Rewiring of primary metabolism for ammonium recycling under short-term low CO2 treatment - its implication for C4 evolution

The dramatic decrease in atmospheric CO2 concentration during Oligocene was proposed as directly linked to C-4 evolution. However, it remains unclear how the decreased CO2 concentration directly facilitate C-4 evolution, besides its role as a selection pressure. We conducted a systematic transcriptomics and metabolomics analysis under short-term low CO2 condition and found that Arabidopsis grown under this condition showed 1) increased expression of most genes encoding C-4-related enzymes and transporters; 2) increased expression of genes involved in photorespiration and pathways related to carbon skeleton generation for ammonium refixation; 3) increased expression of genes directly involved in ammonium refixation. Furthermore, we found that in vitro treatment of leaves with NH4+ induced a similar pattern of changes in C-4 related genes and genes involved in ammonium refixation. These data support the view that Arabidopsis grown under short-term low CO2 conditions rewired its metabolism to supply carbon skeleton for ammonium recycling, during which process the expression of C-4 genes were up-regulated as a result of a hitchhiking process. This study provides new insights into the adaptation of the C-3 model plant Arabidopsis under low CO2 conditions and suggests that low CO2 can facilitate the evolution of C-4 photosynthesis beyond the commonly assumed role of being a selection pressure.