ABSTRACT
Cotyledons of broad bean (Vicia faba L.) develop in an apoplasmic environment that shifts in composition from one dominated by hexoses to one dominated by sucrose. During the latter phase of development, sucrose / H+ symporter activity and expression is restricted to cotyledon epidermal transfer cell complexes that support sucrose fluxes that are 8.5-fold higher than those exhibited by the storage parenchyma. In contrast, the flux difference between these cotyledon tissues is only 1.7-fold for hexoses. Glucose and fructose uptake was shown to be sensitive to PCMBS and phloridzin, both of which slow H+-sugar transport. A low Km (or high affinity transporter, HAT) mechanism transports glucose and glucose-analogues exclusively. No HAT system for fructose could be found. A high Km (low affinity transporter, LAT) mechanism transports a broader range of hexoses, including glucose and fructose. Consistent with glucose and fructose transport being H+-coupled, their uptake was inhibited by dissipating the proton motive force (pmf) by treating cotyledons with carbonyl cyanide m-chlorophenol hydrazone, propionic acid or tetraphenylphosphonium ion. Erythrosin B inhibited hexose uptake, indicating a role for the P-type H+-ATPase in establishing the pmf. It is concluded that H+-coupled glucose and fructose transport mechanisms occur at plasma membranes of dermal transfer cell complexes and storage parenchyma cells. These transport mechanisms are active during pre- and storage phases of cotyledon development. However, hexose symport only makes a quantitative contribution to cotyledon biomass gain during the pre-storage stage of development.
ABSTRACT
A proton-sucrose symporter mediates the key step in carbon export from leaves of most plants. Sucrose transport activity and steady-state mRNA levels of BvSUT1, a sugar beet leaf sucrose symporter, are negatively regulated specifically by sucrose. Results reported here show that BvSUT1 mRNA was localized to companion cells of the leaf's vascular system, which supports its role in the systemic distribution of photoassimilate. Immunoblot analysis showed that decreased transport activity was caused by a reduction in the abundance of symporter protein. RNA gel blot analysis of the leaf symporter revealed that message levels also declined, and nuclear run-on experiments demonstrated that this was the result of decreased transcription. Further analysis showed that symporter protein and message are both degraded rapidly. Taken together, these data show that phloem loading is regulated by means of sucrose-mediated changes in transcription of a phloem-specific sucrose symporter gene in a regulatory system that may play a pivotal role in balancing photosynthetic activity with resource utilization.
