Sieve element

The plasmodesmata may be aggregated in primary pit fields or in the pit membranes between pit pahs. The plasmodesmata appear as narrow canals (2 pm) lined by a plasma membrane and are traversed by a des-motubule, a tubule of endoplasmic reticulum. The plasmodemata are dynamic altering their dimensions and are functionally diverse. For example, whereas some transport endogenous plant transcription factors, others transport numerous proteins from companion cells to enucleated sieve elements. 

Evert RE, Derr WF. Callose substance in sieve elements. Am JBot 1964 51 552-559. 

Figure 4.1 The mustard oil bomb in flower stalks of Arabidopsis thaliana consists of S-cells (with glucosinolates) and adjacent myrosin cells (with myrosinase). This is illustrated by transverse (A,C) and longitudinal (B) sections of a pedicel, analyzed by light microscopy (A,B) and transmission electron microscopy (C). The myrosin cells (m) are in contact with the S-cells (S-c), situated inside the starch sheath ( ) (A,B,C). The myrosin cells are located peripherally in the phloem tissue other cells of the phloem include sieve elements (s) and companion cells (cc, in (C) only).

The main components of inner bark are sieve elements, parenchyma cells, and sclerenchymatous cells. Sieve elements perform the function for transportation of liquids and nutrients. More specifically and according to their shape the sieve elements are divided into sieve cells and sieve tubes. The former types are present in gym nosperms, the latter in angiosperms. The sieve elements are arranged in longitudinal cell rows which are connected through sieve areas. The sieve cells are comparatively narrow with tapering ends, whereas the sieve tubes are thicker and cylindrical. After 1 -2 years, or after a longer time in the monocotyledons, the activity of the sieve elements ceases and they are replaced by new elements. 

Parenchyma cells have the function of storing nutrients and are located between the sieve elements in the inner bark. Both vertical parenchyma cells and horizontal phloem rays are present. The latter are direct continuations of the xylem rays, but much shorter. 

In addition, a highly branched arabinan probably occurs in many barks, and especially pines. The connecting strands of the sieve elements are surrounded by a polysaccharide called callose, which is a (1 —> 3)-/3-D-glucan. 

Figure 1. Source leaf minor vein phloem. (A) Autoradiograph of leaf tissues following l C-sucrose accumulation showing radioactivity (white) in veins. (B) Tracing of an electron micrograph of a cross section of minor vein, x, xylem, vp, vascular parenchyma cc, companion cell se, sieve element pp, phloem parenchyma, me, mesophyll cell. Reproduced with permission from Ref. 6. Copyright 1983. Annual Reviews.

Rather than the solute speed in the phloem, we are sometimes more interested in how much matter is translocated. For example, if the sieve elements contain 0.5 m (500 mol m-3) sucrose moving at an average speed of 0.6 m hour-1, what is the transfer rate of sucrose in kg m-2 hour-1 By Equation 3.7 (Jj = vjcj), the flux density of sucrose is... 

In the current case, slightly more than half of the hydrostatic pressure drop along the phloem is necessary to overcome the resistance of the sieve plate pores. When the end walls are steeply inclined to the axis of the sieve element, the pores of the sieve plate can occupy an area that is greater than the cross section of the sieve tube. This causes Jv in the pores to be less than in the lumen and tends to reduce the resistance to flow in the phloem. 

Water enters and leaves the phloem by passively moving toward regions of lower water potential ( P = P - n + pwgh Eq. 2.13a). The conducting cells of the xylem generally have a low and relatively constant osmotic pressure (here 0.1 MPa). Solutes either diffuse or are actively transported into and out of the sieve elements, leading to a high phloem osmotic pressure of 1.7 MPa in the leaf and a decrease to 0.7 MPa in the root the much lower n in the sink leads to a lower P there, which favors the delivery of more solutes. 

Such a large osmotic pressure, caused by the high concentrations of sucrose and other solutes, suggests that active transport is necessary at some stage to move certain photosynthetic products from leaf mesophyll cells to the sieve elements of the phloem. From the definition of water potential, = P — II + pwgh (Eq. 2.13a), we conclude that the hydrostatic pressure in the phloem of a leaf that is 10 m above the ground is... 

Patr ick, J.W. 1997. Phloem unloading Sieve element unloading and post-sieve element transport. Annu. Rev. Plant Physiol. Plant Mol. Biol. 48 191—222. 

Bird, D.A., Franceschi, V.R. and Facchini, PJ. (2003) A tale of three cell-types alkaloid bios)mthesis is localised to sieve elements in opium poppy. Plant Cell, 15, 2626-35. 

The order Caryophyllales embraces families which have a characteristic ultrastructure of their sieve-element plastids, namely, the P-III subtype (104). In addition, the widespread occurrence of C4 photosynthesis as well as DNA-RNA hybridization data support this taxonomic treatment (23). Within this order, the occurrence of betalains is restricted to nine of the eleven families of the Caryophyllales. The two exceptions, Caryophyllaceae and Molluginaceae, produce anthocyanins instead. There is controversy regarding the phylogenetic importance of this phenomenon (105). It has been suggested that a division of the Caryophyllales into two phylogenetic lines is possible, the betalain-producing Chenopodiineae and the anthocyanin-producing Caryophyllineae (103) (see Scheme 9). The presence of betalains has been an important criterion in the classification of questionable taxa as demonstrated by various examples (13). 

Samanani N, Alcantara J, Bourgault R, Zulak KG, Facchini 70. PJ. The role of phloem sieve elements and laticifers in the biosynthesis and accumulation of alkaloids in opium poppy. Plant... 

An unusual and taxonomically useful trait found in the Myrtaceae involves the vascular system of the stem. In most dicotyledonous plants the food conducting cells of the vascular system, the sieve elements of the phloem, surround the water conducting cells, or xylem. In young stems there is usually another group of large cells that appear open in sections viewed under a light micro-... 

Sieve elements of Dioteorea (D. buUt era. D. macroura, D. reticulata, Tamus communis) Lanioe-like membrane D.G (311.(32)... 

Sieve elements of UInms americana Complex netwoikfmaze G (36)... 

Sieve elements of Phaseolus vulgaris Convoluted membranes G (37)... 

Sieve elements of Acer ( uasi-ciystalline membranes G,P (35)... 

---