Downregulating the sucrose transporter VpSUT1 in Verbascum phoeniceum does not inhibit phloem loading

Abstract

Sucrose is loaded into the phloem in the minor veins of leaves before export. Two active, species-specific loading mechanisms have been proposed. One involves transporter-mediated sucrose transfer from the apoplast into the sieve element-companion cell complex, so-called apoplastic loading. In the putative second mechanism, sucrose follows an entirely symplastic pathway, and the solute concentration is elevated by the synthesis of raffinose and stachyose in the phloem, not by transporter activity. Several sucrosetransporting plants have been shown to be apoplastic loaders by downregulating sucrose transporter 1 (SUT1), leading to accumulation of sugars and leaf chlorosis. In this study we compared the effect of downregulating SUT1 in Nicotiana tabacum, a sucrose transporter, and Verbascum phoeniceum, a species that transports raffinose and stachyose. To test the effectiveness of RNAi downregulation, we measured SUT1 mRNA levels and sucrose-H ؉ symport in leaf discs. Mild NtSUT1 downregulation in N. tabacum resulted in the pronounced phenotype associated with loading inhibition. In contrast, no such phenotype developed when VpSUT1 was downregulated in V. phoeniceum, leaving minimal sucrose transport activity. Only those plants with the most severe VpSUT1 downregulation accumulated more carbohydrate than usual and these plants were normal by other criteria: growth rate, photosynthesis, and ability to clear starch during the night. The results provide direct evidence that the mechanism of phloem loading in V. phoeniceum does not require active sucrose uptake from the apoplast and strongly supports the conclusion that the loading pathway is symplastic in this species.plasmodesmata ͉ raffinose ͉ stachyose P hloem loading provides the driving force for long-distance transport from leaves to sink organs in many plants by elevating the carbohydrate content and hydrostatic pressure in the sieve elements (SE) and companion cells (CC) of minor veins (1-3). In a large number of species, phloem loading is mediated by transporters on the plasma membranes of the SEs and CCs. These transporters drive sucrose, and in some cases sugar alcohol, from the apoplast into the SE-CC complex. The most direct proof of apoplastic loading has come from studies in which sucrose transporter 1 (SUT1/SUC2) is downregulated by antisense technology or DNA insertion (4-9). Such downregulation inhibits phloem loading and results in accumulation of sugars and starch, stunted growth, diminished photosynthesis, and leaf chlorosis. These experiments confirm the necessity of transmembrane transport of sucrose in these species and the indispensable role of SUT1 in apoplastic phloem loading.In species that load from the apoplast, relatively few plasmodesmata connect the SE-CC complex to surrounding cells (10,11). This is unsurprising given that extensive symplastic continuity would create a futile cycle of active loading from the apoplast and passive leakage back to mesophyll cells. However, in many other species plasmodesmata in the minor veins are numer...

Phloem Loading through Plasmodesmata: A Biophysical Analysis

“…However, available evidence does not support the presence of complementary apoplastic and symplastic mechanisms in intermediary cells: The Suc transporter does not immunolocalize to the intermediary cell plasma membrane (Voitsekhovskaja et al, 2009). It is possible, even likely, that apoplastic loading occurs in other companion cells in RFO plants (those that do not express the RFO pathway), but if so this contribution is apparently minimal since blocking the Suc transporter chemically (Turgeon and Gowan, 1990) or by RNAi (Zhang and Turgeon, 2009) does not noticeably inhibit growth. Nonetheless, the effect of additional Suc flux at the mesophyll/phloem interface due to apoplastic loading could be implemented in the model.…”

Sucrose transporter NtSUT1 confers aluminum tolerance on cultured cells of tobacco (Nicotiana tabacumL.)

“…Sucrose transporters localized at the plasma membrane have been reported to have important roles in normal plant growth and development including root growth (Bürkle et al 1998;Sauer 2007;Schmitt et al 2008;C.K. Zhang and Turgeon 2009).…”

An Overview of Plant Photosynthesis Modulation by Pathogen Attacks

“…As a result of photosynthetic CO 2 reduction during the day, starch granules accumulate in the chloroplast while an excess of assimilates are continuously allocated, mostly in the form of sucrose, to sink tissues such as developing leaves, roots, meristems, fruits, and flowers, that are unable to produce sufficient amounts of assimilates by themselves and therefore require their net import via the phloem (Kocal et al, 2008). Sucrose is loaded into the phloem in the minor veins of leaves before export (Zhang and Turgeon, 2009). Recently, Zhang and Turgeon (Zhang and Turgeon, 2009) have proposed two active, species-specific loading mechanisms.…”

“…Sucrose is loaded into the phloem in the minor veins of leaves before export (Zhang and Turgeon, 2009). Recently, Zhang and Turgeon (Zhang and Turgeon, 2009) have proposed two active, species-specific loading mechanisms. One involves transporter-mediated sucrose transfer from the apoplast into the sieve element-companion cell complex, so-called apoplastic loading.…”