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Methylation of Gibberellins by Arabidopsis GAMT1 and GAMT2^[W]

^a Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109-1048
^b RIKEN Plant Science Center, Tsurumi-ku, Yokohama, Kanagawa 230-0045, Japan
^c Smith Institute for Plant Sciences and Genetics, Faculty of Agriculture, Hebrew University, Rehovot 76100, Israel
^d Department of Genetics, Development, and Cell Biology, Iowa State University, Ames, Iowa 50011-3260
^e Howard Hughes Medical Institute, Jack H. Skirball Chemical Biology and Proteomics Laboratory, Salk Institute for Biological Studies, La Jolla, California 92037
^f Virginia Bioinformatics Institute, Virginia Tech, Blacksburg, Virginia 24061
^g Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia

¹ To whom correspondence should be addressed. E-mail lelx{at}umich.edu; fax 734-647-0884.

Arabidopsis thaliana GAMT1 and GAMT2 encode enzymes that catalyze formation of the methyl esters of gibberellins (GAs). Ectopic expression of GAMT1 or GAMT2 in Arabidopsis, tobacco (Nicotiana tabacum), and petunia (Petunia hybrida) resulted in plants with GA deficiency and typical GA deficiency phenotypes, such as dwarfism and reduced fertility. GAMT1 and GAMT2 are both expressed mainly in whole siliques (including seeds), with peak transcript levels from the middle until the end of silique development. Within whole siliques, GAMT2 was previously shown to be expressed mostly in developing seeds, and we show here that GAMT1 expression is also localized mostly to seed, suggesting a role in seed development. Siliques of null single GAMT1 and GAMT2 mutants accumulated high levels of various GAs, with particularly high levels of GA₁ in the double mutant. Methylated GAs were not detected in wild-type siliques, suggesting that methylation of GAs by GAMT1 and GAMT2 serves to deactivate GAs and initiate their degradation as the seeds mature. Seeds of homozygous GAMT1 and GAMT2 null mutants showed reduced inhibition of germination, compared with the wild type, when placed on plates containing the GA biosynthesis inhibitor ancymidol, with the double mutant showing the least inhibition. These results suggest that the mature mutant seeds contained higher levels of active GAs than wild-type seeds.

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