The PINHEAD/ZWILLE gene acts pleiotropically in Arabidopsis development and has overlapping functions with the ARGONAUTE1 gene

Several lines of evidence indicate that the adaxial leaf domain possesses a unique competence to form shoot apical meristems, Factors required for thi s competence are expected to cause a defect in shoot apical meristem format ion when inactivated and to be expressed or active preferentially in the ad axial leaf domain. PINHEAD, a member of a family of proteins that includes the translation factor eIF2C, is required for reliable formation of primary and axillary shoot apical meristems, In addition to high-level expression in the vasculature, we find that low-level PINHEAD expression defines a nov el domain of positional identity in the plant. This domain consists of adax ial leaf primordia and the meristem, These findings suggest that the PINHEA D gene product may be a component of a hypothetical meristem forming compet ence factor. We also describe defects in floral organ number and shape, as well as aberrant embryo and ovule development associated with pinhead mutan ts, thus elaborating on the role of PINHEAD in Arabidopsis development. In addition, we find that embryos doubly mutant for PINHEAD and ARGONAUTE1, a related, ubiquitously expressed family member, fail to progress to bilatera l symmetry and do not accumulate the SHOOT MERISTEMLESS protein. Therefore PINHEAD and ARGONAUTE1 together act to allow wild-type growth and gene expr ession patterns during embryogenesis.