Secondary plasmodesmata formation in the minor-vein phloem of Cucumis melo L. and Cucurbita pepo L.

Numerous branched plasmodesmata (pd) are present between bundle-sheath cells (BSCs) and specialized companion cells known as intermediary cells (ICs) in the minor-vein phloem of melon (Cucumis melo L.) and squash (Cucurbita pepo L.). These pd were found to be secondary, i.e., they form across existing walls. Sink, sink-source transition, and source tissues were sampled from developing and mature leaves. In sink tissue, IC precursors divide to produce the two to four ICs and associated sieve elements which are present by the time of the sink-source transition. Plasmodesmata along the interface between the IC precursor and adjacent BSCs in sink tissue are unbranched and few in number. Before the leaf tissue undergoes the sink-source transition, the number of pd channels (individual branches of pd) becomes more numerous. This increase in number of pd channels occurs at least in part and perhaps entirely by branching, resulting in more channels on the IC-side than on the BSC-side. In melon there is a 12-fold increase in the number of pd channels within the IC-side of the interface and a corresponding 9-fold increase in pd channels within the BSC-side. Thus, secondary pd form by the time of the sink-source transition and may be involved in phloem loading and photoassimilate export. The system described is well-defined and amenable to experimental manipulation: secondary pd form in large numbers, at a particular interface, over a short period of time, and in a highly predictable manner.Abbreviations BSC bundle-sheath cell - DAP days after planting - IC intermediary cell - LPI leaf plastochron index - pd plasmodesmata - PI plastochron interval We thank Edith Haritatos, Rich Medville, Esther Gowan, and Nancy Dussault for expert technical assistance. This research was supported by an NSF/DOE/USDA Cornell Plant Science Center fellowship (G.M.V.), Natural Sciences and Engineering Research Council Grant GP0138401 and Université de Montréal, Fonds internes de recherche (D.U.B.), and NSF grant IBN-9419703 (R.T.).