PLASMODESMATA BETWEEN SIEVE ELEMENT AND COMPANION CELL OF EXTRAFASCICULAR STEM PHLOEM OF CUCURBITA-MAXIMA PERMIT PASSAGE OF 3-KDA FLUORESCENT-PROBES

Fluorescent probes of differing M(r) were introduced into the sieve el ement-companion cell (SE-CC) complexes of the extrafascicular stem phl oem of Cucurbita maxima by either immersing the cut ends of internode segments in aldehyde-fixable probes, or directly by micro-injection. I n both cases probes of up to M(r) 3000 (3 kDa) were able to move freel y along the extrafascicular SE and, more significantly, between SE and CC. No movement occurred out of the SE-CC complex. In the case of the fascicular phloem, occlusion of the sieve plates was ineffectual at i mpeding the longitudinal movement of low molecular mass probes such as Lucifer Yellow CH (LYCH), but was effective at preventing the movemen t of probes of M(r) 3000 or greater. The results show that the molecul ar exclusion limit of plasmodesmata can be considerably in excess of M (r) 800 and provide direct evidence for the long-held suspicion that t he plasmodesmata which connect the CC with the enucleate SE have a spe cial permeability to allow the intercellular passage of macromolecules essential for the maintenance of the SE.