The phloem pathway: new issues and old debates

Phloem loading, plant growth form, and climate

Plasmodesmatal frequencies in the phloem of leaf minor veins vary considerably, suggesting that photoassimilate is loaded into the phloem by different strategies. The ecophysiological basis for multiple loading types is unknown. We updated the analysis of van Bel and Gamalei (Plant Cell Environ 15: 265-270, 1992) with more current phylogenetic data and by treating separately two symplastic loading types, those that load actively by polymer trapping (synthesis of raffinose family oligosaccharides--RFOs), and those that load passively, by diffusion. The results indicate a stronger association between passive, symplastic loading and the tree growth form than previously recognized. Apoplastic loading is highly correlated with the herbaceous habit. There is no correlation between RFO families and growth form. At the family level, there are no correlations between minor vein types and climate that cannot be explained by the dearth of woody plants in the arctic for reasons unassociated with phloem loading. However, at the species level, a floristic analysis uncovered a correlation between the RFO trait and species frequency in tropical and subtropical regions of the world. The correlations between loading types and both growth form and climate are subtle, probably indirect, and poorly understood.

Amborella trichopoda, plasmodesmata, and the evolution of phloem loading

Phloem loading is the process by which photoassimilates synthesized in the mesophyll cells of leaves enter the sieve elements and companion cells of minor veins in preparation for long distance transport to sink organs. Three loading strategies have been described: active loading from the apoplast, passive loading via the symplast, and passive symplastic transfer followed by polymer trapping of raffinose and stachyose. We studied phloem loading in Amborella trichopoda, a premontane shrub that may be sister to all other flowering plants. The minor veins of A. trichopoda contain intermediary cells, indicative of the polymer trap mechanism, forming an arc on the abaxial side and subtending a cluster of ordinary companion cells in the interior of the veins. Intermediary cells are linked to bundle sheath cells by highly abundant plasmodesmata whereas ordinary companion cells have few plasmodesmata, characteristic of phloem that loads from the apoplast. Intermediary cells, ordinary companion cells, and sieve elements form symplastically connected complexes. Leaves provided with (14)CO(2) translocate radiolabeled sucrose, raffinose, and stachyose. Therefore, structural and physiological evidence suggests that both apoplastic and polymer trapping mechanisms of phloem loading operate in A. trichopoda. The evolution of phloem loading strategies is complex and may be difficult to resolve.

Binding properties of the N-acetylglucosamine and high-mannose N-glycan PP2-A1 phloem lectin in Arabidopsis

Phloem Protein2 (PP2) is a component of the phloem protein bodies found in sieve elements. We describe here the lectin properties of the Arabidopsis (Arabidopsis thaliana) PP2-A1. Using a recombinant protein produced in Escherichia coli, we demonstrated binding to N-acetylglucosamine oligomers. Glycan array screening showed that PP2-A1 also bound to high-mannose N-glycans and 9-acyl-N-acetylneuraminic sialic acid. Fluorescence spectroscopy-based titration experiments revealed that PP2-A1 had two classes of binding site for N,N',N''-triacetylchitotriose, a low-affinity site and a high-affinity site, promoting the formation of protein dimers. A search for structural similarities revealed that PP2-A1 aligned with the Cbm4 and Cbm22-2 carbohydrate-binding modules, leading to the prediction of a beta-strand structure for its conserved domain. We investigated whether PP2-A1 interacted with phloem sap glycoproteins by first characterizing abundant Arabidopsis phloem sap proteins by liquid chromatography-tandem mass spectrometry. Then we demonstrated that PP2-A1 bound to several phloem sap proteins and that this binding was not completely abolished by glycosidase treatment. As many plant lectins have insecticidal activity, we also assessed the effect of PP2-A1 on weight gain and survival in aphids. Unlike other mannose-binding lectins, when added to an artificial diet, recombinant PP2-A1 had no insecticidal properties against Acyrthosiphon pisum and Myzus persicae. However, at mid-range concentrations, the protein affected weight gain in insect nymphs. These results indicate the presence in PP2-A1 of several carbohydrate-binding sites, with potentially different functions in the trafficking of endogenous proteins or in interactions with phloem-feeding insects.