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

Abstract

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.

The Role of Xylem Parenchyma in the Storage and Utilization of Nonstructural Carbohydrates

“…8.4 ). Transcript and protein levels of several of these putative transporters have been studied in walnut (Decourteix et al 2006(Decourteix et al , 2008. While the sucrose transporter JrSUT1 was strongly up-regulated in xylem parenchyma cells during bud break, two hexose transporters, JrHT1 and JrHT2, were abundant during the period of intense radial growth.…”

“…Expanding buds and the active cambial zone represent the strongest sinks during this period. Both of these tissues receive carbohydrates stored in sapwood (Hill et al 1995 ;Bonhomme et al 2010 ;Decourteix et al 2008 ). While little is known about the partitioning of reserves between these two tissues, it can be expected that it is closely related to the offset between the cambium and bud phenology.…”

Walnut (Juglans spp.) ecophysiology in response to environmental stresses and potential acclimation to climate change

“…Experiments by Alves et al (2004Alves et al ( , 2007 found that plasma membrane H + -ATPase plays an important role in the uptake of carbohydrates from xylem vessels during growth resumption in English walnut. Sucrose and hexose transporters (JrSTU1, JrHT1, JrHT2) involved in parenchyma cells of xylem vessels were also identified in English walnut (Decourteix et al 2006(Decourteix et al , 2008.…”

“…4). Sucrose demand by developing buds, leaves, and shoots provides a sink needed to move carbohydrates into the xylem (Decourteix et al 2008;Bonhomme et al 2010). This positive xylem pressure refills vessels, which regain hydraulic function .…”

Atividade de enzimas relacionadas com a mobilização de carboidratos durante a dormência da nogueira (Juglans Regia)

“…Contudo, as bases bioquímicas e/ou fisiológicas dos fatores causadores da acrotonia são ainda hipotéticas, pois as abordagens experimentais focadas nos gradientes são pouco desenvolvidas. Alguns estudos relativos aos gradientes de brotação abordaram as bases tróficas (Brunel, 2001;Leite, 2004;Alves et al, 2007;Decourteix et al, 2008), os hormônios (Crabbé, 1987) e o estado hídrico (Leite, 2004).…”