Functional Study of Plant SABATH Methyltransferases in the Biosynthesis of Methyl Cinnamate, Juvenile Hormone III and Methyl Gibberellins

SABATH family of methyltransferases (MTs) is a group of plant MTs that are capable of methylating phytohormones and other small molecular compounds. This dissertation investigates biochemical function and evolution of SABATH genes with special interests in the biosynthesis of methyl cinnamate, juvenile hormone III, and methyl gibberellins. Methyl cinnamate is a fragrant volatile compound that occurs in a variety of land plants including great scent liverwort, Conocephalum salebrosum Szweykowski, Buczkowska and Odrzykoski. Using a comparative transcriptomic approach, we compare biosynthesis in liverworts and flowering plants and identified a cinnamic acid methyltransferase (CAMT) from C. salebrosum SABATH MTs. Structural and phylogenetic evidence indicate that methyl cinnamate biosynthesis in liverwort and flowering plants originated through convergent evolution. Juvenile hormones (JHs) are important in insect development and reproduction, however, JH III has been detected in some plants including Cyperus iria L. Unlike JH III biosynthetic pathway in insects that has been largely resolved, our understanding of JH III biosynthesis in plants remains limited. To reveal key enzymes involved in JH III biosynthesis in plants, a comparative transcriptomic approach was undertaken and identified C. iriahomologs of those enzyme-coding genes in insects, but no homolog for the biosynthesis of methyl farnesoate, the immediate precursor of JH III in plants. However, C. iria SABATH MTs were used to identify a farnesoic acid methyltransferase (FAMT) that can produce methyl farnesoate.This discovery implied the independent evolution of JH III biosynthetic pathway in plants and insects. Gibberellins (GAs) are a class of plant hormones that have multiple roles in different physiological processes. GA methylation is a recently discovered route of maintaining GA homeostasis which is catalyzed by gibberellic acid methyltransferase (GAMT). Because GAMT genes have previously identified only in the model plant Arabidopsis, the importance of this GA catabolic mechanism in other plants has remained unclear. To investigate the distribution, evolution, and biochemical functions of GAMT genes in land plants, we systemically analyzed 260 plant genomes and found that the GAMT gene arose early in the evolution of seed plants and was subsequently got lost in many flowering plants.