PROJECT SUMMARY/ABSTRACT The mechanisms by which organisms alter their growth and development in response to changes in their ambient environment are largely unknown. Plants exhibit an enormous array of phenotypic plasticity because most plant organs do not arise until after the seed germinates, allowing organ size and shape to be optimized to the local environment. Because plants are sessile and photosynthetic, they are especially attuned to their light environment. Light influences every developmental transition, from seed germination to flowering, having particularly dramatic effects on the morphogenesis of seedlings. During this stage of development, light can alter the expression of thousands of genes within a few hours. Light signals do not act autonomously, but are integrated with seasonal/diurnal changes in temperature and intrinsic programs to specify correct spatial and temporal regulation of gene expression, organelle development, and cellular differentiation. The proposed studies aim to answer the following questions: (1) What are the key signaling pathways and mechanisms that translate information from the environment into changes in growth rate or the initiation of flowering? (2) Are there multiple routes to the same response? (3) In the absence of a central nervous system, how do multicellular organisms integrate a multitude of signals that are spatially and temporally separated and often send conflicting messages? (4) Can we develop predictive models of plant growth? These questions will be addressed using three stages of the plant life cycle that are well studied and known to be highly sensitive to environmental input, namely shade-induced activation of growth, light intensity-induced inhibition of growth, and shade-induced early flowering. In these contexts we will ask how a diverse set of transcriptional responses is generated by the activation of a small number of signaling pathways. The resolution of the studies will be increased using genetic sensors for specific hormone signal transduction pathways. These data will provide an in-depth analysis of hormonal responses to combinations of abiotic stresses. The diverse responses that plants exhibit to light and other environmental parameters provide a unique model system for understanding signaling pathways that regulate phenotypic plasticity. The considerable genetic resources available for each individual signaling pathway make these experiments feasible and timely. Our efforts should contribute significantly to knowledge of complex signal transduction networks and our emerging understanding of how they modulate metabolism in a spatial and temporal manner in multiple kingdoms.