Primary carbon metabolism in developing seeds:
The main storage product in the mature embryo of Arabidopsis seeds is oil (triacylglycerol, TAG). The dedicated pathway of fatty acid synthesis and TAG synthesis is well understood, but much less is known about how sucrose entering the developing embryo is converted to the substrates for fatty acid and hence TAG synthesis. Sucrose also provides substrates for almost all of the other metabolic pathways in the developing embryo, including synthesis of starch (abundant in early embryo development) and of the ATP and reducing power required for TAG synthesis.
To understand the precise fates of sucrose in the developing embryo, and the general relationship of primary metabolism to seed development and final composition, we are studying mutant and transgenic plants defective in specific aspects of primary carbon metabolism in the embryo. Important findings about embryo metabolism include:
- Flux from sucrose to phosphoenolpyruvate, required for fatty acid synthesis, occurs largely or solely in the cytosol of the developing embryo. PEP then enters the plastid for conversion to fatty acids.
- Import of glucose 6-phosphate (Glc6P) derived from sucrose into the plastid is vital for the normal development of the embryo. In embryos with reduced activity of the Glc6P transporter, development is arrested at the globular stage and morphogenesis is blocked. We conclude that Glc6P is the substrate for the generation of essential reductant or signalling molecules inside the plastid at this crucial stage of development.
- Substantial starch turnover occurs during embryo development. Defects in this process do not affect the final seed weight and composition, although they may cause slowing of the rate of maturation. Starch turnover is probably associated with cellular differentiation in the embryo, rather than with seed-specific storage processes.
- Defects in starch turnover in the maternal plant can cause significant reductions in seed oil content. This is because leaf starch provides carbon for export to the growing seed at night. Failure to mobilise adequate amounts of leaf starch results in starvation of developing fruits and a strong reduction in fruit and seed growth rate specifically at night.
Left, developing embryo containing starch (brown) in a zone of differentiating cells close to the root meristem.
Right, developmental abnormality leading to abortion in an embryo deficient in the Glc6P transporter (age equivalent to mid-torpedo stage in wild-type embryos).
Andriotis VME, Kruger NJ, Pike MJ, Smith AM (2010) Plastidial glycolysis in developing Arabidopsis embryos. New Phytol 185, 649-662
Andriotis VME, Pike MJ, Bunnewell S, Hills MJ, Smith AM (2010) The plastidial glucose 6-phosphate/phosphate antiporter GPT1 is essential for morphogenesis in Arabidopsis embryos. Plant J 64, 128-139
Andriotis VME, Pike MJ, Kular B, Rawsthorne S, Smith AM (2010) Starch turnover in developing oilseed embryos. New Phytol 187, 791-804
Andriotis VME, Pike MJ, Schwarz SL, Rawsthorne S, Wang TL, Smith AM (2012) Altered starch turnover in the maternal plant has major effects on Arabidopsis fruit growth and seed composition Plant Physiol, doi:10.1104/pp.112.205062