A symplasmic flow of sucrose contributes to phloem loading in Ricinus cotyledons

Orlich, Gabriele; Hofbrückl, Markus; Schulz, Alexander

doi:10.1007/s004250050380

Cited by 26 publications

(27 citation statements)

References 29 publications

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“…However, the requirement for Suc synthesis in the scutellum of germinating wheat seeds dictates that sugars first must be taken up by scutellum ground tissues. This situation contrasts with the operation of parallel apo-and symplasmic pathways for phloem loading in germinating castor bean seeds in which Suc is supplied directly from the endosperm (Orlich et al, 1998).…”

Section: Discussionmentioning

confidence: 83%

Pathway of Sugar Transport in Germinating Wheat Seeds

et al. 2006

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Three homeologous genes encoding a sucrose (Suc) transporter (SUT) in hexaploid wheat (Triticum aestivum), TaSUT1A, 1B, and 1D, were expressed in germinating seeds, where their function is unknown. All three TaSUT1 proteins were confirmed to be capable of transporting both Suc and maltose by complementation tests with the SUSY7/ura3 yeast (Saccharomyces cerevisiae) mutant strain. The role of Suc transporters in germinating grain was examined by combining in situ hybridization, immunolocalization, fluorescent dye tracer movement, and metabolite assays. TaSUT1 transcript and SUT protein were detected in cells of the aleurone layer, scutellar epidermis, scutellar ground cells, and sieve element-companion cell complexes located in the scutellum, shoot, and root. Ester loading of the membrane-impermeable fluorescent dye carboxyfluorescein into the scutellum epidermal cells of germinating seeds showed that a symplasmic pathway connects the scutellum to the shoot and root via the phloem. However, the scutellar epidermis provides an apoplasmic barrier to solute movement from endosperm tissue. Measurements of sugars in the root, shoot, endosperm, and scutellum suggest that, following degradation of endosperm starch, the resulting hexoses are converted to Suc in the scutellum. Suc was found to be the major sugar present in the endosperm early in germination, whereas maltose and glucose predominate during the later stage. It is proposed that loading the scutellar phloem in germinating wheat seeds can proceed by symplasmic and apoplasmic pathways, the latter facilitated by SUT activity. In addition, SUTs may function to transport Suc into the scutellum from the endosperm early in germination and later transport maltose.

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Section: Discussionmentioning

confidence: 83%

Pathway of Sugar Transport in Germinating Wheat Seeds

et al. 2006

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“…For instance, infiltration of broad bean (Vicia faba) leaf apoplast by 2 mM pCMBS completely blocks phloem loading of assimilates without significantly affecting CO 2 assimilation or the transmembrane potential difference generated by the plasma membrane H 1 -ATPase (Bourquin et al, 1990). pCMBS is known as a potent inhibitor of Suc and oligopeptide carriers by reacting with Cys residues in the external part of these transporters (Delrot et al, 1980;Bush, 1993;Jamai et al, 1994;Orlich et al, 1998;Lemoine, 2000;Knop et al, 2004). This inhibiting effect cannot be generalized to other nutrient carriers, such as monosaccharide transporters (Noiraud et al, 2001), and depends on the location of Cys residues in the extracellular domains (Ramsperger-Gleixner et al, 2004).…”

Section: Structure-phloem Mobility Relationshipsmentioning

confidence: 99%

Salicylic Acid Transport in Ricinus communis Involves a pH-Dependent Carrier System in Addition to Diffusion

et al. 2009

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Despite its important functions in plant physiology and defense, the membrane transport mechanism of salicylic acid (SA) is poorly documented due to the general assumption that SA is taken up by plant cells via the ion trap mechanism. Using Ricinus communis seedlings and modeling tools (ACD LogD and Vega ZZ softwares), we show that phloem accumulation of SA and hydroxylated analogs is completely uncorrelated with the physicochemical parameters suitable for diffusion (number of hydrogen bond donors, polar surface area, and, especially, LogD values at apoplastic pHs and D LogD between apoplast and phloem sap pH values). These and other data (such as accumulation in phloem sap of the poorly permeant dissociated form of monohalogen derivatives from apoplast and inhibition of SA transport by the thiol reagent p-chloromercuribenzenesulfonic acid [pCMBS]) lead to the following conclusions. As in intestinal cells, SA transport in Ricinus involves a pH-dependent carrier system sensitive to pCMBS; this carrier can translocate monohalogen analogs in the anionic form; the efficiency of phloem transport of hydroxylated benzoic acid derivatives is tightly dependent on the position of the hydroxyl group on the aromatic ring (SA corresponds to the optimal position) but moderately affected by halogen addition in position 5, which is known to increase plant defense. Furthermore, combining time-course experiments and pCMBS used as a tool, we give information about the localization of the SA carrier. SA uptake by epidermal cells (i.e. the step preceding the symplastic transport to veins) insensitive to pCMBS occurs via the ion-trap mechanism, whereas apoplastic vein loading involves a carrier-mediated mechanism (which is targeted by pCMBS) in addition to diffusion.

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“…the cell-to-cell transport of sugars from the mesophyll to the vascular bundles). A symplasmic pathway for prephloem transport is assumed (Ayre, 2011;Chen et al, 2012), but all cells on that pathway seem to be able to take up Suc from the apoplast, which would allow for apoplasmic steps or a mixed apoplasmic/symplasmic prephloem transport (Orlich et al, 1998).…”

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confidence: 99%

In Vivo Quantification of Cell Coupling in Plants with Different Phloem-Loading Strategies

Liesche

Schulz

2012

Plant Physiology

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Uptake of photoassimilates into the leaf phloem is the key step in carbon partitioning and phloem transport. Symplasmic and apoplasmic loading strategies have been defined in different plant taxa based on the abundance of plasmodesmata between mesophyll and phloem. For apoplasmic loading to occur, an absence of plasmodesmata is a sufficient but not a necessary criterion, as passage of molecules through plasmodesmata might well be blocked or restricted. Here, we present a noninvasive, whole-plant approach to test symplasmic coupling and quantify the intercellular flux of small molecules using photoactivation microscopy. Quantification of coupling between all cells along the prephloem pathways of the apoplasmic loader Vicia faba and Nicotiana tabacum showed, to our knowledge for the first time in vivo, that small solutes like sucrose can diffuse through plasmodesmata up to the phloem sieve element companion cell complex (SECCC). As expected, the SECCC was found to be symplasmically isolated for small solutes. In contrast, the prephloem pathway of the symplasmic loader Cucurbita maxima was found to be well coupled with the SECCC. Phloem loading in gymnosperms is not well understood, due to a profoundly different leaf anatomy and a scarcity of molecular data compared with angiosperms. A cell-coupling analysis for Pinus sylvestris showed high symplasmic coupling along the entire prephloem pathway, comprising at least seven cell border interfaces between mesophyll and sieve elements. Cell coupling together with measurements of leaf sap osmolality indicate a passive symplasmic loading type. Similarities and differences of this loading type with that of angiosperm trees are discussed.

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A symplasmic flow of sucrose contributes to phloem loading in Ricinus cotyledons

Cited by 26 publications

References 29 publications

Pathway of Sugar Transport in Germinating Wheat Seeds

Pathway of Sugar Transport in Germinating Wheat Seeds

Salicylic Acid Transport in Ricinus communis Involves a pH-Dependent Carrier System in Addition to Diffusion

In Vivo Quantification of Cell Coupling in Plants with Different Phloem-Loading Strategies

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