Studies On The Mechanisms Of Growth Stimulation And Enhanced Stress Tolerance Induced By Low-dose Gamma Irradiation In Arabidopsis Thaliana

It is generally accepted that ionizing radiation differentially affects the morphology, anatomy, biochemistry, and physiology of plants in a dose-dependent manner. Radiation hormetic effects are defined as stimulation by small doses of ionizing radiation and inhibition at large doses, and this definition has been validated in a variety of organisms. The power of gammaray penetration facilitates its wide application in techniques for the mutation breeding. Recently, an increasing body of evidence shows that the growth and development of various plant species are obviously improved in response to gamma irradiation at relatively low doses. Moreover, gamma rays at low doses stimulate the tolerance to stress in plants, thus alleviating the damages of stress to plants. However, our knowledge regarding the molecular mechanism underlying the growth stimulation and enhanced stress tolerance induced by low-dose gamma irradiation remains quite limited.In the present study, the model plant Arabidopsis thalianais used as the plant materials. We aim to explore the mechanisms of plant growth promotion in response to low-dose gamma irradiation through analyzing the physiological responses and genes expression associated with signaling pathways in Arabidopsis seedlings germinated from control and gamma-irradiated seeds. Firstly, we observed the growth parameters of seedlings germinated from seeds irradiated with a range of gamma rays. The results showed that low-dose gamma irradiation(less than 100 Gy) obviously promoted the seed germination and increase of root length and fresh weight in seedling compared with control samples. In particular, 50-Gy gamma irradiation presented maximal beneficial effects; while, 150-Gy gamma irradiation severely affected the seedling growth. Furthermore, the Arabidopsis adult plants displayed increased plant height, rosette leaves and seedpods in response to 50-Gy gamma irradiation, indicating that the growth stimulation of low-dose gamma irradiation can persist from seedling period to adult plant.In normal growth condition, we found that the H2O2 contents were increased in the Arabidopsis seedlings germinated from seeds irradiated with 50-Gy gamma irradiation compared with the non-irradiated samples. The analysis of effects of low-dose gamma irradiation on gene expression of Arabidopsis RBOHs that encode the NADPH oxidases revealed that the transcription levels of Atrboh B and Atrboh F were notably enhanced compared with the controls, and this treatment resulted in maximal Atrboh D expression. In addition, the enzymatic activities of POD, SOD, and CAT were all significantly increased to different extents in response to low-dose gamma irradiation compared with the controls. In order to determine whether ROS have a role in the growth stimulation of low-dose gamma irradiation, we examined the effects of the treatment of ROS scavenger DMSO on seedling growth. The results showed that germination index, primary root length, and fresh weight of the irradiated samples were significantly inhibited or reduced, whereas these growth parameters were notinfluenced in the non-irradiated samples by the DMSO treatment.We further analyzed the ABA contents in Arabidopsis seedlings and found that gamma irradiation with the dose of 50 Gy notably induced ABA accumulation compared with the wild-type controls. The responses of the ABA deficient mutant line aba2-1 to low-dose gamma irradiation revealed that the germination index, primary root length, and fresh weight of 50 Gy gamma-irradiated mutant seedlings were not significantly different from those of the mutant control samples, whereas wild-type plants showed growth stimulation under the same irradiation conditions compared with the wild-type controls. The relationship between ABA and ROS under low-dose gamma irradiation was also determined. The results showed that H2O2 concentration was not increased by low gamma rays in the ABA-deficient mutant aba2-1 compared with the mutant control lines. Moreover, the scavenging of ROS with DMSO treatment had no obvious influence on the ABA concentration in both the aba2-1 and wild-type plants. The analysis of the expression of genes related to ABA synthesis, catabolism, and transport revealed that the expression of selected genes for ABA synthesis were obviously upregulated in response to low-dose gamma irradiation, and the expression of genes for ABA catabolism was not significantly stimulated. Interestingly, the expression of At ABCG40, which encodes an ABA-importing transporter, was significantly upregulated, whereas the expression of At ABCG25, which encodes an ABA-exporting transporter, was repressed. Our results suggest that ABA signaling is involved in the growth stimulation induced by low-dose gamma irradiation.In response to the treatments with 50 m M or 100 m M Na Cl, 50-Gy gamma irradiation presented maximal beneficial effects on germination index and root length in response to salt stress compared with controls. The contents of H2O2 and MDA in irradiated seedlings under salt stress were significantly lower than those of controls. The activities of antioxidant enzymes POD, SOD and CAT were markedly increased compared with the controls. In particular, CAT showed much higher activities than SOD and POD in both the irradiated and non-irradiated samples. The proline levels in the irradiated seedlings were also markedly accumulated. Furthermore, transcriptional expression analysis of selected genes revealed that some components of salt stress signaling pathways, such as the genes for SOS pathways including SOS1, SOS2, SOS3 and salts tress-responsive genes AREB1, DREB2, RD29 A and RD29 B, were stimulated by low-dose gamma irradiation under salt stress. Our results suggest that gamma irradiation at low doses alleviates the salt stress probably by modulating the physiological responses as well as stimulating the gene expression related to stress signal transduction in Arabidopsis seedlings.In response to the treatments with 75 μMCd Cl2 or 0.5 m M Pb(NO3)2, 50-Gy gamma irradiation maximally promoted the seedling growth, such as germination index and root length, in response to cadmium/lead stress. The contents of H2O2 and MDA in irradiated seedlings under cadmium/lead stress were significantly decreased compared with the contents in control seedlings. Moreover, 50-Gy gamma irradiation markedly stimulated the antioxidant enzyme activities of SOD, POD and CAT, and induced the accumulation of proline levels compared with the controls in response to cadmium/lead stress. A transcriptional expression analysis of selected genes revealed that some components of heavy metal detoxification, such as At PDR8, At PDR12, At ATM3 and At PCR1, were stimulated by low-dose gamma irradiation under cadmium/lead stress. Our results indicate that low-dose gamma irradiation enhances tolerance to the stress of cadmium and lead in Arabidopsis seedlings.In summary, our results indicate that ROS and ABA signaling are involved in the stimulatory effects on the seedling growth in Arabidopsis. The pretreatments with low-dose gamma irradiation can enhance the tolerance of Arabidopsis seedlings to salt and heavy metal stress by decreasing the concentrations of H2O2 and MDA, increasing the antioxidant enzyme activities which, in turn, enhance the antioxidant ability and protect Arabidopsis seedlings from the oxidative stress, triggering the proline accumulation, and up-regulating the expression of genes related to stress signal transduction.