REVOLUTA regulates meristem initiation at lateral positions

While the shoot apical meristem (SAM) is indirectly responsible for the initiation of all above-ground postembryonic organs, in most plants the vast majority of these organs are directly initiated by lateral meristems. In Arabidopsis thaliana, the lateral meristems include flower meristems (FMs), which form on the flanks of the SAM, and lateral shoot meristems (LSMs), which develop in leaf axils. While significant progress has been made on the molecular genetic basis of SAM initiation during embryo development, relatively little is known about the initiation of meristems at lateral positions. Here we have characterized the phenotypic consequences and genetic interactions of mutations in the REVOLUTA (REV) gene, with an emphasis on the role of REV in lateral meristem initiation. Our observations indicate that REV is required for initiation of both LSMs and FMs, and likely acts in the same pathway as, and upstream of, known meristem regulators. We identified the REV gene and found it encodes a predicted homeodomain/leucine zipper transcription factor that also contains a START sterol-lipid binding domain. REV is the same as the IFL gene. REV was expressed at the earliest stages of LSM and FM formation. Within the inflorescence shoot meristem, REV expression appeared to predict 3--5 incipient flower primordia on the flanks of the SAM, and REV expression at stage 1 and stage 2 matched that of WUS and STM, respectively. We propose that REV acts at lateral positions to activate the expression of known meristem regulators.