The shoot apical meristem (SAM) produces all of the cells that make up the above-ground tissues of the plant. After cells are made in the SAM, they begin to differentiate and participate in forming the organs of the shoot such as the leaves and the stem. Thus, in addition to being the source of cells for new organs, the SAM is the site of pattern formation for the shoot. The SHOOTMERISTEMLESS (STM) gene is required for establishing a SAM in Arabidopsis since STM mutants do not generate a SAM in the space between the cotyledons during embryogenesis. STM is expressed in both the central and peripheral zones of the SAM and is down-regulated in leaf primordia. STM is sufficient for establishing a meristem since overexpression of the STM gene leads to the formation of ectopic meristems. Arabidopsis plants mutant for the PINHEAD (PNH) gene make defective SAMs which fail to function as continuous sources of new organs, and instead terminate in a single central leaf or pin. Low level PNH expression marks a novel domain of positional identity in the plant which consists of the adaxial half of a leaf primordium and the SAM, and PNH expression in vascular traces marks leaf primordia before the down-regulation of STM expression. Therefore, PNH expression may be a source of positional cues for STM expression. We are using a STM::GUS reporter gene construct to study in vitro SAM formation in wild-type and pnh root explants. In this poster we present initial studies of the timing and pattern of expression of the STM gene during in vitro SAM formation in pnh and wild-type root explants. This work will give us insight into the roles of STM and PNH in both in vitro and in vivo SAM formation.

Key words: SHOOTMERISTEMLESS, Arabidopsis, pinhead, shoot organogenesis