Changes in structure and dimension of plasmodesmata in the phloem loading pathway in Tecoma capensis (Bignoniaceae) - locating the polymer trap

Abstract

Tecoma capensis is one of relatively few dicotyledonous plants in which the phloem in the veins of leaves, contain specialized parenchyma cells (called intermediary cells) in addition to sieve tubes and companion cells. These plants convert sucrose to the raffinose family oligosaccharides (RFOs), which form the primary transport sugar. The fact that the interface between bundle sheath and intermediary cells in RFO-transporting species always contain high numbers of plasmodesmata (Pd) in the BS-IC interface raises questions on the mechanism driving RFO accumulation and the subsequent loading of the RFOs into the phloem. Our primary focus was thus to make a detailed study of the plasmodesmata in the phloem loading pathway in Tecoma to determine (a) plasmodesmatal frequencies at all cell-to-cell interfaces in the phloem loading pathway and (b) to undertake a comprehensive study of the ultrastructure of plasmodesmata in the loading pathway. We noted that the average plasmodesmatal total diameter decreased from 49.5 nm in the common walls between mesophyll cells, to 47.5 nm between mesophyll and bundle sheath cells, to 31.2 nm at the interface between bundle sheath and intermediary cells and was reduced further to 9.7 nm in the IC. From our observations the radius of the cytoplasmic sleeve (through which intercellular transport takes place), was condensed to approximately 1.5 nm across, and there are indications that the plasma membrane and/ or the desmotubule may be lost within these PD.