Plasma Membrane Vesicles from Source and Sink Leaves

Abstract

Plasma membrane vesicles (PMVs) were prepared by phase partitioning from microsomal fractions of either sink or source leaves of sugar beet (Beta vulgaris L.). The purity, the internal volume, the sidedness, and the sealingness of PMVs prepared from sink leaves did not differ from those measured with PMVs from source leaves. Yet, in response to an imposed proton motive force, PMVs from source leaves accumulated about 4-fold more sucrose than PMVs from sink leaves. The developmental stage did not affect the uptake of glucose and valine in PMVs prepared from leaf tissues. It was concluded that the sink/source transition is accompanied either by the incorporation into the plasma membrane of leaf cells of proteins mediating proton-sucrose cotransport, or by their activation. N-ethylmaleimide and a polyclonal ascitic fluid directed against the 42-kD region of the plasma membrane containing a putative sucrose carrier inhibited the uptake of sucrose in PMVs from source leaves, but not in PMVs from sink leaves. Sodium dodecyl sulfate gel electrophoresis and western blot suggested that the 42 polypeptide was more abundant in the PMVs from source leaves than in the PMVs from sink leaves.

Molecular Cloning, Immunochemical Localization to the Vacuole, and Expression in Transgenic Yeast and Tobacco of a Putative Sugar Transporter from Sugar Beet

“…The signal increased with the transition from very young to mature leaves. This was intriguing because Sue transport activity has previously been shown to increase in a similar pattern (Lemoine et al, 1992;D.R. Bush, unpublished data).…”

Symplasmic Transport in Phloem Loading and Unloading

“…All investigations point to a completely symplasmic mechanism of phloem unloading as shown by high abundance of PD at all interfaces of the post-phloem pathway (Ding et al 1988 ;Roberts et al 2001 ), GFP and CF diffusing out of CC into neighboring cells (Roberts et al 1997 ;Oparka et al 1999 ), apoplasm fl uid analysis, and test of sucrose-transporter activity (Schmalstig and Geiger 1985 ;Turgeon 1987 ;Lemoine et al 1992 ).…”

The biochemical and molecular basis for photosynthetic acclimation to elevated atmospheric CO2

“…Antisense inhibition of the symporter protein in potato resulted in an altered phenotype with mature leaves having bleached areas, reduced photosynthesis, and an accumulation of carbohydrates that often resulted in plant stunting (Riesmeier, Willmitzer & Frommer 1994; Kühn et al 1996). Phloem expression of the sucrose symporter protein is enhanced during leaf maturation (Lemoine et al 1992) and is diurnally regulated at the mRNA and protein levels (Kühn et al 1997). Furthermore, the mRNA abundance and carrier activity of the sucrose symporter protein may be under metabolic control by sucrose abundance (Chiou & Bush 1998).…”