As tools for studying adventitious organogenesis, three temperature-sensitive mutants of Arabidopsis thaliana, designated rrd1, rrd2, and rrd4, were isolated by screening with callus-mediated root redifferentiation from hypocotyl explants as an index phenotype. Phenotypes of these mutants were characterized with special regard to their tissue-culture responses. The obtained results suggest that the RRD1 and RRD2 genes participate in some fundamental processes required for active cell proliferation and that the RRD4 gene is involved in the acquisition step of competence for cell proliferation during callus initiation in hypocotyl explants.