Metabolite transport and associated sugar signalling systems underpinning source/sink interactions

Abstract

Metabolite transport between organelles, cells and source and sink tissues not only enables pathway co-ordination but it also facilitates whole plant communication, particularly in the transmission of information concerning resource availability. Carbon assimilation is co-ordinated with nitrogen assimilation to ensure that the building blocks of biomass production, amino acids and carbon skeletons, are available at the required amounts and stoichiometry, with associated transport processes making certain that these essential resources are transported from their sites of synthesis to those of utilisation. Of the many possible posttranslational mechanisms that might participate in efficient co-ordination of metabolism and transport only reversible thiol-disulphide exchange mechanisms have been described in detail. Sucrose and trehalose metabolism are intertwined in the signalling hub that ensures appropriate resource allocation to drive growth and development under optimal and stress conditions, with trehalose-6-phosphate acting as an important signal for sucrose availability. The formidable suite of plant metabolite transporters provides enormous flexibility and adaptability in inter-pathway coordination and source-sink interactions. Focussing on the carbon metabolism network, we highlight the functions of different transporter families, and the important of thioredoxins in the metabolic dialogue between source and sink tissues. In addition, we address how these systems can be tailored for crop improvement.

Keywords: Phloem loading; Redox regulation; Source-sink interactions; Sucrose transporters; Sugar signalling; Trehalose.

Graphical abstract

Fig. 1

The central role of thiol-disulphide exchange regulation via thioredoxins in plant biology.

Fig. 2

Apoplastic loading of sucrose from the mesophyll to the phloem in leaf tissue. Sucrose produced during photosynthesis is stored in the vacuole and transported into the mesophyll by SUT2/4. Sucrose moves to the phloem parenchyma through plasmodesmata where it is transported to the apoplast by SWEETS. SUT1/3 transport sucrose from the apoplast to the companion cell of the phloem and from the companion cell to the sieve element by plasmodesmata, SUT2/4 and SUT1/3. Arrows represent direction of transport.

Fig. 3

Sugar signalling via trehalose 6-phosphate (T6P)/SnRK1 interactions. The sucrose status of plant tissue is relayed through T6P to SnRK1. The activity of SnRK1 which is inhibited by T6P determines gene expression for starvation or satiety responses and maintains sucrose homeostasis. There is strong cell and developmental specificity of expression of genes regulating T6P content. Cell specific changes in T6P may enable modification of plant process and particularly productivity and resilience of crops.

Fig. 4

Source-sick and T6P-related processes throughout the life cycle. Changes in T6P accumulation, and source-sink relationships affect cellular processes in seed germination, vegetative and reproductive plant growth. Source tissues generally have a low T6P content, and sink tissues a high T6P content. Arrows indicate direction of sucrose transport.

Fig. 5

OsMADS6-TPP maize. Expression of TPP in female reproductive tissues of maize leads to increased yield in drought-stressed conditions. TPP is expressed in maize ear vasculature (a). In wild type plants, drought stress causes yield loss (b), in OsMADS6-TPP maize, yield loss due to drought-stress is decreased, resulting in higher yield in comparison to wild-type during drought-stress (c).