AMP1 and MP antagonistically regulate embryo and meristem development in Arabidopsis

AUXIN RESPONSE FACTOR (ARF)-mediated signaling conveys positional information during embryonic and postembryonic organogenesis and mutations in MONOPTEROS (MP/ARF5) result in severe patterning defects during embryonic and postembryonic development. Here we show that MP patterning activity is largely dispensable when the presumptive carboxypeptidase ALTERED MERISTEM PROGRAM 1 (AMP1) is not functional, indicating that MP is primarily necessary to counteract AMP1 activity. Closer inspection of the single and double mutant phenotypes reveals antagonistic influences of both genes on meristematic activities throughout the Arabidopsis life cycle. In the absence of MP activity, cells in apical meristems and along the paths of procambium formation acquire differentiated identities and this is largely dependent on differentiation-promoting AMP1 activity. Positions of antagonistic interaction between MP and AMP1 coincide with MP expression domains within the larger AMP1 expression domain. These observations suggest a model in which auxin-derived positional information through MP carves out meristematic niches by locally overcoming a general differentiation-promoting activity involving AMP1.