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The dramatic decrease in atmospheric CO 2 concentration during Oligocene was proposed as directly linked to C 4 evolution. However, it remains unclear how the decreased CO 2 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 CO 2 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 NH 4 + 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 CO 2 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 CO 2 conditions and suggests that low CO 2 can facilitate the evolution of C 4 photosynthesis beyond the commonly assumed role of being a selection pressure.
Miao et al. (Thu,) studied this question.