Freeze-fracture analysis of phloem structure in plant tissue cultures. III. P-Protein, sieve area pores, and wounding.

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.