Submitted to: International Conference on Legume Genomics and Genetics
Publication Type: Proceedings
Publication Acceptance Date: 5/6/2004
Publication Date: 6/7/2004
Citation: Stacey, G., Huang, S., Wan, J., Zou, J., Clough, S.J., Scheffler, B.E., Kim, S., Nguyen, H. 2004. DNA microarray and proteomic analysis of root hair infection. International Conference on Legume Genomics and Genetics. p. 183-184. Interpretive Summary: Plants need nitrogen in the form of ammonia which is often limiting. Legumes have developed a reaction with soil rhizobia bacteria that 'fix' atmospheric nitrogen to ammonia. We are utilizing DNA microarray and proteomic methods to study legume root hair infection by rhizobia. These methods are allowing for identification of specific genes and proteins that play a role in nitrogen fixation. Soybean is the legume of choice for these studies since its large size permits the isolation of purified root hairs in gram quantities making the application of proteomic methods more practical. This information will be useful to any scientist interested in breeding, genetics, gene structure, and gene function and can lead to improved plant health by enhancing nitrogen fixation.
Technical Abstract: In most legumes, initiation of nitrogen fixing symbiosis occurs by infection of root hair cells by the compatible rhizobium. This infection results in the de novo formation of a novel organ, the nodule, in which the bacterium fixes nitrogen providing its host plant an advantage in environments where this element is limiting. The establishment of the symbiosis involves a complex interplay between host and symbiont, which is orchestrated by the exchange of diffusible signal molecules. Root hair infection is remarkable since it involves a unique series of cellular changes. For example, the infection thread, by which the bacteria gains entry into the cell, is a novel structure in plant biology. The molecular mechanisms that give rise to this structure and, indeed, to most of the cell biological changes occurring in the root hair are unknown beyond the microscopic level. We are using functional genomic tools to unravel the fascinating cell biology occurring during root hair infection of soybean by its compatible symbiont Bradyrhizobium japonicum. This plant is preferred for our studies since its large root size allows the isolation of gram quantities of highly pure root hairs allowing both DNA microarray and proteomic studies. In addition, facile genomic tools (e.g., reverse genetic methods) are also available for this plant, which will allow detailed studies of gene function. Recently, considerable attention has been focused on model legumes species, Medicago truncatula and Lotus japonicus. Although these species clearly present advantages for genetic studies, their small size makes them less than ideal for biochemical investigations. Therefore, a comparative approach utilizing soybean for gene identification, but utilizing both soybean and the model legumes to study gene function will allow us to take full advantage of the genetic tools (e.g., mutants) of the model legumes, while benefiting from the larger size of soybean. Root hairs are specialized projections from modified epidermal cells of the root, which are thought to increase root surface area allowing greater nutrient and water uptake. The infection process occurs by the rhizobia entering the plant via the root hair cell and inducing the reprogramming of root cortical cell development and the formation of a nodule. The first observable event in the infection process is the curling of the root hair, which likely occurs via the gradual and constant reorientation of the direction of root hair growth. The bacteria become enclosed within the root hair curl where the plant cell wall is degraded, the cell membrane invaginated and an intracellular, tubular structure (infection thread) is initiated. It is within this infection thread that the bacteria enter the root hair cell, ramify into the root cortex, and eventually populate the nodule.