Trans-methylation reactions are vital in basic metabolism, epigenetic
regulation, RNA metabolism, and posttranslational control of protein
function and therefore fundamental in determining the physiological
processes in all living organisms. The plant kingdom is additionally
characterized by the production of secondary metabolites that undergo
specific hydroxylation, oxidation and methylation reactions to obtain a
wide array of different chemical structures. Increasing research efforts
have started to reveal the enzymatic pathways underlying the
biosynthesis of complex metabolites in plants. Further engineering of
these enzymatic machineries offers significant possibilities in the
development of bio-based technologies, but necessitates deep
understanding of their potential metabolic and regulatory interactions.
Trans-methylation reactions are tightly coupled with the so-called
activated methyl cycle (AMC), an essential metabolic circuit that
maintains the trans-methylation capacity in all living cells. Tight
regulation of the AMC is crucial in ensuring accurate trans-methylation
reactions in different subcellular compartments, cell types,
developmental stages and environmental conditions. This review addresses
the organization and posttranslational regulation of the AMC and
elaborates its critical role in determining metabolic regulation through
modulation of methyl utilization in stress-exposed plants.