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.