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 element-companion cell (SE-CC) complexes of the extrafascicular stem phloem of Cucurbita maxima by either immersing the cut ends of internode segments in aldehyde-fixable probes, or directly by micro-injection. In both cases probes of up to M(r) 3000 (3 kDa) were able to move freely 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 impeding the longitudinal movement of low molecular mass probes such as Lucifer Yellow CH (LYCH), but was effective at preventing the movement of probes of M(r) 3000 or greater. The results show that the molecular exclusion limit of plasmodesmata can be considerably in excess of M(r) 800 and provide direct evidence for the long-held suspicion that the plasmodesmata which connect the CC with the enucleate SE have a special permeability to allow the intercellular passage of macromolecules essential for the maintenance of the SE.