Abstract
Although primarily studied in relation to photorespiration, serine metabolism in chloroplasts may play a key role in plant CO(2) fertilization responses by linking CO(2) assimilation with growth. Here, we show that the phosphorylated serine pathway is part of a 'photosynthetic C(1) pathway' and demonstrate its high activity in foliage of a C(3) tree where it rapidly integrates photosynthesis and C(1) metabolism contributing to new biomass via methyl transfer reactions, imparting a large natural (13)C-depleted signature. Using (13)CO(2)-labelling, we show that leaf serine, the S-methyl group of leaf methionine, pectin methyl esters, and the associated methanol released during cell wall expansion during growth, are directly produced from photosynthetically-linked C(1) metabolism, within minutes of light exposure. We speculate that the photosynthetic C(1) pathway is highly conserved across the photosynthetic tree of life, is responsible for synthesis of the greenhouse gas methane, and may have evolved with oxygenic photosynthesis by providing a mechanism of directly linking carbon and ammonia assimilation with growth. Although the rise in atmospheric CO(2) inhibits major metabolic pathways like photorespiration, our results suggest that the photosynthetic C(1) pathway may accelerate and represents a missing link between enhanced photosynthesis and plant growth rates during CO(2) fertilization under a changing climate.