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.
"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. "
[Show abstract][Hide abstract] ABSTRACT: Plants in nature, which are continuously challenged by diverse insect herbivores, produce constitutive and inducible defenses to reduce insect damage and preserve their own fitness. In addition to inducing pathways that are directly responsible for the production of toxic and deterrent compounds, insect herbivory causes numerous changes in plant primary metabolism. Whereas the functions of defensive metabolites such as alkaloids, terpenes, and glucosinolates have been studied extensively, the fitness benefits of changes in photosynthesis, carbon transport, and nitrogen allocation remain less well understood. Adding to the complexity of the observed responses, the feeding habits of different insect herbivores can significantly influence the induced changes in plant primary metabolism. In this review we summarize experimental data addressing the significance of insect feeding habits, as related to herbivore-induced changes in plant primary metabolism. Where possible, we link these physiological changes with current understanding of their underlying molecular mechanisms. Finally, we discuss the potential fitness benefits that host plants receive from altering their primary metabolism in response to insect herbivory.
"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. "
[Show abstract][Hide abstract] ABSTRACT: The perception of aphid infestation induces highly coordinated and sequential defensive reactions in plants at the cellular and molecular levels. The aim of the study was to explore kinetics of induced antioxidative defence responses in leaf cells of Pisum sativum L.cv. Cysterski upon infestation of the pea aphid Acyrthosiphon pisum at varying population sizes, including accumulation of flavonoids, changes of carbon metabolism, and expression of nuclear genes involved in sugar transport. Within the first 96 h, after A. pisum infestation, flavonoid accumulation and increased peroxidase activity were observed in leaves. The level of pisatin increased after 48 h of infestation and reached a maximum at 96 h. At this time point, a higher concentration of flavonols was observed in the infested tissue than in the control. Additionally, strong post-infestation accumulation of chalcone synthase (CHS) and isoflavone synthase (IFS) transcription products was also found. The levels of sucrose and fructose in 24-h leaves infested by 10, 20, and 30 aphids were significantly lower than in the control. Moreover, in leaves infested by 30 aphids, the reduced sucrose level observed up to 48 h was accompanied by a considerable increase in the expression level of the PsSUT1 gene encoding the sucrose transporter. In conclusion, A. pisum infestation on pea leads to stimulation of metabolic pathways associated with defence.
"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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