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PHLOEM LOADING

Low sugar, under pressure?

Nature Plants volume 8, pages 102–103 (2022)Cite this article

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The difference in phloem pressure is the driving force that moves photosynthetic products and other solutes from the source to the sink. Measurements with emerging technologies reveal that sugar loading is not essential for maintaining phloem pressure and phloem bulk flow in the maize sugar-loading-defective mutant sut1.

Long-distance phloem transport in plants is proposed to be driven by osmotically generated hydrostatic pressure. This widely accepted pressure-flow model was introduced by Ernst Münch1 in 1930. Recent evidence collected from Ipomoea nil demonstrates that conductance of sieve tubes (STs), an arrayed subset of transport-specialized living cells in the vein that transport organic and inorganic nutrients, is sufficient for pressure bulk flow, as the conductivity and turgor pressure increase in proportion to the distance between the source (for example, mature leaves) and the sink (for example, roots, flowers and seeds)2, supporting the Münch hypothesis. In source tissues, photoassimilates and other solutes enter the STs of the minor vein and accumulate to high concentrations through the process of phloem loading. Then water enters through osmosis, generating high pressure at the source. In sink tissues, photoassimilates and other solutes exit the STs through plasmodesmata that connect to companion cells to enter expanding or storage cells of the sink, through the process of phloem unloading3. As a consequence, the hydrostatic pressure at the end of STs in the sinks is reduced. The hydrostatic pressure differences between the source and sink ends of STs drive solute bulk flow through the phloem. Maintenance of this hydrostatic pressure difference requires coordination of phloem loading and unloading, especially of the major osmotic compounds3. The abundance of sugars present in ST sap accounts for up to 80% of the sap osmotic content3. Sugars enter sieve element/companion cell (SE/CC) complexes through the process of sugar loading, which was commonly thought to be required for the maintenance of hydrostatic pressure. However, in this issue of Nature Plants, Babst et al.4 demonstrate in maize that sugar loading is not essential to maintain the hydrostatic pressure gradient for bulk flow in the phloem.

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Fig. 1: Model of the phloem loading and major solutes contributing to the hydrostatic pressure at a rank-2 intermediate vein of a mature leaf of maize.

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  1. Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL, USA

    Li-Qing Chen

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Correspondence to Li-Qing Chen.

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Chen, LQ. Low sugar, under pressure?. Nat. Plants 8, 102–103 (2022). https://doi.org/10.1038/s41477-021-01034-5

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  • DOI: https://doi.org/10.1038/s41477-021-01034-5

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