Abstract
Photosynthetically derived sugars provide carbon skeletons for metabolism and carbon signals that favor anabolism. The amount of sugar available for fatty acid (FA) and triacylglycerol (TAG) synthesis depends on sugar compartmentation, transport, and demands from competing pathways. We are exploring the influence of sugar partitioning between the vacuole and cytoplasm on FA synthesis in Arabidopsis by building on our previous finding that reduced leaf sugar export in the sucrose-proton symporter2 (suc2) mutant, in combination with impaired starch synthesis in the ADP-glucose pyrophosphorylase (adg1) mutant, accumulates higher sugar levels and increased total FA and TAG compared to the wild type parent. Here we sought to relocalize sugar from the vacuole to the cytoplasm to drive additional FA/TAG synthesis and growth. Arabidopsis suc2 adg1 was therefore crossed with tonoplast monosaccharide transporter mutants tmt1 and tmt2 and overexpression of the sucrose/proton cotransporter SUC4 in which tmt1 tmt2 impairs sugar transport to the vacuole from the cytoplasm and SUC4 overexpression enhances sugar transport in the reverse direction from the vacuole to the cytoplasm. A resulting homozygous suc2 adg1 tmt1 tmt2 SUC4 line was used to test the hypothesis that increased intracellular carbon supply in the form of sugars would increase both FA and TAG accumulation. The data shows that relative to suc2 adg1, suc2 adg1 tmt1 tmt2 SUC4 significantly increases leaf total FA content by 1.29-fold to 10.9% of dry weight and TAG by 2.4-fold to 2.88%, supporting the hypothesis that mobilizing vacuolar sugar is a valid strategy for increasing vegetative oil accumulation.
Highlights
Maintaining sugar homeostasis in the cell requires coordinated control between production, distribution, storage, and metabolism of sugar, which is essential for plant growth and its development
Adg1 double mutant, impaired in both sugar loading into the FIGURE 2 | suc2 adg1 tmt1 tmt2 SUC4 shows partial rescue of the suc2 adg1 dwarf phenotype. (A) Comparison of growth of 5-week-old soil-grown WT, SUC4, tmt1 tmt2, tmt1 tmt2 SUC4, suc2 adg1, and suc2 adg1 tmt1 tmt2 SUC4 plants. (B) Rosette leaves of suc2 adg1 and suc2 adg1 tmt1 tmt2 SUC4 in (A). (C)
The decrease in size associated with sucrose phloem loading in suc2 is severe, and we previously reported that it can be partially mitigated by increasing growth of both shoots and roots by combining with adg1 (Zhai et al, 2017b), which blocks starch synthesis thereby increasing sugar levels and stimulating TAG accumulation by the mechanisms described above
Summary
Maintaining sugar homeostasis in the cell requires coordinated control between production, distribution, storage, and metabolism of sugar, which is essential for plant growth and its development. Photosynthetically derived sugars get exported from the production site (source tissues) to consumption sites (sink tissues) where it is either used for growth and development or converted to storage compounds (Julius et al, 2017). Sucrose is the predominant sugar for long distance transport through the phloem (Julius et al, 2017). Several decades of studies on sugar transport have identified more than 60 putative monosaccharide transporters in plants (Wormit et al, 2006). Vacuoles occupy as much as 90% of the cell by volume and are capable of both short- and long-term sugar storage.
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