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

Abstract

SUMMARYProtophloem 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. Coichicine-treated PSEs stop elongating and increase in diameter considerably. Ahnormal uitrastructura! 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 invaginations. Profuse paracrystalline material, presumably consisting of tubulin or, more likely, of tubulin-colchicine polymers, accumulates in the cytoplasm of the polyploid cells. Endopiasmic reticulum (ER) cisternae do not aggregate in stacks and mitochondria are not enveloped by ER cisternae, as normal. Golgi bodies appear active and distmct, 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 ceils. Due to the retardation and to the overall stoppage of root 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.

Abnormal structure of protophloem sieve-element cell wall in colchicine-treated roots of Triticum aestivum L.

“…A major effect of colchicine at the concentration used might be the fact that cell expansion stops (Eleftheriou 1994). And it is in non-expanding cells or cell parts that microtubules and microfibrils have been found to be not parallel to each other (Emons et al 1992) and microtubules to be shifted from transverse to longitudinal orientations (Baluska et al 1992).…”

“…Seedlings, 54h old, were transferred to 2 m M (= 0.08%) colchicine solution for 4, 8,12,16,22,30,42,48,72 and 96 h, while untreated seedlings were used as controls (for details, see Eleftheriou 1994). After treatment, root tips were excised, fixed and processed for electron microscopy as was previously described in detail (Eleftheriou 1986(Eleftheriou , 1994.…”

“…After treatment, root tips were excised, fixed and processed for electron microscopy as was previously described in detail (Eleftheriou 1986(Eleftheriou , 1994. Briefly, samples were fixed in a mixture of 3% glutaraldehyde and 3% paraformaldehyde in 0.05 M cacodylate buffer (pH 7.0-7.2) at room temperature for 3 h, postfixed in 1% osmium tetroxide similarly buffered, dehydrated in a graded ethanol series and embedded in Spurr's epoxy resin.…”

“…A simple numbering system has been used to define the position of each element within the series (Eleftheriou 1986(Eleftheriou , 1987(Eleftheriou , 1994. Numbering starts from a readily recognizable sieve plate that separates the last sieve element bearing ribosomes and the first one deficient in stainable material, and proceeds both downwards (for differentiating PSEs) and upwards (for maturing ones).…”

“…They are rather the result of production of large amounts of wall material in cells which do not elongate due to disturbance by colchicine (cf. Eleftheriou 1994). This material is deposited in another, less regular pattern, namely the abnormal wall ingrowths.…”