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A three-step screening procedure to identify the mode of phloem loading in intact leaves

Evidence for symplasmic and apoplasmic phloem loading associated with the type of companion cell

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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

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Authors and Affiliations

  1. Transport Physiology Research Group, Department of Plant Ecology and Evolutionary Biology, University of Utrecht, Sorbonnelaan 16, 3584 CA, Utrecht, The Netherlands

    Aart J. E. van Bel, Ankie Ammerlaan & Anthonie A. van Dijk

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  1. Aart J. E. van Bel
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  2. Ankie Ammerlaan
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  3. Anthonie A. van Dijk
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Additional information

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

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