Summary
Protophloem sieve-element differentiation in roots of the grassAegilops comosa var.thessalica has been studied by light and electron microscopy. Sieve elements are arranged in vertical files and mature close to the root apex (about 450 μm).
Nuclei have a nearly normal appearance until the last immature cell. In this cell, chromatin aggregates and becomes amorphous. Furthermore, nuclei are segmented in many parts with various appearance; the most condensed segments are retained in mature cells and treatment with proteolytic enzymes show that they are composed mostly of proteins. The ER does not aggregate into stacks, but some cisternae of it are appressed to the nuclear envelope. Smooth membranes of ER, plastids and mitochondria are retained at maturity. On the other hand, ribosomes and Golgi apparatus disappear completely. Vacuoles were not observed at any stage of development. Sieve-element cell walls develop wave-like thickenings, which in mature cells gradually become evenly thickened. These thickenings may serve as stores of wall material. Transformation of plasmodesmata canals into sieve plate pores is realized by removal of both callose and ER cisternae from the pore sites. The complete opening of pores occurs in the first mature sieve elements.
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Eleftheriou, E.P., Tsekos, I. Development of protophloem in roots ofAegilops comosa var.thessalica. II. Sieve-element differentiation. Protoplasma 113, 221–233 (1982). https://doi.org/10.1007/BF01280911
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DOI: https://doi.org/10.1007/BF01280911