Phloem Structure and Function

Abstract

The phloem collects photoassimilates in green leaves, distributes them in the plant and supplies the heterotrophic plant organs (e.g. fruits, buds and roots). Phloem structure is specialized for loading, long‐distance transport and unloading of assimilates. The conducting cells, called sieve elements, are highly modified to create a low‐resistance pathway composed of contiguous living cells, whose long‐term viability is maintained through an intimate association with companion (or Strasburger) cells. The difference in turgor pressure that is generated by osmotically active assimilates within this living conduit is the physical force that drives long‐distance transport through sieve elements. Plant species have evolved a variety of strategies to generate and maintain turgor pressure between source tissues, where assimilates are synthesized or released, and sink tissues, where assimilates are utilized or stored. Signalling molecules accessing the phloem are swept along with assimilates and trigger important growth processes such as flowering.

Key concepts

  • Long‐distance transport of assimilates occurs from source to sink, that is, from the green leaves, which produce a surplus of assimilates to the plant organs which consume assimilates.

  • The conducting cells are the sieve elements in the phloem which offer a low‐resistance pathway, characterized by a reduced cytoplasm and sieve pores in the connecting cell walls.

  • Reduction of the sieve‐element cytoplasm is accompanied by a dependency of sieve elements on neighbouring cells, called companion cells in angiosperms, and Strasburger cells in gymnosperms, that hold the sieve elements alive.

  • The mechanism of phloem transport is based on a high sugar concentration in the sieve element‐companion cell complexes of source leaves, which osmotically builds up a high turgor pressure.

  • Accumulation of sugars in the source phloem is called phloem loading and depends either on sucrose transporters in the plasma membrane of sieve‐element companion cell complexes, or enzyme activity in the companion cells of the source phloem.

  • Phloem transport is utilized for signal transport, including rapid electropotential waves, mobile noncoding ribonucleic acids (RNAs) and mobile transcription factors that are responsible for wound responses and developmental switches in the target tissue.

Keywords: solute transport in plants; long‐distance signalling; companion cells; sieve elements; plasmodesmata

Figure 1.

Micrographs of transverse sections through phloem tissue. (a) A collateral vascular bundle in the petiole of Plantago major has internal (iP) and external phloem (eP) on either side of the xylem (X) enclosed by several phloem fibre layers with dark appearing cell walls. (b) Sieve elements (se) are linked by a sieve plate (arrow) and intimately associated with companion cells (cc). (c) The abaxial phloem of a seventh order minor vein in a potato leaf is composed of two sieve elements, associated companion cells (cc) and two phloem parenchyma cells (pp), all surrounded by the bundle sheath. (d) A glancing section through a sieve plate in a developing cotyledon of castor bean shows large sieve pores (arrows), sieve‐element plastids (p), one mitochondrion (m) and filamentous P protein. (e) Pore–plasmodesma contacts symplasmically join a sieve element (containing tubular P protein; arrow) and its companion cell in a potato leaf. Note that the sieve element plasma membrane is covered by smooth ER cisternae (er).

Figure 2.

Micrographs of longitudinal sections through phloem tissue. (a) Four sieve elements of a sieve tube in the developing hypocotyl of castor bean Ricinus communis are each associated with several cytoplasmically dense companion cells (cc). (b) Two sieve elements of Cheiranthus cheiry containing parietal sieve‐element plastids (arrows) are connected by a sieve plate. Owing to their identical ontogenetic origin, the ends of the associated companion cells (cc) are inline with their sieve elements at the sieve plate (by courtesy of H.‐D. Behnke, Heidelberg, Germany). (c) A young sieve element–companion cell complex of castor bean shortly after the unequal division of the mother cell. The sieve element still contains a nucleus, vacuoles and all organelles. Note the thin transverse end walls that mark the site of future sieve plates. (d) Sieve areas of living stem phloem in spruce as visualized with confocal laser scanning microscopy. (e) after preparation for electron microscopy. Complexes of smooth ER are covering the sieve area on either side. The pores (small arrows) are connected via cavities (arrowheads) in the cell wall (w).

Figure 3.

Illustrations of the general mechanism of phloem transport and different models of phloem loading and unloading. (a) This simplified drawing shows the generally accepted mechanism of phloem transport where the sieve element–transfer cell complex is symplasmically isolated from the surrounding cells in source leaves. Sucrose (S) produced by photosynthesis in the mesophyll cells moves cell‐to‐cell (symplasmically) through the bundle sheath and phloem parenchyma, where it is released into the apoplast. Apoplastic phloem loading is driven by sucrose/proton cotransporters in the plasma membrane of transfer cells and energized by ATPase activity (see ‘Motor’ inset). Active loading of sucrose into the sieve element–transfer cell complex results in the high concentration of sugars required for turgor‐driven transport (see the graph inset). After loading, sucrose is passively transported down the turgor gradient in sieve elements and unloaded from the phloem symplasmically towards the assimilate‐consuming parenchyma cells in storage organs or towards growing tissue in roots or shoots. (b) Apoplastic loading by active membrane transport also occurs in plants where the conducting tissue is only partially isolated and companion cells do not have transfer cell‐type characteristics (i.e. potato source leaves; see Figure c). Symplasmic loading by ‘polymer trapping’ is suggested for plants translocating the sucrose–galactosyl polymers raffinose, stachyose or verbascose. These larger oligosaccharides are synthesized from sucrose in intermediary‐type companion cells and, due to their increased size, are trapped in the sieve element–companion cell complex. Symplasmic loading by alternative mechanisms involves sucrose moving down a concentration gradient from the mesophyll into the sieve element–companion cell complex of the phloem. This alternative mechanism of phloem loading, which appears to lack a sugar‐concentrating step in the source tissue, has been observed in willow and also suggested in gymnosperm needles. (c) In most cases sucrose unloading occurs down the concentration gradient and is symplasmic. However, where sieve element–companion cell complexes are isolated, unloading involves an apoplastic step and active reuptake (retrieval) of either sucrose or hexoses. Apoplastic unloading typically occurs across the maternal/filial interface in seeds, where assimilates leave the maternal symplasm and are actively taken up in the filial tissue.

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Further Reading

Amiard V, Mueh KE, Demmig‐Adams B et al. (2005) Anatomical and photosynthetic acclimation to the light environment in species with differing mechanisms of phloem loading. Proceedings of the National Society of Sciences of the USA 102: 12968–12973.

Behnke H‐D and Sjolund RD (eds) (1990) Sieve Elements. Comparative Structure, Induction and Development. Berlin: Springer‐Verlag.

Furch ACU, Hafke JB, Schulz A and van Bel AJE (2007) Ca2+‐mediated remote control of reversible sieve tube occlusion in Vicia faba. Journal of Experimental Botany 58: 2827–2838.

Kehr J and Buhtz A (2008) Long distance transport and movement of RNA through the phloem. Journal of Experimental Botany 59: 85–92.

Lin M‐K, Lee Y‐J, Lough TJ, Phinney BS and Lucas WJ (2008) Characterization of phloem‐sap transcription profile in melon plants. Molecular and Cellular Proteomics 8: 343–356.

Martens HJ, Roberts AG, Oparka KJ and Schulz A (2006) Quantification of plasmodesmatal ER coupling between sieve elements and companion cells using fluorescence redistribution after photobleaching (FRAP). Plant Physiology 142: 471–480.

Oparka KJ (ed) (2005) Annual Plant Reviews, Volume 18, Plasmodesmata. Oxford: Wiley‐Blackwell.

Petersen MlC, Hejgaard J, Thompson GA and Schulz A (2005) Cucurbit phloem serpins are graft‐transmissible and appear to be resistant to turnover in the sieve element‐companion cell complex. Journal of Experimental Botany 56: 3111–3120.

Turgeon R and Wolf S (2009) Phloem transport: cellular pathways and molecular trafficking. Annual Review of Plant Biology 60: 207–221.

Will T, Tjallingii WF, Thönnessen A and van Bel AJE (2007) Molecular sabotage of plant defense by aphid saliva. Proceedings of the National Society of Sciences of the USA 104: 10536–10541.

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How to Cite close
Schulz, Alexander, and Thompson, Gary A(Sep 2009) Phloem Structure and Function. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001290.pub2]