Companion cells

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. 

Companion cells Specialized parenchyma May play a role in the... 

Two complex tissues, the xylem and phloem, provide the conducting network or "circulatory system" of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5-9 pm in diameter.154 Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated "companion" cell. 

Kuhn, C., Quick, W.P., Schulz, A., Riesmeier, J.W., Sommewald, U., and Frommer, W.B., Companion cell-specific inhibition of the potato sucrose transporter SUT1, Plant Cell Environ., 19, 1115-1123, 1996. 

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).

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.

Figure 1-3. Idealized longitudinal section through part of a vascular bundle in a stem, illustrating various anatomical aspects of the xylem and the phloem. New cells forming in the xylem initially contain cytoplasm, which is lost as the cells mature and become conducting. Fiber cells, which occur in the xylem, are usually quite tapered and provide structural support. The nucleated companion cells are metabolically involved with the sieve-tube members of the phloem.

Pig. 49.—Stages in the development of sieve tubes, companion cells, and phloem parenchyma. A, o and b. Two rows of plerome cells in c and d, a has divided longitudinally and c is to become companion cells d, a sieve tube, and b, phloem parenchyma. B, c. Companion cells, and d, a beginning sieve tube from c and d, respectively in.A. The cross-walls in d are pitted b, phloem parenchyma grown larger than in A. C, The same as B with the pits in the crosswalls of the sieve tubes become perforations, and the nuclei gone from the cells composing the tube. (From Stevens.)... 

Fig. so.—Vascular elements. A, annular tracheal tube B, spiral trachea tube C, reticulated tracheal tube D, pitted tracheal tube E, cross-section through plate of seive tube, and adjoining companion cell F, length-wise section of sieve tube G, portions of two companion cells. (A, B, C, D, Robbins E, F, and G, after Strasburger.)... 

Pig. 52.—Closed collateral bundle of stem of Zea mays. VG, Bundle sheath Lf intercellular space A, ring from an annular tracheal tube SP, spiral tracheal tube My pitted vessels V, sieve tubes 5, companion cells CP, crushed primary-sieve tubes P, thin-walled i>arenchyma of the ground or fundamental tissue. From Sayre after Strashurger.)... 

Transverse section of a concentric bundle from the rhizome of Iris (a monocotyledon). Xylem surrounding the phloem, Tracheae proto-xylem s, sieve.tubes g, companion cells of the internal phloem portion. (From Sayre after Vines,)... 

Phloem cells Sieve tubes Companion cells... 

Fio. 7. Transverse section of vascular bundle in leaf of Clivia miniata cc companion cells Ic sieve tube cf, cp parenchyma around the bundle, x 270. From Molle (6). 

Fig. 8. Transverse section of vasuclar bundle in flower stalk of Narcissm rugulosus.. lkaloids precipitated in companion cells (a) and in the parenchyma around the bundle. X 100. From Errera, Maistriau and Clautriau (1).

A notable feature of the development and differentiation of the cell walls in phloem tissues is the formation of pores connecting the adjacent sieve-tubes to each other and to the companion cells. At an early stage of development, walls of the sieve tubes are marked by parts of endoplasmic reticulum on both sides where the pore is to be formed. As the sieve plate develops and the wall thickens, normal materials are deposited on the cell wall, except in the areas below the endoplasmic reticulum these areas grow, instead, by the... 

In the development of phloem, the plasma membranes ends of the cells are joined by elaborate plasmodesmata, which make contact through channels in the cell wall. Callose is deposited around these and the sieve tube is the much specialised chain of cells that results, with residual, functional cytoplasm still present, connected across the pores of the sieve plates. Connections to companion cells also remain though the lateral walls and these must serve to sustain the c5hoplasm of the sieve tubes. 

Fig. 3. Putative protein trafficking mechanism between companion cells and sieve elements. In opium poppy, BA biosynthetic enz5mes synthesized in companion cells would be transported to sieve elements through pore-plasmodesma units. Abbreviations ER, endoplasmic reticulum SER, sieve element reticulum.

The apparent biosynthesis and accumulation of sanguinarine within a single cultured opium poppy cell is in contrast to the requirement for different cell types to facilitate biosynthetic gene expression, alkaloid biosynthesis, and product accumulation in the plant 251). The movement of proteins from companion cells to sieve... 

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