Sucrose (JrSUT1) and hexose (JrHT1 and JrHT2) transporters in walnut xylem parenchyma cells: their potential role in early events of growth resumption

In temperate woody species, the vegetative growth period is characterized by active physiological events (e.g., bud break), which require an adequate supply of soluble sugars imported in the xylem sap stream. One-year-old shoots of walnut (Juglans regia L. cv. 'Franquette') trees, which have an acrotonic branching pattern (only apical and distal vegetative buds burst), were used to study the regulation of xylem sugar transporters in relation to bud break. At the end of April (beginning of bud break), a higher xylem sap sucrose concentration and a higher active sucrose uptake by xylem parenchyma cells were found in the apical portion (bearing buds able to burst) than in the basal portion (bearing buds unable to burst) of the sample shoots. At the same time, xylem parenchyma cells of the apical portion of the shoots exhibited greater amounts of both transcripts and proteins of JrSUT1 (Juglans regia putative sucrose transporter 1) than those of the basal stem segment. Conversely, no pronounced difference was found for putative hexose transporters JrHT1 and JrHT2 (Juglans regia hexose transporters 1 and 2). These findings demonstrate the high capacity of bursting vegetative buds to import sucrose. Immunological analysis revealed that sucrose transporters were localized in all parenchyma cells of the xylem, including vessel-associated cells, which are highly specialized in nutrient exchange. Taken together, our results indicate that xylem parenchyma sucrose transporters make a greater contribution than hexose transporters to the imported carbon supply of bursting vegetative buds.