The productivity of crops has been enhanced and maintained by the application of vast quantities of nitrogen fertilizer (over 100 million tons per year worldwide). Much of this nitrogen, in the form of nitrate, leaches from the soil and contaminates drinking water supplies (both ground and surface) and marine habitats. One approach to alleviating this hazard while preserving crop productivity is to improve nitrogen use efficiency (NUE) in plants; however, a good understanding of nitrogen metabolism and regulation is needed to develop such technologies. Much is known about nitrogen metabolism but little is known about nitrogen regulation especially nitrate sensing and induction. The aim of this proposal is to identify key regulators (both cis and trans-acting) that allow plants to sense and respond to nitrate. A genetic approach will identify new genes that are required for nitrate regulation using a nitrate-responsive reporter gene we constructed. A molecular approach will identify new cis-acting DNA regulatory elements and trans-acting transcription factors using a fast expression assay we have developed. We expect to identify new genes that control nitrate metabolism and uncover new regulatory mechanisms that impact NUE. Such tools and information should lead to the development of new technologies that will improve NUE, which, in turn, will improve crop productivity and reduce the environmental load of agriculture. Students and research personnel will be trained in plant molecular biology and genetics, and efforts will be made to educate K-12 students and the public about plant biotechnology and environmental impacts of agriculture.
Nitrate is an important source of nitrogen for crop plants, and the United States consumes 13 million tons of nitrogen fertilizer crops every year, yet much of this nitrogen in the form of nitrate leaches from the soil to contaminate water supplies and marine environments. To understand how plants assimilate nitrate, we have studied how their genes and root systems respond to nitrate. Nitrate acts a signal as well as a metabolite. It can up- or down-regulate the expression of over 1000 genes. It can also affect root development causing lateral roots to grow preferentially in regions of high nitrate, a process called root foraging. These responses enhance the ability of plants to compete for and assimilate nitrate from the soil environment. By searching for mutants that disrupt nitrate induction responses and for gene products that bind to promoter DNA required for this induction, we identified almost 20 genes that are potential key regulators that function in nitrate induction of gene expression. One of these genes encodes a transcription factor and is a member of an ancient, plant-specific gene family named after three seminal genes in the family: TEOSINTE BRANCHED1 /CYCLOIDEA /PROLIFERATING CELL FACTOR1 (or TCP). This gene (TCP20) was found to play a key role in root foraging for nitrate. Mutations in this gene blocked the ability of Arabidopsis roots to preferentially grow in regions of high nitrate, and this phenotype was due to defects in systemic signaling (signals sent from one organ to another). Our studies demonstrate that the transcription factor TCP20 functions in nitrate foraging as an essential part of the systemic signaling pathway that redirects root growth to nitrate-rich zones. An understanding of these molecular mechanisms will assist in efforts to improve nitrogen use efficiency in agriculture and thereby reduce pollution of ecosystems.
