Abstract
A three-step screening method was developed to identify the mode of phloem loading in intact leaves. Phloem loading of 14CO2-derived photosynthate was challenged by p-chloromercuribenzenesulfonic acid (PCMBS) in leaves of dicotyledons with either a symplasmic (type 1, with intermediary cells as companion cells) or apoplasmic (type 2b, with transfer cells as companion cells) minor-vein configuration. Firstly, photosynthate export as the result of phloem loading was measured by collection of phloem exudate from the petiole. The PCMBS had virtually no effect on photosynthate export in representatives of type-1 families (Lamiaceae, Lythraceae, Onagraceae, Saxifragaceae). In contrast, photosynthate export was strongly reduced by PCMBS in representatives of type-2b families (Asteraceae, Balsaminaceae, Dipsacaceae, Linaceae, Tropaeolaceae, Valerianaceae) and type-2b members of polytypical families (Fabaceae, Scrophulariaceae). Secondly, densitometric measurements of leaf autoradiographs demonstrated that the contrast between the mesophyll and the lower-order veins was hardly affected by PCMBS treatment in type-1 species, whereas PCMBS strongly reduced the contrast in type-2b species. Thirdly, separate 14C-radioassays of vein and mesophyll tissues confirmed this observation. The three-step procedure thus revealed a strong and consistent reduction of phloem loading by PCMBS in type-2b species which was absent in type-1 species. In conclusion, phloem loading in type-2b species occurs via the apoplast and type-1 species execute an alternative — most likely symplasmic — mode of phloem loading.
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Abbreviations
- PCMBS:
-
p-chloromercuribenzenesulfonic acid
- SE/CC-complex:
-
sieve element/companion cell complex
References
Aloni, B., Wyse, R.E., Griffith, S. (1986) Sucrose transport and phloem unloading in the stem of Vicia faba: possible involvement of a sucrose carrier and osmotic regulation. Plant Physiol. 81, 482–486
Bourquin, S., Bonnemain, J.L., Delrot, S. (1990) Inhibition of loading of 14C assimilates by p-chloromercuribenzenesulfonic acid. Localization of the apoplastic pathway in Vicia faba. Plant Physiol. 92, 97–102
Delrot, S. (1987) Phloem loading: apoplastic or symplastic? Plant Physiol. Biochem. 25, 667–676
Delrot, S., Despeghel, J.P., Bonnemain, J.L. (1980) Phloem loading in Vicia faba leaves: effect of N-ethylmaleimide and parachloromercuribenzenesulfonic acid on H+ extrusion, K+ and sucrose uptake. Planta 149, 144–148
Farrar, J., Van der Schoot, C., Drent, P., Van Bel, A. (1992) Symplastic transport of Lucifer Yellow in mature leaf blades of barley: potential mesophyll-to-sieve tube transfer. New Phytol. 120, 191–196
Gamalei, Y.V. (1989) Structure and function of leaf minor veins in trees and herbs. Trees 3, 96–110
Gamalei, Y.V. (1991) Phloem loading and its development related to plant evolution from trees to herbs. Trees 5, 50–63
Giaquinta, R.T. (1976) Evidence for phloem loading from the apoplast. Chemical modification of membrane sulfhydryl groups. Plant Physiol. 57, 872–875
Giaquinta, R.T. (1979) Phloem loading of sucrose. Involvement of membrane ATPase and proton transport. Plant Physiol. 63, 744–748
Giaquinta, R.T. (1983) Phloem loading of sucrose. Annu. Rev. Plant Physiol. 34, 347–387
Girousse, C., Bonnemain, J.L., Delrot, S., Bournoville, R. (1991) Sugar and amino acid composition of phloem sap of Medicago sativa: a comparative study of two collecting methods. Plant Physiol. Biochem. 29, 41–48
Grimm, E., Bernhardt, G., Rothe, K., Jacob, F. (1990) Mechanism of sucrose retrieval along the phloem path —a kinetic approach. Planta 182, 480–485
Hayes, P.M., Patrick, J.W., Offler, C.E. (1987) The cellular pathway of radial transfer of photosynthates in stems of Phaseolus vulgaris L.: effects of cellular plasmolysis and p-chloromercuribenzenesulphonic acid. Ann. Bot. 59, 635–642
Jahnke, S., Bier, D., Estruch, J.J., Beitran, J.P. (1989) Distribution of photoassimilates in the pea plant: chronology of events in nonfertilized ovaries and effects of gibberellic acid. Planta 180, 53–60
Lichtner, F.T., Spanswick, R.M. (1981) Sucrose uptake by developing soybean cotyledons. Plant Physiol. 68, 693–698
Madore, M.A., Lucas, W.J. (1987) Control of photoassimilate movement in source-leaf tissues of Ipomoea tricolor Cav. Planta 171, 197–204
Maynard, J.W., Lucas, W.J. (1982) Sucrose and glucose uptake into Beta vulgaris leaf tissues. A case from general (apoplastic) retrieval systems. Plant Physiol. 70, 1436–1443
M'Batchi, B., Delrot, S. (1984) Parachloromercuribenzenesulfonic acid. A potential tool for differential labeling of the sucrose transporter. Plant Physiol. 75, 154–160
M'Batchi, B., El Ayadi, R., Delrot, S., Bonnemain, J.L. (1986) Direct versus indirect effects of p-chloromercuribenzenesulphonic acid on sucrose uptake by plant tissues: the electrophysiological evidence. Physiol. Plant. 68, 391–395
Minchin, P.E.H., Thorpe, M.R. (1987) Measurement of unloading and reloading of photo-assimilate within the stem of bean. J. Exp. Bot. 38, 211–220
Servaites, J.C., Schrader, L.E., Jung, D.M. (1979) Energy-dependent loading of amino acids and sucrose into the phloem of soybean. Plant Physiol. 64, 546–550
Thompson, R.G., Dale, J.E. (1981) Export of 14C-and 11C-labelled assimilate from wheat and maize leaves: effects of parachloromercuribenzylsulphonic acid and fusicoccin and of potassium deficiency. Can J. Bot. 59, 2439–2444
Turgeon, R. (1991) Symplastic phloem loading and the sink-source transition in leaves: a model. In: Recent advances in phloem transport and assimilate compartmentation, pp. 18–22, Bonnemain, J.L., Delrot, S., Lucas, W.J., Dainty, J., eds. Ouest Editions Presses Académiques, Nantes, France
Turgeon, R., Beebe, D.U. (1991) The evidence for symplastic phloem loading. Plant Physiol. 96, 348–354
Turgeon, R., Gowan, E. (1990) Phloem loading in Coleus blumei in the absence of carrier-mediated uptake of export sugar from the apoplast. Plant Physiol. 94, 1244–1249
Turgeon, R., Hepler, P.K. (1989) Symplastic continuity between mesophyll and companion cells in minor veins of mature Cucurbita pepo leaves. Planta 179, 24–31
van Bel, A.J.E. (1987) The apoplast concept of phloem loading has no universal validity. Plant Physiol. Biochem. 25, 677–686
van Bel, A.J.E. (1992a) Different phloem-loading machineries correlated with the climate. Acta Bot. Neerl. 41, 121–141
van Bel, A.J.E. (1992b) Pathways and mechanisms of phloem loading. In: Carbon partitioning within and between organisms, pp 53–70, Farrar, J.F., Gordon, A.J., Pollock, C.J., eds. BIOS, Oxford
van Bel, A.J.E. (1992c) Mechanisms of sugar translocation. In: Crop photosynthesis. Spatial and temporal determinants, pp. 177–211, Baker, N.R., Thomas, H., eds. Elsevier, Amsterdam, The Netherlands
van Bel, A.J.E., Gamalei, Y.V. (1991) Multiprogrammed phloem loading. In: Recent advances in phloem transport and assimilate compartmentation, pp. 128–139, Bonnemain, J.L., Delrot, S., Lucas, W.J., Dainty, J., eds. Ouest Editions, Nantes, France
van Bel, A.J.E., Gamalei, Y.V. (1992) Ecophysiology of phloem loading in source leaves. Plant Cell Environ. 15, 265–270
van Bel, A.J.E., Ammerlaan, A., Blaauw-Jansen, G. (1986) Preferential accumulation by mesophyll cells at low and by veins at high amino acid and sugar concentrations in Commelina benghalensis L. leaves. J. Exp. Bot. 37, 1899–1910
van Bel, A.J.E., Gamalei, Y.V, Ammerlaan, A., Bik, L.P.M. (1992) Dissmilar phloem loading in leaves with symplasmic or apoplasmic minor-vein configurations. Planta 186, 518–525
van Kesteren, W.J.P., Van der Schoot, C., van Bel, A.J.E. (1988) Symplastic transfer of fluorescent dyes from mesophyll to sieve lubes in stripped leaf tissue and partly isolated minor veins of Commelina benghalensis. Plant Physiol. 88, 667–670
Weibull, J., Ronquist, F., Brishammer, S. (1990) Free amino acid composition of leaf exudales and phloem sap. A comparative study in oats and barley. Plant Physiol. 92, 222–226
Wimmers, L.E., Turgeon, R. (1991) Transfer cells and solute uptake in minor veins of Pisum sativum leaves. Planta. 186, 2–12
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We gratefully acknowledge the expert help of Dr. Maarten Terlou, Department of Image Processing and Design, University of Utrecht, in carrying out the densitometric measurements.
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van Bel, A.J.E., Ammerlaan, A. & van Dijk, A.A. A three-step screening procedure to identify the mode of phloem loading in intact leaves. Planta 192, 31–39 (1993). https://doi.org/10.1007/BF00198689
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DOI: https://doi.org/10.1007/BF00198689