Sucrose is a signal molecule in assimilate partitioning

Abstract

The proton-sucrose symporter mediates the key transport step in the resource distribution system that allows many plants to function as multicellular organisms. In the results reported here, we identify sucrose as a signaling molecule in a previously undescribed signal-transduction pathway that regulates the symporter. Sucrose symporter activity declined in plasma membrane vesicles isolated from leaves fed exogenous sucrose via the xylem transpiration stream. Symporter activity dropped to 35-50% of water controls when the leaves were fed 100 mM sucrose and to 20-25% of controls with 250 mM sucrose. In contrast, alanine symporter and glucose transporter activities did not change in response to sucrose treatments. Decreased sucrose symporter activity was detectable after 8 h and reached a maximum by 24 h. Kinetic analysis of transport activity showed a decrease in Vmax. RNA gel blot analysis revealed a decrease in symporter message levels, suggesting a drop in transcriptional activity or a decrease in mRNA stability. Control experiments showed that these responses were not the result of changing osmotic conditions. Equal molar concentrations of hexoses did not elicit the response, and mannoheptulose, a hexokinase inhibitor, did not block the sucrose effect. These data are consistent with a sucrose-specific response pathway that is not mediated by hexokinase as the sugar sensor. Sucrose-dependent changes in the sucrose symporter were reversible, suggesting this sucrose-sensing pathway can modulate transport activity as a function of changing sucrose concentrations in the leaf. These results demonstrate the existence of a signaling pathway that can control assimilate partitioning at the level of phloem translocation.

Figure 1

Concentration-dependent inhibition of sucrose transport activity in purified plasma membrane vesicles. Sucrose, alanine, and glucose transport activities were assayed in membrane vesicles that were isolated after 24 h of transpirational feeding of solutions containing the indicated concentrations of sucrose. Error bars represent ± standard deviation.

Figure 2

Symporter-mediated sucrose transport activity in plasma membrane vesicles isolated from sugar beet leaves treated in H₂O or in 100 mM sucrose for 4, 8, 12, or 24 h.

Figure 3

Sucrose, alanine, and glucose transport activities in plasma membrane vesicles isolated from leaves fed H₂O (stippled bars) or isoosmotic solutions of 100 mM sucrose (solid bars), 100 mM glucose (gray bars), or 50 mM KCl (herringbone bars) for 12 h.

Figure 4

RNA gel blot probed with cDNA of the sugar beet proton–sucrose symporter. Leaves were fed H₂O, 25 mM sucrose, 100 mM sucrose, or 250 mM sucrose for 24 h. The Arabidopsis actin gene was used as a control message. The percentage changes of the normalized symporter signals are presented.

Figure 5

Sucrose-mediated changes in symporter activity were reversible. Four leaves were cut from an individual plant, and two of them were placed in H₂O and the other two in 100 mM sucrose. Plasma membranes were purified from one leaf of each treatment after 24-h transpirational feeding. At the same time, petioles of the other two leaves were recut, and those leaves were placed in the opposite treatment solution [i.e., H₂O-treated leaves into 100 mM sucrose (H₂O/Suc), and sucrose-treated leaves into H₂O (Suc/H₂O)] for another 24 h before isolating plasma membrane vesicles.