Abstract: Salicylic acid methyltransferase (SAMT), benzoic acid methyltransferase (BAMT) and theobromine methyltransferase (TH) (henceforth, SABATH) family proteins belong to a unique class of methyltransferase that can methylate small molecular compounds including indole-3-acidic acid (IAA), salicylic acid (SA) and jasmonic acid (JA), in plants. Here we report that the GAMT2 protein, which has 34.2% similarity with IAMT1 in the amino acid sequence, can methylate gibberellic acid (GA). Bioinformatics analysis suggests that GAMT2 may be able to methylate one molecule larger than SA. GAMT2 is predominantly expressed in the developing seed embryo and endosperm in Arabidopsis. During seed germination, the expression of GAMT2 decreases until the cotyledons expand out of the seed coat. Overexpression of GAMT2 in Arabidopsis resulted in multiple phenotypes, including dwarfism, retarded growth, late flowering, and reduced fertility, which are similar to the phenotypes of GA-deficient mutants. Seed germination assay showed that GAMT2 overexpression in plants was hypersensitive to GA biosynthesis inhibitor (ancymidol) and abscisic acid (ABA) treatments, whereas the GAMT2 null mutant (SALK_075450) was slightly insensitive to such treatments, suggesting that GAMT2 may methylate GA or ABA. Enzyme activity analysis indicated that GAMT2 was able to methylate GA3 into Methyl-GA3 in vitro, but could not methylate ABA. Microarray analysis on GAMT2 overexpression plants suggested that Methyl-GA may be an inactive form of GA in Arabidopsis. These data suggest that GAMT2 is involved in seed maturation and germination by modulating GA activity.

Key words: GAMT2, GA-deficient, seed germination, seed maturation, Me-GA, methyl acceptors, SABATH methyltransferase.