetizados nas folhas deve ser preferencialmente transportada para os tecidos não fotossintetizantes como, por exemplo, o xilema, onde servirão como fonte de energia para a síntese de biomoléculas (KONISHI et al, 2004). Duas enzimas catalisam a síntese de sacarose nas folhas, a sacarose sintetase (SUSY) e a sacarose fosfato sintetase (SPS), ambas apresentaram baixo nível de expressão nas folhas de Populus (GEISLER-LEE et al, 2006) semelhante ao encontrado no presente trabalho.…”

“…Essa seqüência de eventos, entretanto, pode apresentar muitas variações devido à necessidade de adaptação das plantas á diferentes condições ambientais (PLAXTON & PODESTÁ, 2006). Por exemplo, a habilidade observada em muitos tecidos de dirigir o piruvato, produto da glicólise, para produçãode acetaldeído e etanol, em condições de baixa pressão de oxigênio, tem sido freqüentemente associada com a adaptação a condições de hipoxia (KREUZWIESER et al, 2004). …”