Plant Physiology Preview
Published on April 22, 2005; 10.1104/pp.104.058800
Received December 22, 2004
Returned for revision February 11, 2005
Accepted February 11, 2005
Nematode Infection Triggers the de Novo Formation of Unloading Phloem That Allows Macromolecular Trafficking of Green Fluorescent Protein into Syncytia
Stefan Hoth , Alexander Schneidereit , Christian Lauterbach , Joachim Scholz-Starke , and Norbert Sauer *
Molekulare Pflanzenphysiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, D-91058 Erlangen, Germany
* Corresponding author; email: nsauer{at}biologie.uni-erlangen.de.
Syncytial feeding complexes induced by the cyst nematode Heterodera schachtii represent strong metabolic sinks for photoassimilates. These newly formed structures were described to be symplastically isolated from the surrounding root tissue and their mechanism of carbohydrate import has repeatedly been under investigation. Here, we present analyses of the symplastic connectivity between the root phloem and these syncytia in nematode-infected Arabidopsis (Arabidopsis thaliana) plants expressing the gene of the green fluorescent protein (GFP) or of different GFP fusions under the control of the companion cell (CC)-specific AtSUC2 promoter. In the same plants, phloem differentiation during syncytium formation was monitored using cell-specific antibodies for CCs or sieve elements (SEs). Our results demonstrate that free, CC-derived GFP moved freely from the phloem into the syncytial domain. No or only marginal cell-to-cell passage of GFP was observed into other root cells adjacent to these syncytia. In contrast, membrane-anchored GFP variants as well as soluble GFP fusions with increased molecular masses were restricted to the SE-CC complex. The presented data also show that nematode infection triggers the de novo formation of phloem containing an approximately 3-fold excess of SEs over CCs. This newly formed phloem exhibits typical properties of unloading phloem previously described in other sink tissues. Our results reveal the existence of a symplastic pathway between phloem CCs and nematode-induced syncytia. The plasmodesmata responsible for this symplastic connectivity allow the cell-to-cell movement of macromolecules up to 30 kD and are likely to represent the major or exclusive path for the supply of assimilates from the phloem into the syncytial complex.
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