Arabidopsis mutants with increased organ regeneration in tissue culture are more competent to respond to hormonal signals

Shoots and roots can be regenerated through organogenesis in tissue culture by subjecting plant explants to the appropriate regime of hormone treatmen ts. In an effort to understand the control of shoot organogenesis, we scree ned for mutants in Arabidopsis thaliana (L.) Heynh. Columbia ecotype for en hanced shoot development at sub-optimal concentrations of cytokinin. Mutant s in four different complementation groups were identified, one of which re presents a new locus named increased organ regeneration1 (ire1) and another that is allelic to the previously identified pom1/erh2 mutant. Although th e mutants were selected for their response to cytokinin, they were neither hypersensitive to, nor were they over-producers of cytokinins. The mutation s identified in this study not only promote more robust shoot production in tissue culture, but also enhance green-callus and root formation. We inter pret this to mean that, in tissue culture, IRE genes act before organ speci fication during the time when root explants acquire the competency to respo nd to organ formation signals. In normal plant development, IRE genes may d own-regulate the competency of vegetative tissue to respond to hormonal sig nals involved in shoot and root organogenesis.