Distribution of Plasmodesmata in Leaves. A Comparison of Cananga Odorata with Other Species Using Different Measures of Plasmodesmatal Frequency

Fisher, David G.

doi:10.1007/978-3-642-83971-9_14

Cited by 16 publications

(17 citation statements)

References 27 publications

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“…Though species-specific differences in plasmodesmatal distribution and frequencies have been found, which indicate an apoplasmic loading in some species and a symplasmic loading in others, and also a combination of both (Fisher 1986(Fisher , 1990, and references therein), the pathway(s) of loading cannot be deduced solely on structural grounds and must be confirmed by a functional analysis Delrot 1989;Orlich and Komor 1989).…”

Section: Introductionmentioning

confidence: 94%

Phloem loading in Ricinus cotyledons: sucrose pathways via the mesophyll and the apoplasm

Orlich

Komor

1992

Planta

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Ricinus communis cv. Carmencita seedlings with their cotyledons incubated in sucrose solution and their hypocotyls cut to induce exudation of phloem sap, constitute a system of sucrose fluxes into and out of the cotyledons. This system was characterized with respect to quasi-steady-state conditions of sucrose uptake and export and then used to investigate the pathways of sucrose during phloem loading. The redistribution of (14)C-labelled internal sucrose between the three "compartments", cotyledons (mesophyll), exudate (sieve tubes) and incubation medium (cell-wall space), was measured in the presence or absence of external nonlabelled sucrose. It was found that mesophyll-derived labelled and external sucrose compete at uptake sites in the apoplasm. On the basis of the specific radioactivity of sucrose which reflects the proportionate intermixture of mesophyll-derived and external sucrose in the three "compartments", it was determined that about 50% of the sucrose exported is loaded directly from the apoplasm, while the other half takes the route via the mesophyll. It was confirmed that mesophyll-derived sucrose is released into the apoplasm, so that the existence of an indirect apoplasmic loading pathway is established. Calculations depending on the concentration gradients of labelled and non-labelled sucrose in the cell-wall space are presented to quantify tentatively the proportions of direct and indirect apoplasmic as well as symplasmic loading.

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Section: Introductionmentioning

confidence: 94%

Phloem loading in Ricinus cotyledons: sucrose pathways via the mesophyll and the apoplasm

Orlich

Komor

1992

Planta

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“…B, Data arranged according to growth habit. The data in A and B are from the following: 1 and 2, this study; 3, Fisher (1990); 4, Goggin et al (2001); 5, Russin and Evert (1985); 6, Haritatos et al (2000); 7, Beebe and Evert (1992);8, McCauley and Evert (1989); 9, Warmbrodt and VanDerWoude (1990); 10, Evert and Mierzwa (1986); 11, Wimmers and Turgeon (1991);12, Bourquin et al (1990);13, Fisher (1991);14, this study;15, Fisher (1986).…”

Section: This Work)mentioning

confidence: 93%

Phloem Loading. A Reevaluation of the Relationship between Plasmodesmatal Frequencies and Loading Strategies

Turgeon

Medville

2004

Plant Physiology

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(R.T.); and Electron Microscopy Services and Consultants, Phoenix, Arizona 85022 (R.M.)The incidence of plasmodesmata in the minor vein phloem of leaves varies widely between species. On this basis, two pathways of phloem loading have been proposed: symplastic where frequencies are high, and apoplastic where they are low. However, putative symplastic-loading species fall into at least two categories. In one, the plants translocate raffinose-family oligosaccharides (RFOs). In the other, the primary sugar in the phloem sap is sucrose (Suc). While a thermodynamically feasible mechanism of symplastic loading has been postulated for species that transport RFOs, no such mechanism is known for Suc transporters. We used p-chloromercuribenzenesulfonic acid inhibition of apoplastic loading to distinguish between the two pathways in three species that have abundant minor vein plasmodesmata and are therefore putative symplastic loaders. Clethra barbinervis and Liquidambar styraciflua transport Suc, while Catalpa speciosa transports RFOs. The results indicate that, contrary to the hypothesis that all species with abundant minor vein plasmodesmata load symplastically, C. barbinervis and L. styraciflua load from the apoplast. C. speciosa, being an RFO transporter, loads from the symplast, as expected. Data from these three species, and from the literature, also indicate that plants with abundant plasmodesmata in the minor vein phloem have abundant plasmodesmata between mesophyll cells. Thus, plasmodesmatal frequencies in the minor veins may be a reflection of overall frequencies in the lamina and may have limited relevance to phloem loading. We suggest that symplastic loading is restricted to plants that translocate oligosaccharides larger than Suc, such as RFOs, and that other plants, no matter how many plasmodesmata they have in the minor vein phloem, load via the apoplast.Phloem loading is an energy-requiring process that elevates the solute content of sieve elements (SEs) and companion cells (CCs) to levels far above those of surrounding cells (Geiger et al., 1973;van Bel, 1993;Grusak et al., 1996;Turgeon, 1996Turgeon, , 2000Beebe and Russin, 1999;Hellmann et al., 2000;Komor, 2000;Lalonde et al., 2003). The pressure gradient between source and sink organs drives long-distance transport. It has been suggested that there are two loading strategies-apoplastic and symplastic. This hypothesis derived originally from the observation that there is great variation in the number of plasmodesmata in the phloem of minor veins (Turgeon et al., 1975;Gamalei, 1989Gamalei, , 1990Gamalei, , 1991Gamalei, , 2000van Bel, 1993). Gamalei has classified over 1,000 species on this basis and finds that it is a relatively consistent feature at the family level. He terms those with the highest number of minor vein plasmodesmata as type 1.The physical basis for phloem loading via the apoplast is reasonably well understood; Suc enters the cell wall space and is driven across the plasma membranes of CCs and/or SEs by cotransport with protons (Lalonde ...

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“…These diagrams enable an instant comparison of symplastic connectivity to be made between plant species. The plasmodesmograms, which are transformations of tabulated plasmodesmatal frequencies (Gunning et al 1974;Turgeon et al 1975;Evert et al 1978;Kaneko et al 1980;Fisher & Evert 1982;Chonan et al 1985;Russin & Evert 1985;Evert & Mierzwa 1986;Fisher 1986Fisher , 1990Botha & Evert 1988;McCauley & Evert 1989;Bourquin et al 1990;Warmbrodt & VanDerWoude 1990), disclose a wide variety in plasmodesmatal coupling between the various cell elements. The assortment of plasmodesmatal contacts suggests a great diversity of mechanisms for phloem loading.…”

Section: Plasmodesmograms Of the Phloem-loading Zonementioning

confidence: 99%

Section: Diagnostic Value Ofplasmodesmogramsmentioning

confidence: 99%

“…The diagnostic value of plasmodesmograms for the assessment of the phloem-loading pathway has been disputed (Fisher 1990) or relativized ). Fisher (1990) put forward that plasmodesmograms of the phloem-loading zone were inadequate to predict the intercellular assimilate fluxes. He stated that the plasmodesmograms, based on the percentage distribution of plasmodesmata per 11m2 of interfaces between various cell elements, greatly underestimated the absolute number of plasmodesmata at the mesophyll-bundle-sheath border in relation to those at other interfaces.…”

Section: Diagnostic Value Ofplasmodesmogramsmentioning

confidence: 99%

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Different phloem-loading machineries correlated with the climate

Bel

1992

Acta Botanica Neerlandica

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Distribution of Plasmodesmata in Leaves. A Comparison of Cananga Odorata with Other Species Using Different Measures of Plasmodesmatal Frequency

Cited by 16 publications

References 27 publications

Phloem loading in Ricinus cotyledons: sucrose pathways via the mesophyll and the apoplasm

Phloem loading in Ricinus cotyledons: sucrose pathways via the mesophyll and the apoplasm

Phloem Loading. A Reevaluation of the Relationship between Plasmodesmatal Frequencies and Loading Strategies

Different phloem-loading machineries correlated with the climate

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