Fig. 5: Simplistic model of the ‘Photosynthetic C₁’ pathway and methanol emission during growth.

The ‘Photosynthetic C₁’ pathway (steps 1–7) results in light-dependent methionine synthesis followed by the activation of methionine to AdoMet in the cytosol (step 8) and its utilization as a major source of cellular methyl transfer reactions including in the Golgi during the methylation of newly synthesized pectin polysaccharides (step 9). Following the export and incorporation of newly synthesized, highly methylated pectin into the primary cell wall (step 10), cell wall expansion during growth processes is regulated by pectin demethylation (step 11). Pectin methyl ester hydrolysis results in the change in cell wall elasticity via formation of calcium pectinate “egg-box”⁹⁹ and release of methanol into the apoplast. Methanol partitions between the plant aqueous and gas phases and rapidly escapes the plant via stomatal emission to the atmosphere. Blue balls represent carbon atoms recently assimilated by the Calvin–Benson cycle during photosynthesis as CO₂; Black balls represent stored or non-recently assimilated carbon atoms; Orange balls represent the sulfur atom of homocysteine, methionine and AdoMet. For abbreviations of metabolites and enzymes, see Fig. 6 for a more detailed schematic of the ‘Photosynthetic C₁’ pathway as well as Note 2 in the supplementary information document. Created in BioRender. K. Jardine (2023) BioRender.com/p03i521.