Changes in the Endoplasmic Reticulum During Sieve Element Differentiation in Triticum aestivum

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Abstract
The changes in structure of the endoplasmic reticulum (ER) and its associations with other cell components have been studied in differentiating protophloem sieve elements of root tips of Triticum aestivum. In the young sieve elements single ER cisternae bearing ribosomes are dispersed in the cytoplasm. As differentiation progresses ER increases in amount while a small proportion of it aggregates into stacks or becomes associated with the nuclear envelope and the mitochondria. These modifications occur in the last two sieve elements containing ribosomes and coincide with most dramatic changes in the degenerating nucleus. Stacks consist of relatively few ER cisternae and may be encountered free in the cytoplasm or applied to the nuclear envelope. Electron-dense material accumulates between the contiguous cisternae of the stacks. ER-attached ribosomes persist even in nearly mature sieve elements, but their pattern of arrangement becomes changed. The structural evidence indicates that only a few highly degraded ER elements are retained in fully mature sieve elements.