Auxin regulates gene expression in Arabidopsis through Auxin Response Factors (ARFs). While most ARF functions remain elusive, patterning functions have been assigned to some, including MONOPTEROS(MP/ARF5), which promotes stem cell formation in the root and shoot apical meristem. Thomas Berleth's group now show that MP primarily functions to counteract the activity of the carboxypeptidase ALTERED MERISTEM PROGRAM 1 (AMP1), which restricts meristem size. Their analysis of single and double mutants in Arabidopsis, reported on , reveals that in the absence of AMP1 activity, MP patterning activity is largely dispensible, and that in MP mutants, meristem cells differentiate because of unimpeded AMP1 activity. These researchers propose that MP represses AMP1's activity and maintains niches, an idea that is supported by the two genes' overlapping expression domains: where they overlap, antagonism occurs. As AMP1 transcript levels are normal in MP mutants, this antagonism is not transcriptionally regulated. Moreover, MP and AMP1 localise to different cellular compartments, so exactly how this antagonism occurs remains unknown.
