Researches On Radio-adaptive Response In Arabidopsis Root Tips Regulated By Microgravity And Bystander Effect

Space radiation and microgravity are the main factors which have inevitable impact on living organisms during space flights.Due to both low-dose background and sudden high-dose radiation exposure during solar storms,radio-adaptive response(RAR)is a critical space radiation effect,Meanwhile,space particles at low dose and fluence rate,directly affect only a fraction of cells in the whole organism,which implement radiation-induced bystander effects(RIBE)in cellular response to space radiation exposure.Thus in this study,based on experimental system for radio-adaptive response of Arabidopsis thaliana root tip,the regulations of microgravity and bystander effect on radio-adaptative response were investigated,using the primary root elongation and the expression level of AtRAD54 and AtKU70 genes as biological endpoints.The main results were as follows:1)Establishment of an experimental system for radio-adaptive response of root Arabidopsis root tipsIn this research,the radio-adaptive responses of Arabidopsis thaliana root growth induced by whole body y-directed-irradiation and local X-ray irradiation were investigated with the primary root elongation as a biological endpoint.The results showed that RAR of root growth could be significantly induced by ? direct irradiation at the dose combinations of 10+25 Gy,10+50 Gy,10+75Gy,10+100 Gy and 10+125 Gy.Moreover,in the dose combination of 10+100 Gy,RAR can be induced at time intervals of 4,8 and 12 hours.In addition,local X-ray irradiation of the aerial parts of Arabidopsis thaliana seedlings can induce the radio-adaptive response of root growth at the combinations of 5+100 Gy and 5+125 Gy through RIBE.And the time intervals of 4,8 and 12 hours between the priming dose and the challenge dose were appropriate.2)Regulation of microgravity on the radio-adaptive response of plant root growthBased on the experimental systems of y-directed-irradiation-induced RAR and modeled microgravity simulated by a two-dimensional rotation clinostat,the regulation of microgravity on RAR of plant root growth was further demonstrated,using primary root elongation,the expression level of AtRAD54 and AtKU70 genes and GUS activity of DR5 as biological endpoints.Results showed that 8 hours microgravity treatment between the priming dose and challenge dose could significantly inhibit the RAR of the root growth in the dose combinations of 10+50 Gy,10+75 Gy and 10+100 Gy.Nevertheless,only the RAR of the root growth in the combination of 10+100 Gy in the mutant pgm-1 was inhibited.It indicates that in addition to the sediment of starch statoliths,higher plants also use other gravity-sensing mechanisms.Furthermore,treatment with exogenous IAA was shown to inhibit the induction of RAR of root growth.The expression level of AtRAD54 and AtKU70 genes induced by the priming dose was reduced by the treatment of microgravity and exogenous IAA.The concentration and distribution of auxin in DR5 root tips and the time course of DNA damage repair gene expression were disturbed by microgravity.These results indicated that auxin might affect the induction of RAR of root growth by changing the radio-sensitivity and/or DNA repair of RAM cells and microgravity suppresses the induction of RAR of root growth by interfering with the level and distribution of auxin in root tips.3)Researches on regulation of radiation target volume on radiation adaptive response of root tips induced by shoot-root bystander effectRadiation-induced bystander effects(RIBE)entail a cascade of bystander signals produced by the hit cells to the neighboring cells to regulate various biological processes including DNA damage repair.However,there is little clarity regarding the effect of radiation-targeted volume(hit cell amount)on the DNA repair potential of the bystander cells.This is especially important to understand in the context of the whole organism,where the target usually consists of multiple types of cells/tissues.To address this question,model plant Arabidopsis thaliana was locally irradiated,and the DNA repair potential of bystander root-tip cells was assessed based on their radio-resistance to subsequent high-dose radiation,i.e.radio-adaptive responses(RAR).We found that X-ray irradiation of the aerial parts(AP)of A.thaliana seedlings(5 Gy)initiated RAR in the root-tip cells,which exhibited an alleviated repression of root growth and root cell division,and reduced amount of DNA strand breaks.We also observed an improvement in the repair efficiency of the homologous recombination(HR)and non-homologous end joining(NHEJ)pathways in the bystander root tip cells.We further expanded the X-ray targeted volume to include the aerial parts with upper parts of the primary root and compared it with X-ray irradiated aerial parts alone.Comparative analysis revealed that RAR for these end points either disappeared or decreased;specifically,the repair efficiency of HR was significantly reduced,indicating that radiation-targeted volume negatively modulates the bystander DNA repair potential.In contrast,X-ray irradiation of upper part of the primary root alone did not induce RAR of the root tip cells.Thus,we propose that additional X-ray irradiation of upper part of the primary root reduces the bystander DNA repair.In summary,the research on the process and related mechanisms of radio-adaptive response in Arabidopsis root tips regulated by microgravity and bystander effect has important theoretical and practical significance in the biological effects of space radiation and the protection against space radiation.