Source–Sink Relationships

Abstract

Life on earth depends on the growth and survival of plants. In order for plants to grow and develop effectively, coordination between sources and sinks is required. Source organs provide a net uptake of resources whilst sink organs have a net drawdown of resources. Molecular mechanisms regulate the relationship between sources and sinks. These molecular mechanisms include carbon‐ and nitrogen‐containing metabolites, plant hormones and genes. Sources and sinks for both carbon and nitrogen are key contributors to plant growth, and these regulate themselves and one another via feedback, feedforward and crosstalk mechanisms. Our understanding of the relationships between sources and sinks is increased by experimental manipulations of the source–sink balance. To bring about increases in crop growth and yield, a holistic view of sources and sinks must be developed, including the molecular mechanisms underpinning the relationships between them. Mathematical modelling can be an effective tool for providing this unified perspective.

Key Concepts

  • Sources have net uptake of a resource and sinks have net utilisation of a resource.
  • Plant growth is underpinned by sources and sinks.
  • Sources and sinks for both carbon and nitrogen are vital components of plant development.
  • Molecular mechanisms including metabolites, genes and phytohormones regulate the source–sink balance.
  • An understanding of source:sink relationships can help improve crop yield.

Keywords: carbon; growth; nitrogen; phytohormone; signalling; sink; source

Figure 1. Net sources and sinks for carbon (green) and nitrogen (blue), in a simplified plant system. Deepening colour illustrates the gradient for each element, indicated by the grey arrows. Sugars and amino acids are transported in the phloem.
Figure 2. Sources (a, b, c) and sinks (d, e, f) of three crop plants grown in the glasshouse at Brookhaven National Laboratory: Helianthus annuus (sunflower) leaves (a) and flower with developing seeds (d); Raphanus sativus (radish) leaves (b) and edible tuber (e); Curcubita pepe (courgette) leaves (c) and flowers with edible fruits (f). Photographs taken at Brookhaven National Laboratory by Angela C Burnett (a, b, c, e, f) and photograph courtesy of Erin O'Connor (d).
Figure 3. Schematic overview of nitrogen (N) transport processes and source–sink relationships at the whole‐plant level. N fluxes from soil to root to leaf to sinks involve short‐ and long‐distance transport of inorganic N (nitrate (NO3), ammonium (NH4+) and di‐nitrogen (N2)) and organic N (amino acids (AA) and ureides (Ur)). The xylem and phloem connect sources with sinks and are essential for N mobilization. The smaller font size of xylem NH4+ and phloem NO3 refers to their lower concentration compared with other transported N compounds. Grey arrows indicate feedback controls exerted by source and sink on N uptake and partitioning, respectively. Tegeder and Masclaux‐Daubresse . Reproduced with permission of John Wiley and Sons.
Figure 4. A range of feedback mechanisms enables fine‐tuning of the balance between sources and sinks for carbon (green) and nitrogen (blue). These mechanisms include metabolites derived from carbon and nitrogen; genetic regulation; and control by phytohormones. Feedbacks operate at the tissue level (arrows 1–4 and 6–9) and at the whole‐plant level (arrows 5 and 10). White et al. . Reproduced with permission of Oxford University Press.
Figure 5. Trehalose 6‐phosphate (T6P), synthesized by trehalose phosphate synthase (TPS) and subsequently catalysed to trehalose by trehalose phosphate phosphatase (TPP), signals sucrose availability through the feast–famine protein kinase, SnRK1, which regulates genes involved in metabolism, growth and development. An intermediary factor (IF) is necessary for inhibition of SnRK1 by T6P (Zhang et al., ). Low T6P results in activation of genes for famine responses; high T6P results in activation of genes for feast responses. Decreases in T6P through genetic modification (Nuccio et al., ) and marker‐assisted selection (Kretzschmar et al., ), or increases in T6P through chemical intervention (Griffiths et al., ), have resulted in improved performance and large yield improvements in maize, rice and wheat. Paul et al. . Reproduced with permission of Oxford University Press.
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Further Reading

Chang T and Zhu X (2017) Source‐sink interaction: a century old concept under the light of modern molecular systems biology. Journal of Experimental Botany 68: 4417–4431.

Heldt H and Piechulla B (2011) Plant Biochemistry, 4th edn. Elsevier: New York.

Taiz L, Zeiger E, Møller IM and Murphy A (2014) Plant Physiology and Development, 6th edn. Sinauer Associates: Sunderland, MA.

Tegeder M and Masclaux‐Daubresse C (2017) Source and sink mechanisms of nitrogen transport and use. New Phytologist 217: 35–53.

White AC, Rogers A, Rees M and Osborne CP (2016) How can we make plants grow faster? A source‐sink perspective on growth rate. Journal of Experimental Botany 67: 31–45.

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Burnett, Angela C(May 2019) Source–Sink Relationships. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001304.pub2]