|Proposed Statement of Research
Judith A. Latch
University of Texas Health Science Center at Houston
Mineral elements needed by plants are taken up by the roots in solution and are carried through the plant body by the transpiration system. All but one of the minerals essential for plants are derived from the weathering of rocks; the exception is nitrogen, the nutrient for which plants have the highest requirement. Most plants cannot use gaseous nitrogen and are dependent upon nitrogen-containing ions--ammonium and nitrate--from the soil. If nitrogen lost from soil were not continually replaced, virtually all life on the earth would cease. Nitrogen is returned to the soil by nitrogen-fixation, the process by which atmospheric nitrogen is incorporated into organic compounds. Most nitrogen-fixation is carried out by a few types of prokaryotes; symbiotic bacteria are the most important of these microbes in terms of total nitrogen fixed on a world-wide basis.
The beneficial effects to the soil of growing leguminous plants have been apparent for centuries. Where these plants are cultivated, excess nitrogen is released into the soil; it is then accessible to other plants. In modern agriculture, it is common to rotate a leguminous crop, such as alfalfa, with a nonleguminous one. Legumes are either harvested, leaving the nitrogen-rich roots, or--even better--they are plowed back into the soil. This method often eliminates the need for chemical fertilizers. The cost of these synthetic chemicals is great--from both monetary and environmental standpoints. More than 50 million tons of them are incorporated into soil each year, and they are produced by commercial nitrogen-fixation processes in which the requisite energy is provided by non-renewable fossil fuels. More than one-third of the total energy required to produce a crop of corn is used in manufacture, transportation, and application of chemical fertilizers. From this perspective, the use of recombinant DNA technology to increase biological nitrogen-fixation acquires a considerable practical significance. The optimization of this natural process should be of particular interest to NASA both for furthering "Project Earth" and for development of a self-sustaining agricultural system in space.
The most common nitrogen-fixing bacterium is Rhizobium, which invades roots of legumes; Rhizobium melilotispecifically infects alfalfa (Medicago saliva) and establishes this symbiosis. Communicating by chemical signals, the alfalfa plant and R. melilotiset up an intricate developmental program in which specialized structures are formed that are essential for nitrogen-fixation; plant roots develop nodules and the rhizobia- after penetrating the cytoplasm of nodule cells--differentiate into branched, non-dividing bacteroids that synthesize the enzyme nitrogenase.
What causes bacteroids to stop dividing? My project addresses this question by focusing on the cell division gene ftsZ, which may be a key player in the cell cycle alterations characteristic of differentiating bacteroids. FtsZ is essential for initiating cell division in other bacteria and is a target for various cell division inhibitors. Rhizobium possesses two copies of this gene. My hypothesis is that the product of ftsZ1is essential for free-living bacteria, while that of ftsZ2 is required for differentiation in planta. The two main thrusts of this study are to investigate the regulation of ftsZ by transcription analyses and to examine differential expression and localization of the two FtsZs--in free-living cells and in bacteroids--by fusions of the genes to *green fluorescent protein (GFP) and blue fluorescent protein (BFP) and by other experiments. I am also conducting in vitro studies to characterize R. meiloti's cell branching phenotype; the purpose of this work is to demonstrate the specificity of the branching phenomenon under various physiological constraints and to determine how branching is initiated. (Under some of these conditions Rhizobium fix nitrogen.)
*These fluorescent tags could also be useful in NASA's on-board experiments, as they provide safe, easily-traced markers.
The work being conducted in this laboratory should carry significance for U.S. agriculture--on Earth and in space, since it will contribute to knowledge of the physiology and genetics of Rhizobium. This information is vital for expanding the usefulness of nitrogen-fixing symbioses in agriculture so that biological nitrogen-fixation can supplant expensive synthetic nitrogen fertilizers. Also new genetic tools for manipulation of this organism will become available as a result of this work. Since the work is toward understanding master cell cycle switches in a controlled pathogen, it should give a basis for understanding and controlling a variety of bacterial infection processes in both animals and plants. Furthermore, the method of using fluorescent protein tags should have a wide variety of applications.
Wednesday, 26-Mar-2003 22:09:43 CST
CSR/TSGC Team Web