Irradiation With Low-Dose Gamma Ray Enhances Tolerance To Heat Stress In Arabidopsis Seedlings

High temperature is one of the major limiting factors that affect plant growth.At the cellular level,heat stress differentially affects the stability of various proteins,fluidity of membrane lipids,RNA synthesis and organization of cytoskeleton,and inhibits activities of metabolic enzymes.These injuries lead to reduced ion flux,metabolic imbalance,accumulation of toxic by-products,such as reactive oxygen species?ROS?,and eventually starvation and inhibition of growth.Evidences have showed that gamma irradiation at low doses can stimulate growth and development,alleviate the damage,and enhance the tolerance to environmental stress in plants.However,the knowledge regarding the mechanisms underlying the enhanced tolerance induced by low-dose gamma irradiation is far from fully understood.In this study,we use model plant Arabidopsis as research materials to investigate the physiological and molecular mechanisms of heat stress alleviated by low-dose gamma irradiation.Firstly,the Arabidopsis seeds were exposed to a range of gamma rays before subjected to heat treatment,and the growth of Arabidopsis thaliana seedlings under heat stress was observed.The results showed that in response to heat stress,the low-dose gamma irradiation?25-75 Gy?significantly increased the germination index,root length,survival rate,and fresh weight compared with controls.Particularly,gamma ray treatment with a dose of 50 Gy exhibited maximal heat tolerance in terms of the overall growth parameters compared with other doses.Thus,we used seeds that were irradiated with the dose of 50 Gy for the subsequent studies.Abiotic stress usually stimulates the production of reactive oxygen species and lipid peroxidation.In order to reveal the redox state of Arabidopsis seedlings,we use the colorimetric analysis method to determine the production rate of super oxygen anion?O₂^·–?,concentration of H2O2 and MDA level in control?0 Gy?and 50 Gy gamma-irradiated samples in response to heat stress.The results showed that the production rate of superoxide radical and contents of hydrogen peroxide and malondialdehyde in the seedlings irradiated with 50-Gy gamma rays under heat stress were significantly lower than those of controls.Our results suggest that low-dose gamma irradiation can alleviate the ROS damage of Arabidopsis seedlings in response to heat stress.Activities of several representative antioxidant enzymes,including SOD,POD,and CAT were determined to assess the physiological mechanism of low-dose gamma irradiation in regulation of these antioxidant enzymes upon heat stress.The results showed that the SOD,POD,CAT in the seedlings irradiated with 50-Gy gamma rays under heat stress were significantly higher than those of controls.Our results indicated that that low-dose gamma irradiation can enhance the ability of Arabidopsis seedlings in response to heat stress.To explore whether the increased tolerance to heat treatment under low-dose gamma irradiation is related to the production of glutathione?GSH?and proline,the GSH content and proline level in seedlings derived from control and gamma-irradiated seeds in response to heat stress were analyzed.The results showed that the GSH content and proline level in the gamma-irradiated seedlings were significantly increased compared with the controls.To gain more insights into the molecular mechanisms underlying the tolerance to heat stress induced by low-dose gamma irradiation,we analyzed the expression of several selected genes associated with heat stress response,such as Hsp70,Hsp18.2,Apx1,Apx2,ABA2,ABA4,AAO3 and MBF1c.The results showed that Hsp70 and Hsp18.2 were differentially induced by heat treatment.The enhanced transcript level of Hsp70 was observed only under heat shock for 6 h in the irradiated seedlings compared to the levels in the control seedlings.Similarly,higher expression level of Hsp18.2 was recorded in the irradiated seedlings compared with the controls in response to heat stress.The expression levels of Apx1 and Apx2,were highly up-regulated in seedlings irradiated with 50-Gy gamma rays in response to heat treatment compared with the controls.The higher transcript levels of abscisic acid?ABA?synthesis genes,such as ABA2,ABA4 and AAO3,were detected in gamma-irradiated seedlings exposed to heat stress compared with control seedlings,although enhanced expression of AAO3 was detected only by heat treatment for 6 h.Furthermore,the expression of MBF1 c was significantly up-regulated in gamma-irradiated seedlings upon heat stress.Particularly,the transcript of MBF1 c was markedly induced by heat shock for 40°C 6 h.The results showed that the expression of several selected genes associated with heat stress response in the gamma-irradiated seedlings were significantly up-regulated compared with the controls.In conclusion,our results suggest that low-dose gamma irradiation can modulate the physiological responses as well as gene expression related to heat tolerance,thus alleviating the stress damage in Arabidopsis seedlings,and enhancing the the heat tolerance of plants...