| Research Abstract |
In plants, polyamines function in cell division, growth and differentiation because the polyamine contents were found to be higher in actively dividing cells and in reproductive organs. Expression of the genes encoding polyamine biosynthesis enzymes is often enhanced in abiotic and biotic stress condition. Thus polyamines are expected to function for defending host plants against various environmental stresses. Before starting this project, the applicant did two important findings in plant polyamine research ; i. e.,(1)spermine-deficient mutant Arabidopsis plant became hypersensitive to high salinity and dehydration stresses, suggesting that spermine has a defensive role against these two stresses.(2)spermine has a signaling role to evoke the expression of a subset of genes involved in pathogen defense. In December, 2007, a research group reported that ACL5 gene which was previously thought to encode a spermine synthase, encodes a thermospermine (an isomer of spermine)synthase. With th
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is background, first we established a method to separate those two tetraamines, spermine and thermospermine. Second we analyzed the spatio-temporal expression of spermine synthase gene and thermospermine synthase gene in Arabidopsis by monitoring β-glucuronidase reporter gene expression under the promoters of these genes in Arabidopsis. In spermine-signaling pathway, the involvement of polyamine oxidase(s)is strongly suggested. Third, therefore, characterization of polyamine oxidase (PAO)genes of Arabidopsis thaliana and Oryza sativa was performed. In Arabidopsis, four of the PAO genes were cloned into Escherichia coli expression vector. All of the recombinat PAOs catalyzed the back-conversion reaction but not the polyamine terminal catabolism reaction. Rice genome contains 7 PAO genes. Of them, at least 3 PAO genes catalyzed the polyamine back-conversion reaction, strongly suggesting that rice plant has two catabolic pathways for polyamines. Fourth, spermine has a defensive role in heat shock response (submitted, revision prepared). We also demonstrated that bZIP60, one of the master genes in the unfolded stress response (UPR), is upregulated by spermine. The product of bZIP60 resides in endoplasmic reticulum (ER)in physiological condition, while, once UPR is triggered, it is shown that bZIP60 transcript is spliced by RNase activity of IRE1. Fifth, bZIP60 transcript is spliced by spermine and the resulting transcript produces the active bZIP60 protein which targets into the nucleus (manuscript in preparation). In this activation process, IRE1 is also involved. Lastly we showed that thermospermine has a similar signaling activity to induce the defense genes and inhibits phytoviral particle multiplication as spermine does. Less
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