The CUP-SHAPED COTYLEDON genes promote adventitious shoot formation on calli

In Arabidopsis, shoots regenerate on calli derived from hypocotyl explants. Mutations in CUC1 and CUC2 (CUP-SHAPED COTYLEDON) reduce the induction of adventitious shoots on calli. To elucidate the function of CUC1 and CUC2 during this process, these genes were overexpressed in calli. Our results indicate that CUC1 and CUC2 promote adventitious shoot formation on calli. To clarify their functions, the concentrations of auxin and cytokinin in the shoot-inducing medium were changed. Calli of the single and double mutants of cuc1 and cuc2, as well as calli overexpressing either of the CUC genes, responded similarly. This suggests that neither of the genes are involved in synthesis or sensitivity of these hormones. During embryogenesis, CUC1 and CUC2 induce shoot apical meristem formation through activation of STM (SHOOT MERISTEMLESS). Our analyses using the stm mutant and an STM::GUS construct suggest that CUC1 and CUC2 also function upstream of STM even in calli.