Mutation in the Arabidopsis PASTICCINO1 Gene,Which Encodes a New FK506-Binding Protein-Like Protein,Has a Dramatic Effect on Plant Development

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 molecular level are unknown. Steroid-like plant growth factors termed brassinosteroids (BR) were first characterized as inducing cell elongation in synergy with auxin, but recently these hormones have also been found to control plant cell divisions and morphogenesis (15; reviewed in reference 11).The genetic and molecular analysis of hormonal mutants is proving to be a powerful tool for unraveling the mode of action of these molecules. In an attempt to understand the mode of action of CKs and their molecular relationships with auxins in promoting plant cell division, we looked for Arabidopsis mutants with phenotypes which were affected by exogenously applied CKs. We have previously reported the isolation of the pasticcino mutants (pas1, pas2, and pas3) which are affected in both embryonic and vegetative development. Their phenotypes are similar to that of wild-type shoots which have been regenerated in vitro from explants, in the presence of an unbalanced auxin/CK ratio in the medium (12).The pas1-1 mutant was isolated from the transfer DNA (T-DNA) mutant collection of INRA-Centre de Versailles (2, 12). Here we describe the cloning of the PAS1 gene from the T-DNA-tagged pas1-1 allele. PAS1 codes for an immunophilin-like protein similar to the FK506-binding proteins (FKBP). We also demonstrate that the PAS1 mRNA steady-state level is increased in the presence of CK and that PAS1 gene expression is affected in the other pas mutants.