DNA Damage Induces Stem Cell Formation From Differentiated Leaf Cells In Physcomitrella Patens

DNA damage can result from intrinsic cellular processes and from exposure to stressful environments.Such DNA damage generally threatens genome integrity and cell viability.Wounding is a general trigger for reprogramming differentiated plant cells into stem cells.In early land plant,Physcomitrella patens(Physcomitrella),leaf excision can cause differentiated leaf cells next to the excision site(edge cells)reprogram into protonema apical stem cells without phytohormones.During the stem cell induction through reprogramming somatic cells,the DNA damage resulted in cell cycle arrest or PCD is generally regarded as interference of this process.To analyze the role of DNA breaks in the reprogramming,we performed related experiments in Physcomitrella and got following results:1.During the investigation of the function of Topoisomerasel in the moss Physcomitrella,we found that the transient induction of DNA strand breaks can trigger the reprogramming of differentiated leaf cells into stem cells.When we exposed intact leafy shoots(gametophores)to Zeocin,an inducer of DNA strand breaks,cell cycle was reactivated in some leaf cells,which subsequently initiated tip growth and underwent cell divisions to form protonema apical stem cells,which have the ability to form new gametophores.2.This reprogramming induced by DNA strand breaks was not always accompanied by cell death,which can serve as the wounding signal,indicating that the DNA strand break-induced reprogramming is not evoked by wounding,a general trigger of the reprogramming3.Temporal BLM addition induces both DSBs and SSBs when it stably induces reprogramming.On the other hand,Temporal zeocin or CPT does SSBs but we could not detect DSBs,we chose zeocin for the further analyses4.Damaged DNA continuously accumulated during zeocin treatment and 1 hour after the treatment,then decreased gradually.Damaged DNA was repaired to the initial level at 30 hours after the removal of zeocin which is around 30-40 hours earlier than the protrusion of protonema apical stem cells from leaf cells5.We found that the promoter activity of STEM CELL-INDUCING FACTOR 1(STEMIN1),a reprogramming factor,was activated in some of the leaf cells exposed to the DNA break inducer,which eventually became stem cells6.Deletion of STEMIN1 and its two closely related genes(STEMIN2 and STEMIN3)caused the loss of the stem cell formation ability in response to the DNA strand breaks.7.Molecular analysis revealed that the DNA damage sensor Ataxia Telangiectasia Mutated and RAD3-related(ATR)kinase,but not Ataxia Telangiectasia Mutated kinase(ATM),is indispensable for this process8.Furthermore,In ?atr mutants,the DNA strand breaks could not induce the expression of STEMIN1,suggesting that the induction of STEMIN1 is dependent on ATRTaken together,our work revealed that DNA strand breaks,which are usually considered to pose a severe threat to cells,can trigger cellular reprogramming towards stem cells from differentiated cells,and the DNA damage sensor ATR and reprogramming factor STEMINs are indispensable in this process.