Soon-Young Choi

Expression of srab7 and SCaM genes required for endocytosis of Rhizobium in root nodules

Symbiotic nitrogen fixation requires the internalization of rhizobia into root cells of legumes and subsequent formation of symbiosomes, rhizobia-containing organelles inside the plant cells. A small GTP-binding protein from soybean, sRab7, was shown to be essential for nodule development. When yeast ypt7 (Rab7) null mutant was transformed with the srab7 gene, highly fragmented vacuoles seen in the mutant were replaced with a prominent central vacuole, as in the wild-type cells. These results confirm that sRab7 is an authentic homolog of Ypt7p. When nodules obtained at different stages of development were hybridized with srab7 cDNA, 7-day-old nodules showed the most intense hybridization signal. Hybridization was only detected in the infection zone of forming nodules, implying that sRab7 may be involved in endocytosis of rhizobia. SCaM-1 and SCaM-4, soybean calmodulin genes, were also expressed in the same region at this stage of nodulation as shown by in situ hybridization. RT-PCR analysis confirmed the expression of these genes. These results indicate that both the srab7 and SCaM genes were expressed at an early stage of nodulation. Since membrane fusion of vesicles has been shown to be dependent on calmodulin and Ypt7p, the similarity of the expression patterns between the two genes in soybean nodules may indicate that sRab7 and SCaM are essential for vesicular fusion during rhizobial endocytosis and symbiosome formation.

Possible dual regulatory circuits involving AtS6K1 in the regulation of plant cell cycle and growth

The role of Arabidopsis S6 Kinase 1 (AtS6K1), a downstream target of TOR kinase, in controlling plant growth and ribosome biogenesis was characterized after generating transgenic plants expressing AtS6K1 under auxin-inducible promoter. Down regulation of selected cell cycle regulatory genes upon auxin treatment was observed in the transgenic plants, confirming the negative regulatory role of AtS6K1 in the plant cell cycle progression reported earlier. Callus tissues established from these transgenic plants grew to larger cell masses with more number of enlarged cells than untransformed control, demonstrating functional implication of AtS6K1 in the control of plant cell size. The observed negative correlation between the expression of AtS6K1 and the cell cycle regulatory genes, however, was completely reversed in protoplasts generated from the transgenic plants expressing AtS6K1, suggesting a possible existence of dual regulatory mechanism of the plant cell cycle regulation mediated by AtS6K1. An alternative method of kinase assay, termed “substrate-mediated kinase pull down”, was employed to examine the additional phosphorylation on other domains of AtS6K1 and verified the phosphorylation of both amino- and carboxy-terminal domains, which is a novel finding regarding the phosphorylation target sites on plant S6Ks by upstream regulatory kinases. In addition, this kinase assay under the stress conditions revealed the salt- and sugar-dependencies of AtS6K1 phosphorylations.

n-Alkane dissimilation by Rhodopseudomonas sphaeroides transferred OCT plasmid.

The OCT plasmid from Pseudomonas maltophilia N246-1 was transferred to Rhodopseudomonas sphaeroides M1 with very low frequency (1.4–1.9 × 10−5 per recipient cell at pH 7–8 for a 3-hour reaction time). P. maltophilia N246-1 was able to utilize C8-C14 of n-alkanes, whereas R. gas-liquid chromatography determined that the broad range of carbon numbers of n-alkanes in crude oil was remarkably degraded by the transconjugant, R. sphaeroides M1-C1, compared with donor strain N246-1. The fact that donor and transconjugant strains simultaneously lost the capacity to utilize n-alkanes on L-broth medium suggests that the OCT plasmids are unstable. It was found that the OCT plasmid of P. maltophilia N246 was incompatible with the IncP-2 group of P. aeruginosa KCTC 11245.

Distribution of Alcohol-tolerant Microfungi in Paddy Field Soils

Ethanol treatment method was attempted for the selective isolation of ethanol-tolerant fungi from two sites of rice paddy fields around Seoul area. The vertical and seasonal fluctuation of the fungal population were also investigated. The ethanoltolerant fungi were Talaromyces stipitatus, T. flavus var. flavus, T. helicus var. major, Eupenicillium javanicum, Emericellopsis terricolor, Pseudourotium zonatum, Aspergillus flavus, Cladosporium cladosporioides, Penicillium frequentans, P. janthinellum, and P. verruculosum. The most dominant species isolated by this method was T. stipitatus. It was found that the numbers of fungal species and colony forming units (CFUs) of ethanol-tolerant fungi were higher in Ascomycota than in Deuteromycota. A particular tendency appeared the highest CFUs in autumn, but lower in spring and winter T. stipitatus was the dominant species of ethanol tolerant microfungi. This result would suggest that membrane lipid composition of ethanoltolerant fungi isolated from the soils may play on important role in the ethanol tolerance.