Companion cell‐specific inhibition of the potato sucrose transporter SUT1

Abstract

In many plants, translocation of sucrose from mesnsophyll to phloem for long‐distance transport is carrier‐mediated. The sucrose H+‐symporter gene SUT1 from potato is expressed at high levels in the phloem of mature, exporting leaves and at lower levels in other organs. Inhibition of SUT1 by expression of an antisense gene in companion cells under control of the rolC promoter leads to accumulation of high amounts of soluble and insoluble carbohydrates in leaves and inhibition of photosynthesis. The distribution of in situ localized starch does not correspond with areas of reduced photosynthesis as shown by fluorescence imaging. Dissection of antisense effects on sink and source organs by reciprocal grafts shows that inhibition of transporter gene expression in leaves is sufficient to produce chlorosis in leaves and reduced tuber yield. In contrast to the arrest of plasmodesmal development found in plants that express yeast invertase in the apoplast, in mature leaves of sucrose transporter antisense plants plasmodesmata are branched and have median cavities. These data strongly support an apoplastic mode of phloem loading in potato, in which the sucrose transporter located at the plasma membrane of the sieve element/companion cell complex represents the primary route for sugar uptake into the long‐distance translocation pathway.

A New Subfamily of Sucrose Transporters, SUT4, with Low Affinity/High Capacity Localized in Enucleate Sieve Elements of Plants

“…In tobacco and potato, the severe phenotype of plants expressing SUT1 genes in antisense orientation has been interpreted to indicate that high-affinity sucrose uptake into the SE/CC complex is required for phloem loading of sucrose (Riesmeier et al, 1994;Kühn et al, 1996;Lemoine et al, 1996;Bürkle et al, 1998). However, phloem transport requires loading sucrose into minor veins as well as the operation of retrieval mechanisms along the transport pathway.…”

SUT2, a Putative Sucrose Sensor in Sieve Elements

“…In solanaceous plants, SUT1 is essential for phloem loading into sieve elements (Riesmeier et al, 1994;Kühn et al, 1996;Bürkle et al, 1998). SUT1 serves as a high-affinity transporter for sucrose ( K m ‫ف‬ 1 mM; Riesmeier et al, 1993), whereas SUT4, with a K m of ‫ف‬ 11 mM, is a low-affinity sucrose transporter (Weise et al, 2000).…”

Expression of Sucrose Transporter cDNAs Specifically in Companion Cells Enhances Phloem Loading and Long-Distance Transport of Sucrose but Leads to an Inhibition of Growth and the Perception of a Phosphate Limitation

“…SUTs are best characterized for their role in apoplastic phloem loading in which Suc, after release into the apoplast from photosynthetic cells, is actively accumulated to high concentrations in phloem companion cells in preparation for long-distance transport to heterotrophic tissues. Reducing the activities of SUTs involved in phloem loading by mutation or transcript reduction strategies results in dramatically stunted plants and carbohydrate accumulation in source leaves (Kühn et al, 1996;Gottwald et al, 2000;Slewinski et al, 2009). However, SUTs are also found throughout the plant, and their regulation and role in carbohydrate distribution remain active research areas.…”