Date of Award
Doctor of Philosophy
Plants, Soils, and Insects
Max Cheng, Hem Bhandari, Hong Guo
Known members of Phytohormone-methylating compounds are plant synthesis compounds that serve as attractants of other living organisms beneficial to the plants or as defense against other biotic as well as abiotic agents. To increase their fitness and survival in a stressful environment plants produce distinct sets of phytohormone-methylating compounds. Plant genomes can encode the necessary enzymes to acquire the ability to make new specialized compounds during evolution. This dissertation aims to investigate the biochemical and biological functions and evolution of SABATH genes in different lineages of plants. Black cottonwood, Brachypodium and Norway spruce genome were used as the model for dicotyledon, monocotyledon and gymnosperm for evolution study. Using a comparative genomic approach 28, 12 and 10 SABATH genes were identified in black cottonwood, Brachypodium and Norway spruce, respectively. In order to understand biochemical and biological of SABATH from dicotyledon plant, selected enzyme members from black cottonwood were encoded and only one protein was determined to methylate phytohormone. PtJMT1 displayed activity with JA over BA. Site-mutation of Ser-153 and Asn-361 were construct in order to reveal a key residue that determine the relative activity of the enzyme with JA and BA. Homology-based structural modeling indicated that substrate alignment, in which Asn-361 is involved, plays a role in determining the substrate specificity of PtJMT1. The expression of PtJMT1 was induced by plant defense signal molecules methyl jasmonate and salicylic acid and a fungal elicitor alamethicin, suggesting that PtJMT1 may have a role in plant defense against biotic stresses. In order to understand the evolution of phytohormone-methylating enzymes entire SABATH genes in Brachypodium and Norway spruce were encoded and recombinant proteins were tested with phytohormone compounds for methyltransferase activities. In Brachypodium three proteins were determined to methylate phytohormone. These three enzymes (BdSABATH5, BdSABATH6 and BdSABATH8) have IAMT activity. All three members also had JMT activity, with BdSABATH5 and 6 having higher activity with JA than with IAA. In Norway spruce, five proteins were determined to methylate phytohormone. Three JMTs (PaJMT, PaJMT2, and PaJMT3), one IAMT (PaIAMT) and one SAMT (PaSAMT) enzymes from Norway spruce.
Chaiprasongsuk, Minta, "Biochemistry and Evolution of the Phytohormone-methylating SABATH Methyltransferase in Plants. " PhD diss., University of Tennessee, 2016.