Sucrose Efflux Mediated by SWEET Proteins as a Key Step for Phloem Transport

Carnegie Institution for Science, 260 Panama Street, Stanford, CA 94305, USA.
Science (Impact Factor: 33.61). 12/2011; 335(6065):207-11. DOI: 10.1126/science.1213351
Source: PubMed


Plants transport fixed carbon predominantly as sucrose, which is produced in mesophyll cells and imported into phloem cells for translocation throughout the plant. It is not known how sucrose migrates from sites of synthesis in the mesophyll to the phloem, or which cells mediate efflux into the apoplasm as a prerequisite for phloem loading by the SUT sucrose-H(+) (proton) cotransporters. Using optical sucrose sensors, we identified a subfamily of SWEET sucrose efflux transporters. AtSWEET11 and 12 localize to the plasma membrane of the phloem. Mutant plants carrying insertions in AtSWEET11 and 12 are defective in phloem loading, thus revealing a two-step mechanism of SWEET-mediated export from parenchyma cells feeding H(+)-coupled import into the sieve element-companion cell complex. We discuss how restriction of intercellular transport to the interface of adjacent phloem cells may be an effective mechanism to limit the availability of photosynthetic carbon in the leaf apoplasm in order to prevent pathogen infections.

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    • "In congruence with this framework, herbivore-induced carbohydrate depletion has coincided with decreased invertase activities in the same tissues (Machado et al., 2013; Robert et al., 2014). Carbohydrate movement in source tissues is facilitated by sets of membrane transporters (Burkle et al., 1998; Gottwald et al., 2000; Ayre, 2011; Chen et al., 2012). Although expression of these transporters is required for herbivore-induced carbohydrate reallocation (Ferrieri et al., 2012), there is so far little evidence showing that their expression is directly influenced by herbivore attack. "
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    • "SUTs play an important role in apoplastic phloem loading in higher plants (Sauer 2007; Reinders et al. 2012). Sucrose enters the phloem for longdistance transport either symplastically via plasmodesmata or apoplastically via SWEET-mediated export from phloem parenchyma cells, followed by SUT-mediated uptake into phloem companion cells (Chen et al 2012; Berthier et al. 2009). As reported by Kühn and Grof (2010), the phloemspecific expression of high affinity group II plasma membrane SUTs is consistent with a role in apoplastic phloem loading and subsequent recovery of sucrose that escapes during phloem transport. "
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    • "In glumes, expression of several carbohydrate transporter genes increased at 8 DAP and then further until 24 DAP. This involved hexose/sugar transporters and members of the SWEET family, which potentially export sugars from Arabidopsis leaves (Chen et al., 2012). In the endosperm, solute transporters related to storage product synthesis (e.g. "
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