The roles auf auxin and cytokinin in cell proliferation

Plants have evolved an amazing repertoire of mechanisms to respond to alterations in their environment. Part of these adaptation processes is the ability to control organogenesis dependent on environmental parameters. Apparently, the enormous flexibility leading to a variety of plant shapes requires tight control mechanisms. Among those, the decision as to whether a plant cell remains meristematic and undergoes further cell divisions or whether it starts to differentiate is one of the most critical events. Two growth regulators, auxin and cytokinin, are key elements required for the signaling events leading to either cell differentiation or cell division. However, elements of a signal transduction cascade, which integrate the signals of both growth regulators, have not been characterized to date. A combined treatment of plant cells with auxin and cytokinin leads to a pronounced induction of cell division. Based on this phenotype we have characterized two arabidopsis mutants in which the response two both growth regulators is altered. Mutants defective in PROPORZ1 (PRZ1) or PROPORZ2 (PRZ2) respond to either one of the principal regulators of cell division with the formation of meristematic tissue. These findings suggest that the corresponding gene products participate in signaling events controlled by the concerted impact of auxin and cytokinin. A detailed characterization of the PRZ genes is goal of the current project application. Moreover, additional elements involved in PRZ1-dependent signaling should be characterized. From the molecular characterization of the PRZ genes we expect to obtain fundamental insight into the mechanism as to how phytohormones interact with the cell cycle machinery - a critical aspect in plant adaptation processes.

Plant development is characterized by a remarkable flexibility in the control of growth rates and organ formation. These characteristics represent a major difference between plants and animals, which undergo pattern formation following a pre-determined morphogenetic blueprint. In contrast, during the entire life cycle (up to several hundred years) plant growth integrates genetic switches that control organ formation with a large spectrum of signaling pathways that sense and respond to variations in environmental parameters. These variations lead to modifications in morphogenetic programs, visible as adaptive growth responses in the course of a plant`s life cycle. A key role in the control of cell proliferation and differentiation processes has been attributed to the concerted impact of auxin and cytokinin, two phytohormones that act as principal regulators of organ initiation and subsequent differentiation. Specifically, the involvement of such phytohormone signals in the control of cell cycle progression was shown to play a central role in the regulation of plant cell proliferation and differentiation. We have identified PROPORZ1 (PRZ1, also known as AtADA2b), as component of a hypothetical Arabidopsis chromatin-remodeling complex involved in the regulation of phytohormone-mediated control of core cell cycle regulators. As a consequence, mutants deficient in PRZ1 are impaired in the control of auxin and/or cytokinin-induced growth responses. We provided evidence for an immediate involvement of PRZ1 in the transcriptional regulation of cell cycle genes, which in turn feeds back on the control of cell differentiation. Additional proteins required for the hormonal control of cell proliferation and that might act in conjunction with PRZ1 have been identified. Mechanisms by which these proteins interact in the control of adaptive plant growth and development are currently investigated.