The plastidial glucose-6-phosphate/phosphate antiporter GPT1 is essential for morphogenesis in Arabidopsis embryos

Abstract

SUMMARYThe glucose-6-phosphate/phosphate antiporter GPT1 is a major route of entry of carbon into nonphotosynthetic plastids. To discover its importance in oilseeds, we used a seed-specific promoter to generate lines of Arabidopsis thaliana with reduced levels of GPT1 in developing embryos. Strong reductions resulted in seed abortion at the end of the globular stage of embryo development, when proplastids in normal embryos differentiate and acquire chlorophyll. Seed abortion was partly dependent on the light level during silique development. Embryos in seeds destined for abortion failed to undergo normal morphogenesis and were 'raspberry-like' in appearance. They had ultrastructural and biochemical defects including proliferation of peroxisomes and starch granules, and altered expression of genes involved in starch turnover and the oxidative pentose phosphate pathway. We propose that GPT1 is necessary for early embryo development because it catalyses import into plastids of glucose-6-phosphate as the substrate for NADPH generation via the oxidative pentose phosphate pathway. We suggest that low NADPH levels during plastid differentiation and chlorophyll synthesis may result in generation of reactive oxygen species and triggering of embryo cell death.

The Import and Export Business in Plastids: Transport Processes across the Inner Envelope Membrane

“…As the lipid content in Arabidopsis seeds with a reduced GPT1 activity is only slightly reduced, an interruption of fatty acid synthesis is not the cause of the lethality. Therefore, it has been suggested that a reduction in the production of NADPH via the oxidative pentose phosphate pathway impairs several NADPH-dependent processes necessary for plastid development and differentiation, loss of which inhibits gametophyte and embryo development (Niewiadomski et al, 2005;Andriotis et al, 2010). However, a direct proof of this hypothesis is still lacking.…”

“…A reduction of GPT1 expression of about 50% in seeds of Vicia narbonensis by an antisense approach with a seed-specific promoter led to a shift in assimilate partitioning from starch/ lipids into storage proteins, indicating that GPT1 exerts a significant control on seed filling and maturation (Rolletschek et al, 2007). A reduction of more than 80% of GPT1 expression in Arabidopsis seeds is lethal because embryo development stops at the globular stage (Andriotis et al, 2010). The reason why a defect in Glc6P transport leads to such severe phenotypes is not clear so far.…”

“…A detailed analysis of heterozygous GPT1/gpt1 plants revealed that the development of both the female and the male gpt1 gametophyte are impaired. Two more recent investigations showed that GPT1 is also involved in embryo development and seed maturation (Rolletschek et al, 2007;Andriotis et al, 2010). A reduction of GPT1 expression of about 50% in seeds of Vicia narbonensis by an antisense approach with a seed-specific promoter led to a shift in assimilate partitioning from starch/ lipids into storage proteins, indicating that GPT1 exerts a significant control on seed filling and maturation (Rolletschek et al, 2007).…”

Starch Turnover and Metabolism during Flower and Early Embryo Development

“…Starchless amyloplasts were reported in gpt1 mutant pollen grains, and a dual function of GPT1 in starch synthesis and the oxidative pentose phosphate pathway was suggested (Kammerer et al, 1998;Niewiadomski et al, 2005;Kunz et al, 2010). However, a seed-specific knockdown of GPT1 activity revealed that starch grains still accumulated in arrested embryos (Andriotis et al, 2010a). Thus, it is still unclear whether starch metabolism in heterotrophic cells of Arabidopsis is based on GPT1-mediated Glc-6-P translocation.…”

“…Our systematic microscopic and transcriptomic analyses of starch and sugar metabolism during flower and early silique development sets the foundation for further research on metabolic pathways using more specific microscopic and molecular approaches, in particular organ-, tissue-, or cell-specific transcriptomic and gene-silencing techniques (Andriotis et al, 2010a;Schmid et al, 2015). However, the simple, effective, and quantitative microscopic technique used here will help to distinguish between models by analyzing mutants, either for genes already known to be essential for sugar and starch metabolism or, to circumvent genetic redundancy, for multiple mutants representing coexpressed genes in specific pathways.…”