Characterization of Source- and Sink-Specific Sucrose/H+ Symporters from Carrot

To understand how sucrose (Suc) is transported from source leaves to developing tap roots of carrot (Daucus carota L.), we cloned two cDNAs (DcSUT1 and DcSUT2) for proteins with homologies to plant Suc/H+ symporters. The deduced polypeptide sequences are 52% identical and have 12 predicted membrane-spanning domains each. Transport activities were confirmed by expression of the clones in yeast cells. Both transporters had optimal activity below pH 5.0 and Michaelis constant values of 0.5 mm. Suc uptake was inhibited by protonophores, suggesting that Suc transport is linked to the proton electrochemical potential across the plasma membrane. DcSUT1 and DcSUT2 had markedly different expression patterns. Transcripts of DcSUT1 were found only in the green parts of plants, with highest levels in the lamina of source leaves, indicating that DcSUT1 is required for the loading of Suc into the phloem. In leaf lamina expression was diurnally regulated, suggesting that Suc export from the leaves is higher during the day than during the night. The mRNA of DcSUT2 was found mainly in sink organs, and no diurnal expression pattern was detected in the storage root. Here, expression was not restricted to the phloem but was much higher in storage parenchyma tissues of phloem and xylem. The close relationship of DcSUT2 with a Suc/H+ symporter from fava bean, which facilitates Suc uptake into the cotyledons of developing seeds, indicates that this carrot Suc transporter may be involved in loading Suc into storage parenchyma cells.

Source and Magnitude of Ammonium Generation in Maize Roots1

Studies with 15N indicate that appreciable generation of NH4+ from endogenous sources accompanies the uptake and assimilation of exogenous NH4+ by roots. To identify the source of NH4+ generation, maize (Zea mays L.) seedlings were grown on 14NH4+ and then exposed for 3 d to highly labeled 15NH4+. More of the entering 15NH4+ was incorporated into the protein-N fraction of roots in darkness (approximately 25%) than in the light (approximately 14%). Although the 14NH4+ content of roots declined rapidly to less than 1 μmol per plant, efflux of 14NH4+ continued throughout the 3-d period at an average daily rate of 14 μmol per plant. As a consequence, cumulative 14NH4+ efflux during the 3-d period accounted for 25% of the total 14N initially present in the root. Although soluble organic 14N in roots declined during the 3-d period, insoluble 14N remained relatively constant. In shoots both soluble organic 14N and 14NH4+ declined, but a comparable increase in insoluble 14N was noted. Thus, total 14N in shoots remained constant, reflecting little or no net redistribution of 14N between shoots and roots. Collectively, these observations reveal that catabolism of soluble organic N, not protein N, is the primary source of endogenous NH4+ generation in maize roots.