High free‐methionine and decreased lignin content result from a mutation in the Arabidopsis S‐adenosyl‐L‐methionine synthetase 3 gene

Abstract

Summary: As an approach to understand the regulation of methionine (Met) metabolism, Arabidopsis Met over‐accumulating mutants were isolated based on their resistance to selection by ethionine. One mutant, mto3, accumulated remarkably high levels of free Met – more than 200‐fold that observed for wild type – yet showed little or no difference in the concentrations of other protein amino‐acids, such as aspartate, threonine and lysine. Mutant plants did not show any visible growth differences compared with wild type, except a slight delay in germination. Genetic analysis indicated that the mto3 phenotype was caused by a single, recessive mutation. Positional cloning of this gene revealed that it was a novel S‐adenosylmethionine synthetase, SAMS3. A point mutation resulting in a single amino‐acid change in the ATP binding domain of SAMS3 was determined to be responsible for the mto3 phenotype. SAMS3 gene expression and total SAMS protein were not changed in mto3; however, both total SAMS activity and S‐adenosylmethionine (SAM) concentration were decreased in mto3 compared with wild type. Lignin, a major metabolic sink for SAM, was decreased by 22% in mto3 compared with wild type, presumably due to the reduced supply of SAM. These results suggest that SAMS3 has a different function(s) in one carbon metabolism relative to the other members of the SAMS gene family.