Yu Ran Moon

Transcriptomic Profile of Arabidopsis Rosette Leaves during the Reproductive Stage after Exposure to Ionizing Radiation

Abstract Kim, J-H., Moon, Y. R., Kim, J-S., Oh, M-H., Lee, J-W. and Chung, B. Y. Transcriptomic Profile of Arabidopsis Rosette Leaves during the Reproductive Stage after Exposure to Ionizing Radiation. Radiat. Res. 168, 267–280 (2007). We attempted to obtain a transcriptomic profile of ionizing radiation-responsive genes in Arabidopsis plants using Affymetrix ATH1 whole-genome microarrays. The Arabidopsis plants were irradiated with 200 Gy γ rays at the early reproduction stage, 33 days after sowing. Rosette leaves were harvested during the postirradiation period from 36 to 49 days after sowing and used for the microarray analysis. The most remarkable changes in the genome-wide expression were observed at 42 days after sowing (9 days after the irradiation). We identified 2165 genes as γ-ray inducible and 1735 genes as γ-ray repressible. These numbers of affected genes were almost two to seven times higher than those at other times. In a comparison of the control and irradiated groups, we also identified 354 differentially expressed genes as significant by applying Welchs t test and fold change analysis. The gene ontology analysis showed that radiation up-regulated defense/ stress responses but down-regulated rhythm/growth responses. Specific expression patterns of 10 genes for antioxidant enzymes, photosynthesis or chlorophyll synthesis after irradiation were also obtained using real-time quantitative PCR analysis. We discuss physiological and genetic alterations in the antioxidative defense system, photosynthesis and chlorophyll metabolism after irradiation at the reproductive stage.

Photosynthetic capacity of Arabidopsis plants at the reproductive stage tolerates γ irradiation.

The developmental stage has an influence on the overall responses of plants under biotic or abiotic stress conditions. However, there is a lack of data about the effects of ionizing radiation in plants at different developmental stages. We examined radiation sensitivity of Arabidopsis plants in terms of photosynthetic ability and oxidative stress resistance at two distinct vegetative and reproductive stages, which correspond to 23 and 43 d after seeding (DAS), respectively. When plants were exposed to γ rays at a dose rate 50 Gy h(-1) for 4 h, they were characterized as various common or differential cellular responses depending on the developmental stage. Radial expansion of leaves, inhibition of non-photochemical quenching, and production of •O(2)(-) and H(2)O(2) under methyl viologen-induced photooxidative stress were commonly more conspicuous in the irradiated leaves of both plants than in the respective control. In contrast, the 23 and 43-DAS plants were explicitly discriminated in growth, chloroplast number & ultrastructure, photosynthetic pigment content & activity, and protein damage after γ irradiation. Natural leaf senescence was thereby enhanced in the irradiated leaves of the 23-DAS plants, while it was reversely alleviated in those of the 43-DAS ones. These results suggest that photosynthetic machineries of Arabidopsis plants at the reproductive stage can be relatively tolerant to γ rays of 200 Gy.

Electron paramagnetic resonance investigation of different plant organs after gamma irradiation

Total reactive oxygen species (ROS) signals in irradiated Arabidopsis plants were examined by electron paramagnetic resonance (EPR) analysis. At 10 kGy, the EPR signal intensity was highest in the root, whereas relatively low intensity levels were observed in the leaf and stem. The relative unit (r.u.) of control plants was 0.38 in the leaf, which was gradually increased to 0.51, 0.71, and 0.95 r.u. at 1, 5, and 10 kGy, respectively. In the stem, the intensity in all irradiated samples was lowest compared with that in other plant organs such as the leaf and root. The r.u. in the root sharply increased from 0.13 r.u. in control samples to 1.58 r.u. at 10 kGy, with 0.30–0.42 r.u. observed in 1–5 kGy irradiated samples. Stem and leaf extracts showed remarkably high levels of radical scavenging activity at 89.12 and 71.45%, respectively, compared with the very low level of activity in the root at 10.75%. These findings were in good agreement with the extraction yield of each plant organ, which was 20.0, 14.8, and 10.0% in the stem, leaf, and root, respectively. Order of EPR signal intensity and radical scavenging activity was as follows: EPR signal intensity: 1) leaf > root > stem at 1 and 5 kGy, 2) root > leaf > stem at 10 kGy; radical scavenging activity: stem > leaf > root. Results showed high or low levels of EPR signal intensity in different plant organs could be caused by the ROS removal power of extracts from different plant organs.

Differential Radiation Sensitivities of Arabidopsis Plants at Various Developmental Stages

Arabidopsis plants were exposed to γ-rays of 200 Gy at one vegetative and three reproductive stages such as 23, 30, 36, and 43 days after sowing the seeds (DAS). The plants irradiated at certain reproductive stages, 30 and 36 DAS, showed a marked delay in leaf senescence in spite of inhibited growth of inflorescence stems and abnormalities in leaf morphology. The delay in leaf senescence was first characterized as increased levels of chlorophylls and carotenoids. The irradiated leaves also showed higher values in the maximum apparent electron transport rate, ETRmax, than the control ones. Moreover, microscopic analysis revealed the increased number of chloroplasts and the integrity of thylakoid membranes in the irradiated leaves of the 30 and 36-DAS plants at the later reproductive stage after ?-irradiation. The obtained results suggest that radiation-induced responses of Arabidopsis plants would be strongly dependent on the developmental stage including differential effects in leaf senescence.

Change of chlorophyll fluorescence transients in Arabidopsis plants irradiated with low-dose radiation using a gamma phytotron.

The present study characterised chlorophyll fluorescence transients significantly altered in Arabidopsis leaves upon exposure to gamma radiation for 7.5 days at a low-dose rate of 0.8 Gy day−1 in a gamma phytotron. The JIP transients of Kautsky fluorescence induction were substantially affected by the consecutive exposure to the low-dose-rate gamma radiation. The performance index of photosynthesis (PI), which is based on the JIP transients, was decreased by up to about 40% in irradiated leaves. However, the maximum photochemical efficiency (Fv/Fm) and phenotypic traits were almost the same between the control and irradiated leaves. Based on these results, the PI is suggested to be a sensitive and reliable parameter for physiological evaluation of irradiated plants after exposure to low-dose gamma radiation.

Characterization of LexA-mediated Transcriptional Enhancement of Bidirectional Hydrogenase in Synechocystis sp. PCC 6803 upon Exposure to Gamma Rays

Influence of gamma rays on the cyanobacterium Synechocystis sp. PCC 6803 cells was investigated in terms of a bidirectional hydrogenase, which is encoded by hoxEFUYH genes and responsible for biohydrogen production. Irradiated cells revealed a substantial change in stoichiometry of photosystems at one day after gamma irradiation at different doses. However, as evaluated by the maximal rate of photosynthetic oxygen evolution, maximal photochemical efficiency of photosystem II, and chlorophyll content, net photosynthesis or photosynthetic capacity was not significantly different between the control and irradiated cells. Instead, transcription of hoxE, hoxH, or lexA, which encodes a subunit of bidirectional hydrogenase or the only transcriptional activator, LexA, for hox genes, was commonly enhanced in the irradiated cells. This transcriptional enhancement was more conspicuously observed immediately after gamma irradiation. In contrast, hydrogenase activities were found to somewhat lower in the irradiated cells. Therefore, we propose that transcription of hox genes should be enhanced by gamma irradiation in a LexA-mediated and possibly photosynthesis-independent manner and that this enhancement might not induce a subsequent increase in hydrogenase activities, probably due to the presence of post-transcriptional and/or post-translational regulatory mechanisms.