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A dual switch in phloem unloading during ovule development in Arabidopsis

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Abstract

Developing flowers are important sinks in Arabidopsis thaliana. Their energy demand is covered by assimilates which are synthesized in source leaves and transported via the vasculature. Assimilates are unloaded either symplastically through plasmodesmata or apoplastically by specific transport proteins. Here we studied the pathway of phloem unloading and post-phloem transport in developing gynoecia. Using phloem-mobile fluorescent tracers, we show that phloem unloading into cells of ovule primordia followed a symplastic pathway. Subsequently, the same tracers could not move out of phloem cells into mature ovules anymore. A further change in the mode of phloem unloading occurred after anthesis. In open flowers as well as in outgrowing siliques, the phloem was again unloaded via the symplast. This observed onset of symplastic phloem unloading was accompanied by a change in frequency of MP17-GFP-labeled plasmodesmata. We could also show that the change in cell–cell connectivity was independent of fertilization and increasing sink demand. The presented results indicate that symplastic connectivity is highly regulated and varies not only between different sink tissues but also between different developmental stages.

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Abbreviations

CC:

Companion cell

CF:

Carboxyfluorescein

CLSM:

Confocal laser scanning microscope

FM4-64:

N-(3-triethylammoniumpropyl)-4-(p-diethylaminophenyl-hexatrienyl) pyridinium dibromide

GFP:

Green fluorescent protein

HPTS:

8-Hydroxypyrene-1,3,6-trisulfonate

SE:

Sieve element

TEM:

Transmission electron microscope

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Acknowledgements

We thank Ingrid Schiessl and Gudrun Steingraeber for excellent technical assistance and Norbert Sauer for support and fruitful discussions. We also thank Magdalena Weingaertner, Kathrin Wippel, and Elisabeth Truernit for discussions and critical comments on the manuscript. We thank Uwe Sonnewald for kindly providing p35S:MP17-GFP plants. This work was supported by the Deutsche Forschungsgemeinschaft (Grants STA 382/1 to R.S.).

Conflicts of interest

The authors declare that they have no conflict of interest.

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Authors and Affiliations

  1. Lehrstuhl Molekulare Pflanzenphysiologie, Universität Erlangen-Nürnberg, Staudtstraße 5, 91058, Erlangen, Germany

    Dagmar Werner, Nadja Gerlitz & Ruth Stadler

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  1. Dagmar Werner
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  2. Nadja Gerlitz
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  3. Ruth Stadler
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Correspondence to Ruth Stadler.

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Handling Editor: Manfred Heinlein

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Fig. S1

Symplastic post-phloem movement of GFP in pSUC2:GFP transgenic gynoecia and MP17-GFP labeling of late stage 12 flowers (not yet pollinated). a Surface view of a gynoecium. GFP is visible in all cells including stigma cells. GFP fluorescence is shown in green; chlorophyll autofluorescence is shown in red. b Cross-section through a funicle. The arrow marks the SE/CC complex. c Cross-section through a funicle of p35S:MP17-GFP transgenic plants. The arrow points to a radial cell wall of the funicular epidermis, which is highly labeled by MP17-GFP. Chlorophyll fluorescence is shown in red; GFP fluorescence is shown in green. Bars are 200 μm in a and 20 μm in b and c (GIF 276 kb)

High resolution image file (TIF 25.4 mb)

Fig. S1

Phloem unloading of carboxyfluorescein and pollen tube staining in a WT gynoecium. Chlorophyll fluorescence is shown in red; GFP fluorescence is shown in green. a Mature ovule of a stage 12 flower. CF is restricted to the SE/CC complex (arrow). b Fertilized ovule of a late stage 13 flower. CF is visible in the phloem unloading domain. The arrow points to the phloem unloading domain. c Gynoecium of a late stage 13 flower. Pollen tubes had been stained with aniline blue; aniline blue fluorescence is shown in white. Bars are 75 μm in a–d and 200 μm in c (GIF 62.2 KB)

High resolution image file (TIF 6.90 mb)

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Werner, D., Gerlitz, N. & Stadler, R. A dual switch in phloem unloading during ovule development in Arabidopsis . Protoplasma 248, 225–235 (2011). https://doi.org/10.1007/s00709-010-0223-8

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