| Variation in rDNA loci producing 45S ribosomal RNA (45S rRNA) in Oryza species (7-18-2008) |
Ribosomal RNA (rRNA) and ribosomal proteins assemble to form functional ribosomes which then perform protein synthesis. In part because large quantities of rRNAs must be transcribed, the rDNAs are organized as tandem repeats at the nucleolus organizing region (NOR). Each copy of the genome has at least one NORs at a specific chromosomal location. Therefore, a typical diploid genome would have two NORs.
Variations in the number and the position of rDNA loci among species can provide valuable information of evolution and genome organization at the chromosomal level. Mei-Chu Chung’s group has reported polymorphisms in the number of rDNA loci and in their chromosomal positions among Oryza species by using fluorescent in situ hybridization (FISH) (Theor Appl Genet 116:745–753, 2008). They thus provided cytological evidence to support the hypothesis that chromosomal inversion as well as amplification and transposition of rDNA can occur during Oryza species evolution. |
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Secret of the Agrobacterium Secretome:
Type VI Protein Secretion System in a plant pathogenic bacterium (07-07-2008)
Erh-Min Lai’s laboratory has been interested in employing cutting-edge proteomics tools to understand the global molecular events that occur between microbes and their hosts. A shotgun proteomics approach was performed to investigate the secretome of Agrobacterium tumefaciens, a plant pathogenic bacterium uniquely adapted for conducting plant transformation by horizontal transmission of DNA. They identified 12 proteins including one known (VirB1*) and 11 new potential secretory proteins (Wu et al., 2008, Journal of Bacteriology 190:2841-2850). Interestingly, one unknown protein, named hemolysin-coregulated protein (Hcp), is predicted to be a soluble protein without a recognizable N-terminal signal peptide.
Further study revealed that Hcp was expressed but not secreted in mutants with deletions of either a conserved gene, icmF, or the entire putative operon encoding a recently identified type VI secretion system, T6SS (Pukatzki et al., 2006; Mougous et al., 2006). Interestingly, in tumor assays on potato tuber discs, deletion of hcp results in about 20-30% reduction of tumorigenesis efficiency, while no consistent difference can be observed in deletions of icmF or t6ss. This pioneering work on the discovery of T6SS in plant pathogenic bacterium furthers our understanding of the conserved T6SS used by both plant and animal pathogenic bacteria.
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Rice Defense Proteome Induced by the Plant Activator Probenazole (07-07-2008) |
Erh-Min Lai’s laboratory, in collaboration with Dr. Su-Jein Chang at the Crop Improvement Section, Miaoli District Agricultural Research and Extension Station, reported the first proteomic analysis of rice defense responses induced by the plant activator probenazole (PBZ) in a recent paper (Lin et al., 2008, Phytochemistry 69:715-728). PBZ is an agricultural chemical that has been widely used to protect rice plants from rice blast and the bacterial blight pathogen.
Two-dimensional gel electrophoresis was utilized to identify a total of 40 protein spots including 9 protein spots that are up-regulated by PBZ and 31 abundant protein spots. A total of 11 unique proteins from these 9 spots were identified by LC–MS/MS, and the majority of them were classified as defense related proteins and/or possessed orthologs with known defense-related functions. Further gene expression analysis revealed that a putative phenylalanine ammonia-lyase (PAL) is likely regulated t the protein level, whereas a putative glutathione S-transferase GSTU17 and a putative caffeic acid 3-O-methyltransferase COMT were regulated at the mRNA level after PBZ application. Interestingly, the mRNA transcripts of two PAL paralogs were found to be up-regulated by PBZ. We propose that PAL, COMT, and GSTU17 are likely to confer PBZ-induced disease resistance via mechanisms involving biosynthesis and transport of flavonoid-type phytoalexins and/or lignin biogenesis. |
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