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 ce lls (ICs) in the minor-vein phloem of melon (Cucumis melo L.) and squa sh (Cucurbita pepo L.). These pd were found to be secondary, i.e., the y form across existing walls. Sink, sink-source transition, and source tissues were sampled from developing and mature leaves. In sink tissu e, IC precursors divide to produce the two to four ICs and associated sieve elements which are present by the time of the sink-source transi tion. Plasmodesmata along the interface between the IC precursor and a djacent BSCs in sink tissue are unbranched and few in number. Before t he leaf tissue undergoes the sink-source transition, the number of pd channels (individual branches of pd) becomes more numerous. This incre ase in number of pd channels occurs at least in part and perhaps entir ely by branching, resulting in more channels on the IC-side than on th e BSC-side. In melon there is a 12-fold increase in the number of pd c hannels 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 phloe m loading and photoassimilate export. The system described is well-def ined and amenable to experimental manipulation: secondary pd form in l arge numbers, at a particular interface, over a short period of time, and in a highly predictable manner.