Overcoming recalcitrance - Auxin response factor functions in plant regeneration.

De novo meristem formation in tissue culture critically depends on the correct organization of hormonal domains, which is followed by expression shoot meristem pattern genes. The genetic basis of plant regeneration is fragmentary, but mutant studies demonstrate that signaling through MONOPTEROS (MP)/AUXIN RESPONSE FACTOR 5 is necessary for the formation of shoots from Arabidopsis calli. Most strikingly, variants of MP, uncoupling MP activity from negative regulation by Aux/IAA proteins, showed that MP is also sufficient for promoting de novo shoot formation even from normally recalcitrant tissues. In this function MP acts through pathways involving the homeobox transcription factor SHOOT MERISTEMLESS (STM) and AP2 domain transcription factor CYTOKININ RESPONSE FACTOR2 (CRF2). Our findings provide an entry point to better address the molecular genetics underlying divergent regeneration properties and demonstrate the potential of ARF-derived constructs as novel genetic tools to develop high frequency regeneration systems in recalcitrant explants and species.