Occurrence of plasmodesmata between differentiating vessels and other xylem cells in Sorbus torminalis L Crantz and their fate during xylem maturation

The development of pit-pairs between differentiating xylem cells has been examined by transmission electron microscopy in young shoots of Sorbus torminalis. In some vessel-to-tracheid pits, as well as in previously studied intertracheid pits, a thickening of the pit membrane containing branched plasmodesmata was observed. A secondary wall-like cap was deposited over the thickening prior to cytoplasmic autolysis; some plasmodesmata, parallel to the plane of section, appeared to perforate the cap. At the end of the cell maturation stage, the central part of the primary wall thickening was hydrolysed, while the cap, including plasmodesmata remnants, appeared unaltered. In half-bordered pit-pairs between a parenchyma cell and a vessel or a tracheid, similar structures could be observed beside the conducting elements. When the vessel or tracheid matured, sealing of the pit membrane plasmodesmata resulted from the formation of a protective layer on the parenchyma-side rather than from the deposition of a cap on the conducting cell-side. These observations provide the first information on the presence of symplasmic connections in pits between differentiating vessels and neighbouring xylem cells. In S. torminalis, xylem differentiation is probably highly coordinated within a symplasmic domain; the persistence of such connections may account for the lack of specialization of Sor bits wood.