The pasticcino (pas) mutants of Arabidopsis thaliana are a new class of plant developmental mutants; members of this class show ectopic cell proliferation in cotyledons, extra layers of cells in the hypocotyl, and an abnormal apical meristem. This phenotype is correlated with both cell division and cell elongation defects. There are three complementation groups of pas mutants (pas1, pas2, and pas3, with, respectively 2, 1, and 4 alleles). Here we describe in more detail the pas1-1 allele, which was obtained by insertional mutagenesis. The PAS1 gene has been cloned and characterized; it encodes an immunophilin-like protein similar to the p59 FK506-binding protein (FKBP52). PAS1 is characterized by an FKBP-like domain and three tetratricopeptide repeat units. Although the presence of immunophilins in plants has already been demonstrated, the pas1-1 mutant represents the first inactivation of an FKBP-like gene in plants. PAS1 expression is altered in pas1 mutants and in the pas2 and pas3 mutants. The expression of the PAS1 gene is increased in the presence of cytokinins, a class of phytohormones originally discovered because of their ability to stimulate cell division. These results are of particular relevance as they show for the first time that an FKBP-like protein plays an important role in the control of plant development.In flowering plants, morphogenesis depends on the control of the pattern and numbers of cell divisions and on the control of cell elongation. Although there are many examples of controlled patterns of cell division, we still know very little about how local patterns of cell division are established and maintained (30). In Arabidopsis thaliana, the roles of cell division control in the development of the embryo, the shoot, and the root have been extensively studied (reviewed in references 29 and 30). In the last few years, much progress has been made in this field by the isolation of mutants in which single-gene mutations affect specific modes of cell division control. Some of the corresponding genes have been cloned from A. thaliana (SHOOT MERISTEMLESS [STM] and SCARECROW [SCR]) maize (KNOTTED1), and petunia (NO APICAL MERISTEM) (reviewed in reference 30). These genes do not seem to specify components of the cell division machinery, but they are thought to act upstream in the control of cell division. The elements at the interface between genes like STM and SCR and cell cycle regulators, such as cyclins and the CDC genes, are still unknown.The growth and differentiation of higher plants is also greatly dependent on environmental stimuli, such as light and temperature, and on endogenous factors, such as phytohormones. Cytokinins (CKs) were originally discovered because of their ability to promote, along with auxins, plant cell division and organogenesis (reviewed in reference 9). Although this discovery initiated a vast amount of fundamental and applied research on the hormonal control of cell proliferation and regeneration, the mechanisms by which auxins and CKs act and interact at the molecu...