Companion Cells

Abstract

Companion cells are responsible for the life maintenance of the sieve element. Based on its ontogeny, the companion cell is an intrinsic part of the phloem.

Keywords: companion cell; transfer cell; intermediary cell; phloem; plasmodesmata

Figure 1.

Companion cells. Electron micrographs of leaf minor vein companion cells in different species. (a) Tomato (Lycopersicon esculentum Mill.). Minor veins from tomato are classified as type 2a with ordinary companion cells and few plasmodesmata at the interface between sieve element–companion cell complex and surrounding cells; × 6000 (b) Onion (Allium cepa L.). Type 2b species like onion have transfer cells with cell wall ingrowths (white arrowheads) and very few plasmodesmata; × 6000. (c) Squash (Cucurbita maxima). Squash is characterized by intermediary cells showing fields of plasmodesmata (black arrowheads). Note small vacuoles and dense cytoplasm; × 8000. BS, bundle‐sheath cell; CC, companion cell; IC, intermediary cell; PP, phloem parenchyma; TC, transfer cell.

Figure 2.

Schematic representation of different transport mechanisms in the phloem in relation to the classes of companion cells. Depending on the density of plasmodesmata and the type of companion cell, respectively, assimilates are loaded into the phloem via apoplasmic routes (upper and middle panels). In plants with intermediary cells, where plasmodesmata are frequent at the bundle‐sheath cell–intermediary cell interface, assimilates are supposed to diffuse through plasmodesmata to the intermediary cell. Here, sucrose is transformed enzymatically to raffinose‐type sugars, which are potentially too big to diffuse back via plasmodesmata. fru, fructose; glu, glucose; sta, stachyose; triose‐P, triose phosphate; suc, sucrose.

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References

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Further Reading

Behnke HD (1989) Structure of the phloem. In: Baker DA and Milburn JA (eds) Transport of Photoassimilates, pp. 79–137. Harlow: Longman Scientific.

Behnke HD and Sjolund RD (1990) Sieve Elements: Comparative Structure Induction and Development. Berlin: Springer‐Verlag.

Gunning BES and Pate JS (1972) Transfer cells. Annual Review of Plant Physiology and Plant Molecular Biology 23: 173–196.

Grusak MA, Beebe DU and Turgeon R (1996) Phloem loading. In: Zamski E and Scheaffer AA (eds) Photoassimilate Distribution in Plants and Crop. Source‐Sink Relationship, pp. 209–227. New York: Marcel Dekker.

Patrick JW (1997) Phloem unloading: sieve element unloading and post‐sieve element transport. Annual Review of Plant Physiology and Plant Molecular Biology 48: 191–222.

Schulz A (1998) Phloem. Structure related to function. Progress in Botany 59: 429–475.

Van Bel A (1993) Strategies of phloem loading. Annual Review of Plant Physiology and Plant Molecular Biology 44: 253–281.

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How to Cite close
Lalonde, Sylvie, Franceschi, Vincent R, and Frommer, Wolf B(Apr 2001) Companion Cells. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0002087]