Differentiation of abnormal sieve elements in roots of wheat (Triticum aestivum L.) affected by colchicine.

Protophloem sieve elements (PSEs) of seminal roots of wheat (Triticum aestivum L.) treated with 2 MM colchicine solution differentiate into an extraordinary cell type not found in normal roots. Colchicine-treated PSEs stop elongating and increase in diameter considerably. Abnormal ultrastructural features appear gradually with increasing time of exposure to the drug. Microtubules disorganize and disappear first. Cell divisions are blocked and nuclei become polyploid, with multiple lobes and deep imaginations. Profuse paracrystalline material, presumably consisting of tubulin or, more likely, of tubulin-colchicine polymers, accumulates in the cytoplasm of the polyploid cells. Endoplasmic reticulum (ER) cisternae do not aggregate in stacks and mitochondria are not enveloped by ER cisternae, as normal. Golgi bodies appear active and distinct, while unusual vesicles with dense contents occur in the cytoplasm. However, development of plastids and degeneration of nuclei do not seem to be significantly affected by colchicine. The rate of development of affected PSEs is retarded considerably, as judged from ultrastructural features (plastid inclusions, wall thickenings), which appear much later in the vertical files of cells. Due to the retardation and to the overall stoppage of Toot elongation, differentiation of PSEs is caught up by other vascular elements such as protoxylem elements and metaphloem sieve elements. The aberrant PSEs undergo autolysis and reach maturity, but plastids, mitochondria, nuclear remnants and ER configurations characterizing normal PSEs are retained. Transverse walls differentiate into abnormal sieve plates, in which sieve pores are not formed. Lateral walls develop unusual thickenings projecting in the cytoplasm. The observations show that: (a) Differentiation of the affected PSEs proceeds in the absence of divisions; (b) the number and degree of aberrations depend largely on the timing of exposure to the drug, and (c) strands of paracrystalline material and vacuoles are formed in microtubule-free PSEs after long and continuous colchicine treatment.