Abstract
Phloem tissue in flowering plants has a distinctive cellular composition in which the conducting elements are a complex of two unique cell types, sieve elements (SE) and companion cells (CC), that are developmentally related and functionally dependent upon one another. The sieve element-companion cell complex (SE-CC) of angiosperms originates from cambial derivatives as the result of an unequal cell division forming the undifferentiated SE and CC (reviewed in Schulz 1998). The differentiating SE undergoes selective autophagy of organelles resulting in apparent loss of the cell’s ability for transcription and translation. Mature SEs are characteristically enucleate and lack ribosomes. Reorganization of the endomembrane system results from degeneration of the tonoplast and dictyosomes and changes in the endoplasmic reticulum (ER) that are distinct enough, according to some authors, to merit the special term sieve element reticulum or SER (Sjolund and Shih 1983). SER, plastids, and mitochondria are retained in a parietal position in close contact with the plasma membrane of the mature SE (Evert 1990).
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