Angiosperms synthesize S-methylmethionine (SMM) from methionine (Met) and 
S-adenosylmethionine (AdoMet) in a unique reaction catalyzed by Met 
S-methyltransferase (MMT). SMM serves as methyl donor for Met synthesis from 
homocysteine, catalyzed by homocysteine S-methyltransferase (HMT). MMT and HMT 
together have been proposed to constitute a futile SMM cycle that stops the free 
Met pool from being depleted by an overshoot in AdoMet synthesis. Arabidopsis 
and maize have one MMT gene, and at least three HMT genes that belong to two 
anciently diverged classes and encode enzymes with distinct properties and 
expression patterns. SMM, and presumably its cycle, must therefore have 
originated before dicot and monocot lineages separated. Arabidopsis leaves, 
roots and developing seeds all express MMT and HMTs, and can metabolize [35S]Met 
to [35S]SMM and vice versa. The SMM cycle therefore operates throughout the 
plant. This appears to be a general feature of angiosperms, as digital gene 
expression profiles show that MMT and HMT are co-expressed in leaves, roots and 
reproductive tissues of maize and other species. An in silico model of the SMM 
cycle in mature Arabidopsis leaves was developed from radiotracer kinetic 
measurements and pool size data. This model indicates that the SMM cycle 
consumes half the AdoMet produced, and suggests that the cycle serves to stop 
accumulation of AdoMet, rather than to prevent depletion of free Met. Because 
plants lack the negative feedback loops that regulate AdoMet pool size in other 
eukaryotes, the SMM cycle may be the main mechanism whereby plants achieve 
short-term control of AdoMet level.