Two Arabidopsis mutants atmyb123 and atkor1 were identified from the T-DNA insertion knockout mutant lines SAIL_005260 and SAIL_2_G11,respectively,and then a double mutant atmyb123/atkor1 was established by crossing method.The two mutants are lacking expression for ATMYB123 and ATKOR1 genes,respectively,which two were found to be tightly related to root development in Arabidopsis thaliana.The results obtained here showed that lack of ATMYB123 gene in expression led to a slow growth of plant rosettes and a yellow skin of seeds in Arabidopsis,while lack of ATKOR1 gene in expression had no marked effects on these two factors.Any one of the two genes ATMYB123 and ATKOR1 knockout extremely repressed the root development in Arabidopsis,especially the knockout of ATKOR1 gene,the mutant atkor1 showed only one third of length of roots as compared to wild type (WT).Interestingly,the double mutant atmyb123/atkor1 exhibited the characteristics of the single mutant atmyb123 has in plant rosette morphology and seed skins but presented intermediated root length between the two single mutants.In addition,the growth trend of roots among the three mutants had no fundamental changes when the plants were cultivated under different pH,NaCl treatments and GA concentration conditions,which imply that these three factors were not concerned in the root shortening event induced by lack of any one of ATMYB123 or/and ATKOR1 proteins in A.thaliana.These results suggest that both ATMYB123 and ATKOR1 genes participate in the root development of Arabidopsis and a specific relationship in functions exist between the two proteins,ATMYB123 and ATKOR1.The transcription factor ATMYB123 might act as a major regulator of ATKOR1 protein for participating the control of root development in Arabidopsis.