Tubular and Fibrillar Components of Mature and Differentiating Sieve Elements

Abstract

An ontogenetic study of the sieve element protoplast of Nicotiana tabacum L. by light and electron microscopy has shown that the P-protein component (slime) arises as small groups of tubules in the cytoplasm. These subsequently enlarge to form comparatively large compact masses of 231 2.5 (SE)A (n = 121) tubules, the P-protein bodies. During subsequent differentiation of the sieve element, the P-protein body disaggregates and the tubules become dispersed throughout the cell. This disaggregation occurs at about the same stage of differentiation of the sieve elements as the breakdown of the tonoplast and nucleus. Later, the tubules of P-protein are reorganized into smaller striated 149 i 4.5 (SE)A (n = 43) fibrils which are characteristic of the mature sieve elements. The tubular P-protein component has been designated Pl-protein and the striated fibrillar component P2-protein.In fixed material, the sieve-plate pores of mature sieve elements are filled with proteinaceous material which frays out into the cytoplasm as striated fibrils of P2-protein. Our observations are compatible with the view that the contents of contiguous mature sieve elements, including the P-protein, are continuous through the sieve-plate pores and that fixing solutions denature the proteins in the pores. They are converted into the electronopaque material filling the pores.

Sieve element occlusion(SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

“…2). In immature tobacco sieve elements, P-protein maturation is characterized by the formation of large protein bodies (5,10), and similar fluorescent protein complexes were identified in the transgenic lines. The most striking demonstration of P-protein-like activity was the behavior of the NtSEO fusions after wounding: The fluorescent complexes in wounded transgenic plants formed distinctive plugs on most sieve plates, and these plugs sometimes even terminated with gel-like transcellular strands, as is consistent with the known characteristics of P-proteins (47,48).…”

“…1 and Fig. S2) mirrored the rapid assembly of conventional P-protein bodies in immature phloem cells (5). Furthermore, the expression of NtSEO genes as hrGFP fusions in a homologous genetic background resulted in the assembly of fluorescent proteinaceous agglomerates that reflected the characteristic morphology of structural P-proteins (Fig.…”

“…Sieve elements are connected to each other through converted plasmodesmata, forming sieve pores that facilitate translocation (4). The synthesis of P-proteins begins in immature, nucleated sieve elements (5), resulting in electron-dense proteinaceous structures. Their ultrastructural characteristics have been described variously as granular, fibrillar, or tubular, even within the same cell (6)(7)(8)(9)(10)(11)(12)(13)(14), and today these structures are thought to represent the various different stages of P-protein differentiation (3).…”

Ultrastructural Observations on Pinaceae Leaf Phloem I. The Spring Condition

“…Srivastava and O'Brien (1966b) reported small fibrils in sieve cells of secondary phloem in the trunks of pine and suggested that they may have had a plastid origin. The fibrils found in the sieve cells of pine leaves are much smaller (5-7 nm) than the 23.1-H-9 nm diameters reported for P-protein in some angiosperms (Cronshaw and Esau, 1967). However, in Coleus the P-protein tubules (20 nm) are converted to 7 nm fibrils (Steer and Newcomb, 1969).…”

La circulation dans le liber

“…Chez le tabac, la future cellule criblée possède un seul et volumineux corps muqueux. Ce dernier est constitué de tubules protéiques dont le diamètre est d'environ 230 Â : il s'agit de protéine-Pl (Cronshaw et Esau, 1967Esau, et 1968.…”