Structure and Function of Chloroplasts - Volume II

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Expression of GPT2 and RRTF1 in response to an increase in PFD from 120 to 500 μmol m–2 s–1 Ca 400 ppm in Col-0 and mutants and model of transcriptional regulation of GPT2. (A) qPCR data of expression of GPT2 in Col-0 and pgm1-1, rrtf1-1, and tpt-3 mutants. n = 5 (B) qPCR data of expression of RRTF1 in Col-0 and tpt-3 mutant. n = 5 (C) Model of transcriptional regulation of GPT2 expression. Triose phosphates in the cytosol and RRTF1 are both required for expression of GPT2 (shown as an “AND” logic gate) while starch synthesis represses GPT2 expression.
Original Research
27 June 2019

The exchange of reduced carbon across the inner chloroplast envelope has a large impact on photosynthesis and growth. Under steady-state conditions it is thought that glucose 6-phosphate (G6P) does not cross the chloroplast membrane. However, growth at high CO2, or disruption of starch metabolism can result in the GPT2 gene for a G6P/Pi translocator to be expressed presumably allowing G6P exchange across the chloroplast envelope. We found that after an increase in light, the transcript for GPT2 transiently increases several 100-fold within 2 h in both the Col-0 and WS ecotypes of Arabidopsis thaliana. The increase in transcript for GPT2 is preceded by an increase in transcript for many transcription factors including Redox Responsive Transcription Factor 1 (RRTF1). The increase in GPT2 transcript after exposure to high light is suppressed in a mutant lacking the RRTF1 transcription factor. The GPT2 response was also suppressed in a mutant with a T-DNA insert in the gene for the triose-phosphate/Pi translocator (TPT). However, plants lacking TPT still had a robust rise in RRTF1 transcript in response to high light. From this, we conclude that both RRTF1 (and possibly other transcription factors) and high amounts of cytosolic triose phosphate are required for induction of the expression of GPT2. We hypothesize that transient GPT2 expression and subsequent translation is adaptive, allowing G6P to move into the chloroplast from the cytosol. The imported G6P can be used for starch synthesis or may flow directly into the Calvin-Benson cycle via an alternative pathway (the G6P shunt), which could be important for regulating and stabilizing photosynthetic electron transport and carbon metabolism.

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