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Phloem sieve elements in small nodules of callus tissue have open sieve pores and scattered P-protein when the tissue is fixed intact. Intentionally slicing the nodules prior to fixation results in the formation of wound responses, including an alteration in P-protein distribution and the formation of plugged sieve pores. Freeze-fracture replicas show well-preserved P-protein as randomly oriented filaments in intact sieve elements. Wounded sieve elements, however, demonstrate dense bundles of oriented P-protein filaments in freeze-fracture replicas. Open pores with few P-protein filaments are seen in replicas of undamaged sieve elements. These callus nodules can be used as a model system to investigate phloem structure, function, and wounding.

A 19S particle was purified from tobacco (Nicotiana rustica) leaf cells. Its density was determined as 1296 g/cm3 in Cs2SO4-DMSO gradients, indicating the presence of RNA and protein. Polyacrylamide gel electrophoresis (PAGE) revealed eight distinct proteins in the range of 20-30 kD and RNA in the range of 70-80 nucleotides. Electron microscopic examination showed the same raspberry-shaped structure with a central depression as described for prosomes. We conclude that tobacco 19S particles represent small cytoplasmic complexes, possessing biochemical and structural characteristics similar to the hither-to known prosomes of animal cells.

The measurement of chromomycin A3/DAPI fluorescence ratios is shown to allow base content determinations in eu- and heterochromatic regions of interphase cell nuclei. The base content values obtained in chromocenters and euchromatin of Scilla sibirica agree with those measured earlier [12] on the band and non-band areas of the chromosomes of this species. In Sinapis alba three different heterochromatin types, with regard to base content, can be discerned. The heterochromatin amplification observed in the polyploid nuclei of Sinapis roots and hypocotyl involves either all three heterochromatin types, with little resulting change of the total nuclear base content, or only one or the other of them, with a measurable shift of the base content.