Phloem loading in
            <i>Verbascum phoeniceum</i>
            L. depends on the synthesis of raffinose-family oligosaccharides

McCaskill, Ashlee; Turgeon, Robert

doi:10.1073/pnas.0707368104

Cited by 81 publications

(69 citation statements)

References 39 publications

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“…It could be argued that V. phoeniceum does not display the same symptoms as tobacco when loading is inhibited, but this is clearly not the case since downregulating RFO synthesis in the same cultivar of this species, using RNAi technology as in the present study, results in the classic symptoms of transport inhibition: excessive accumulation of soluble carbohydrates, inability to transport starch during an extended dark period, stunted growth, and leaf chlorosis (18).…”

Section: Discussionmentioning

confidence: 74%

“…However, Verbascum phoeniceum, an RFO-transporting species, has been found to transform with ease (18). Earlier experiments demonstrated that RFO synthesis is required for efficient phloem loading in V. phoeniceum (18) but these experiments did not address the fundamental question of how sucrose enters the minor veins. The results presented here indicate that severe downregulation of SUT1, leading to almost complete loss of sucrose transporter activity, has little effect on long-distance transport.…”

mentioning

confidence: 89%

“…Seeds of V. phoeniceum L. cv. ''Flush of White'' (Garden Makers) and N. tabacum ''Petite Havana SRI'' were surface-sterilized and germinated as described (18). Plants were kept in a growth chamber with 14-h day/10-h night cycles at approximately 100 mol photons m Ϫ2 s Ϫ1 for plant transformation.…”

Section: Methodsmentioning

confidence: 99%

“…A ''polymer trap'' mechanism (18)(19)(20) has been proposed to explain how sucrose is loaded through plasmodesmata in RFO plants. According to this model, sucrose diffuses through plasmodesmata from mesophyll cells to intermediary cells, where it is converted into RFOs.…”

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confidence: 99%

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

Zhang

Turgeon

2009

Proc. Natl. Acad. Sci. U.S.A.

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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...

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Section: Discussionmentioning

confidence: 74%

mentioning

confidence: 89%

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

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

Zhang

Turgeon

2009

Proc. Natl. Acad. Sci. U.S.A.

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“…The ability to store a secondary metabolite with a chemical structure larger than the precursor (i.e. amygdalin versus prunasin, raffinose versus Suc; McCaskill and Turgeon, 2007) is an option plants have developed to reduce back diffusion through plasmodesmata and thereby to achieve the accumulation of high concentrations of a product in a sink organ, such as the almond embryo or the phloem.…”

Section: Discussionmentioning

confidence: 99%

Prunasin Hydrolases during Fruit Development in Sweet and Bitter Almonds

Sánchez‐Pérez

Belmonte²,

Borch

et al. 2012

Plant Physiology

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Amygdalin is a cyanogenic diglucoside and constitutes the bitter component in bitter almond (Prunus dulcis). Amygdalin concentration increases in the course of fruit formation. The monoglucoside prunasin is the precursor of amygdalin. Prunasin may be degraded to hydrogen cyanide, glucose, and benzaldehyde by the action of the b-glucosidase prunasin hydrolase (PH) and mandelonitirile lyase or be glucosylated to form amygdalin. The tissue and cellular localization of PHs was determined during fruit development in two sweet and two bitter almond cultivars using a specific antibody toward PHs. Confocal studies on sections of tegument, nucellus, endosperm, and embryo showed that the localization of the PH proteins is dependent on the stage of fruit development, shifting between apoplast and symplast in opposite patterns in sweet and bitter cultivars. Two different PH genes, Ph691 and Ph692, have been identified in a sweet and a bitter almond cultivar. Both cDNAs are 86% identical on the nucleotide level, and their encoded proteins are 79% identical to each other. In addition, Ph691 and Ph692 display 92% and 86% nucleotide identity to Ph1 from black cherry (Prunus serotina). Both proteins were predicted to contain an amino-terminal signal peptide, with the size of 26 amino acid residues for PH691 and 22 residues for PH692. The PH activity and the localization of the respective proteins in vivo differ between cultivars. This implies that there might be different concentrations of prunasin available in the seed for amygdalin synthesis and that these differences may determine whether the mature almond develops into bitter or sweet.

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Phloem Transport of Resources

Grof¹,

Byrt²,

Patrick³

2013

Sugarcane: Physiology, Biochemistry, and Functional Biology

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This chapter reviews the principles and concepts of resource transport in the phloem in general and then moves to describe the physiology of phloem translocation in sugarcane. The movement of resources through the phloem is interpreted through the Münch pressure flow hypothesis. The phloem transport rate is determined by magnitudes of pressure differences between the source and sink ends of the pathway and the phloem sap concentration of each transported nutrient. Phloem rate is regulated by loading and unloading activities, sink hydrostatic pressures, and hydraulic conductivities of plasmodesmata interconnecting sieve element/companion cell complexes with phloem parenchyma cells. A description of the leaf structural framework in sugarcane is provided and the phloem pathways in sugarcane are dissected from loading of collection phloem in source leaves, transit through transport phloem, to release phloem where resources are unloaded into growth and storage sinks. Phloem conductivities in sugarcane, mechanisms of loading and unloading of solutes, and the environmental effects on phloem translocation are considered.

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Phloem loading in Verbascum phoeniceum L. depends on the synthesis of raffinose-family oligosaccharides

Cited by 81 publications

References 39 publications

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

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

Prunasin Hydrolases during Fruit Development in Sweet and Bitter Almonds

Phloem Transport of Resources

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