The aim of herein proposed project is to study the effect of signaling molecules, primarily plant growth regulators, on the regulation of plant development at the molecular, genetic, biochemical and physiological levels. Although plant growth regulators play a role in many aspects of plant development, their regulatory pathways, mechanisms of action and interactions remain poorly understood. In continuation of a long laboratory tradition, we will focus on three groups of plant growth regulators: auxins, gibberellins and cytokinins. The Christmas rose flower (Helleborus niger L.) will be used as a model for part of our studies, since it undergoes atypical morphological changes following fertilization. We have recently characterized these changes on the basis of ultrastructure and photosynthetic capacity, and have shown them to be dependent on signaling molecules originating in developing fruits. Preliminary results implicate a significant role of cytokinins and gibberellins, thus, we plan to analyze those plant growth regulators and their metabolism. We will try to identify genes involved in the biogenesis of plant growth regulators and investigate their differential expression. We will also identify and analyze plant growth regulators that orchestrate embryo growth and development as dependent on different nitrogen forms in the growth medium. In these experiments we will use pumpkin somatic embryos (Cucurbita pepo L.) as a model system. Furthermore, our intention is to perform structural and functional characterizations of proteins involved in the metabolism and homeostasis of plant growth regulators, and to monitor molecular interactions (protein-substrate, protein-protein, protein-DNA). We will also investigate oxidative stress of plant tissues as well as phenolic antioxidants as dependent on endogenous and exogenous parameters. The proposed research will be performed using the methods of molecular and classical experimental biology, as well as structural and analytical biochemistry. We expect that the results of our research will contribute to the understanding of the metabolism and mechanism of action of plant growth regulators at the molecular and cellular levels. We predict that the results obtained will shed light on the role of these signaling compounds in plant growth and development. The new basic information on the mechanisms of plant growth and development will be widely applicable in agronomy and biotechnology.