Nitrogen Fixing Bacteria

Nitrogen-Fixing Bacteria

Nodulation in soybean is a highly regulated and interactive process between the soybean plant and the nitrogen-fixing bacteria, Bradyrhizobium japonicum.  Understanding the process of how this interaction occurs could significantly impact agriculture today through reduced use of nitrogen based fertilizers. Nodulation is a highly, host-specific interaction where, with few exceptions, specific rhizobial strains infect a limited range of plant hosts. Plants secrete (iso)flavonoids that are recognized by the compatible bacteria resulting in the induction of nodulation genes. These nodulation genes encode enzymes that synthesize a specific lipo-chitin nodulation signal (Nod signal), which activates many of the early events in the root hair infection process. The infection process occurs by the bacteria entering the plant via the root epidermis and inducing the reprogramming of root cortical cell development and the formation of a nodule. In the most well studied cases, infection occurs through root hairs. 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 and eventually ramify into the root cortex. Before the infection thread reaches the base of the root hair cell, the root cortical cells are induced to dedifferentiate, activating their cell cycle and dividing to form the nodule primordium. In addition to the cortical cells, pericycle cells are also activated leading to some cell divisions. When the infection thread reaches the cells of the developing primordium, the bacteria are released into cells via endocytosis. Inside a plant cell, the bacteria are enclosed in vacuole-like structures (symbiosomes) where they differentiate into bacteroids. It is within these symbiosomes that the bacteria convert N₂ to NH₃. The nodule is a true organ since there is cellular specialization. For example, in addition to infected plant cells, uninfected plant cells also carry out the function of nitrogen assimilation and a well-developed, symplastic transport system allows the exchange of nutrients between the nodule and peripheral vascular tissue (Kijne, 1992).

Research Projects:

• Functional Genomics - A collaborative project with the University of Missouri (Gary Stacey) and University of Illinois (Steve Clough) examining the functional genomics of B. japonicum induced genes.
• Promoter Study - Examining the use of different viral promoters and their expression levels during nodulation.
• Apyrase - Examining the role of extracellur ATP and its role in regulating nodulation.