Date of Award

12-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plants, Soils, and Insects

Major Professor

Feng Chen

Committee Members

Max Cheng, Timothy J. Tschaplinski, M. Reza Hajimorad

Abstract

Known members of the plant SABATH family of methyltransferases (MTs) have important biological functions by methylating hormones, signaling molecules and other metabolites. This dissertation aims to systematically investigate the biochemical and biological functions and evolution of SABATH genes in plants. The genomes of rice and poplar have been fully sequenced, which provides unprecedented opportunities for cross-species comparison of the SABATH family. Using a comparative genomic approach, 41 and 33 SABATH genes were identified in rice and poplar, respectively. The expression of these genes in different tissue was analyzed using RT-PCR approach and some genes highly expressed in multiple tissues were cloned. The cloned cDNAs were expressed in E. coli to produce enzymes. Recombinant proteins were tested with a large number of compounds for MT activities. In poplar, two proteins were determined to have MT activities with indole-3-acetic acid (IAA) and jasmonic acid (JA), respectively. Biochemical and gene expression evidence suggests that poplar IAMT (PtIAMT1) has an important role in poplar development through IAA methylation. Poplar JMT (PtJBMT1) shows high levels of expression at multiple tissues under normal conditions, and the expression of PtJBMT1 was slightly induced by wounding and methyl jasmonate treatment, suggesting it has a role in poplar development and defense. Two SABATH genes, IAMT (OsIAMT1) and BSMT (OsBSMT1), were identified in rice. The high level of OsIAMT1 transcripts in rice roots and panicles implies that OsIAMT1 is involved in root and panicle development. OsBSMT1 has MT activities with both salicylic acid and benzoic acid. The specific induction of OsBSMT1 by herbivory and enhanced emission of methylsalicylate support that it has a specific role in plant response to insects. The evolution of the SABATH gene family was also investigated based on the identified SABATH genes in plants using comparative genomics analysis. Phylogenetic analysis revealed that IAMTs in Arabidopsis, rice and poplar are highly conserved, suggesting IAMTs are evolutionarily ancient. However, SAMTs in these plant species are divergent, indicating it is possible that SAMTs were evolved after the split of these plant lineages.

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