The 'photosynthetic C pathway' links carbon assimilation and growth in California poplar.

Although primarily studied in relation to photorespiration, serine metabolism in chloroplasts may play a key role in plant CO fertilization responses by linking CO assimilation with growth. Here, we show that the phosphorylated serine pathway is part of a 'photosynthetic C pathway' and demonstrate its high activity in foliage of a C tree where it rapidly integrates photosynthesis and C metabolism contributing to new biomass via methyl transfer reactions, imparting a large natural C-depleted signature. Using CO-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 metabolism, within minutes of light exposure.