Plants, unlike animals, are unable to run away from adversity (e.g., herbivores and drought) or toward their next meal (e.g., sunlight and water). Instead plants have evolved strategies that involve organ-bending in order to respond to changes in their environment. Dramatic, rapid, and reversible changes in morphology can result from differential growth -- unequal cellular elongation in one position of an organ relative to an opposing position. One set of differential growth responses that are extremely important for the normal development of plants are the tropic responses, such as phototropism (organ bending in response to directional light) and gravitropism (organ bending oriented by gravity). Analyses of Arabidopsis thaliana (mouse ear cress) mutants that affect various aspects of phototropism have defined several components of the signaling system regulating the response, and have led to the development of a model for the induction of phototropic bending in response to low intensity light. First, it has been proposed that light activates the photoreceptor phototropin 1, which is in a physical complex with a novel plant-specific protein NPH3; this complex influences the activity and/or localization of transporters of the plant growth hormone auxin. This proposed change in transporter activity is believed to lead to the formation of a lateral gradient of hormone across the stem, with auxin accumulating on the "shaded" side away from the directional light stimulus. Next it is hypothesized that the increased level of auxin in the "shaded" side of the stem stimulates the destruction of MSG2/IAA19, a protein that otherwise complexes with and inhibits NPH4/ARF7, releasing a repressed state and allowing for NPH4/ARF7-dependent expression of genes necessary for increased cell elongation on the "shaded" side. This drives the differential growth response. A number of "tropic stimulus-responsive genes" have been identified which are likely controlled by NPH4/ARF7. The current project is aimed at 1) understanding how the expression of these genes is regulated in a temporal and spatial context in Arabidopsis thaliana where mutants can be brought to bear, and 2) determining the function of "tropic stimulus-responsive genes" in the development of phototropism and gravitropism. Results from these studies will provide fundamental new insights into the regulation of stimulus-driven changes in cell growth in plants, as well as how plants integrate signals from multiple sources, both endogenous and exogenous. The broader impacts of these studies are several-fold. This project will provide theoretical and practical training for at least one high school student, two undergraduate students, two PhD students, and one technician. The high school student, one undergraduate, and the technician are all female, and one of the graduate students is Hispanic. A conscious effort will be made to recruit into the remaining undergraduate position a female and/or under-represented minority. Each of the individuals trained will be in a position to add dramatically to the scientific work force in the US when they leave the project. Moreover, insights into the regulation of plant growth resulting from these studies, which will be published in publicly-accessible and high-profile scientific journals, can be utilized in crop improvement programs aimed at increasing economic and ecological aspects of US agriculture.
