Studies of the shoot apical meristem: The PINHEAD gene of Arabidopsis thaliana

Plants produce proximal-distal growth axes with two types of growth potential. An axis can be indeterminate, in which growth continues indefinitely through the action of an apical meristem. Or an axis can be determinate, in which growth is limited to the production of a single organ or a discrete set of organs. The indeterminate shoot axes of Arabidopsis pinhead mutants frequently are transformed to a determinate state. PINHEAD is expressed in the central domain of the developing plant: the developing vascular tissue, the shoot apical meristem and the adaxial (upper) sides of lateral organ primordia. In Arabidopsis there are eight genes with significant similarity, to PINHEAD. In particular, the ARGONAUTE gene is highly similar to PINHEAD and is required for proper leaf development. Axis determinacy defects similar to those caused by pinhead mutations also are found in argonaute mutants. The expression pattern of the ARGONAUTE gene and the phenotype of individuals carrying mutations in both PINHEAD and ARGONAUTE indicate that ARGONAUTE overlaps in function with PINHEAD. The phenotypes of pinhead mutants carrying one or two mutant alleles of argonaute indicate that PINHEAD contributes to the polarity of lateral organs and to pattern formation and gene expression in early stages of embryogenesis, respectively. Ectopic expression of PINHEAD on the abaxial (lower) sides of lateral organs results in involute blades. This phenotype correlates with a loss of coordination of cell number between the two surfaces of the blade, suggesting that PINHEAD has a role in cell division coordination. More strikingly, in pinhead mutants, abaxial expression causes transformation of the determinate cotyledon axis to an indeterminate state. Taken together, the evidence presented seems to indicate that PINHEAD provides positional information in the radial dimension and that new growth axes interpret this information to determine their position with respect to other growth axes. Each axis then performs determinate or indeterminate growth depending on where it perceives its radial location to be in the plant body.