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).
- 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.
Qiao Z, Brechenmacher L, Smith B, Strout GW, Mangin W, Taylor CG, Russel SD, Stacey G, Libault M. 2017. GmFWL1, a regulator of Glycine max nodulation, encodes a plasma membrane microdomain-associated protein. Plant, Cell & Environments (In Press).
Radwan O, Govindarajulu M, Taylor CG, Stacey G, Clough SJ. 2012. A 14-3-3 protein encoded by the SGF14c gene plays a critical role during establishing normal nodulation in soybean. Plant Physiology 160: 2125-2136.
Libault M, Govindarajulu M, Berg RH, Ong YT, Puricelli K, Taylor CG, Stacey G. 2011. A dual-targeted soybean protein is involved in Bradyrhizobium japonicum infection of soybean root hair and cortical cells. Molecular Plant-Microbe Interactions 24:10951-1060. Cover photo: Soybean root nodule cell.
Libault M, Zhang X-C, Govindarajulu M, Qi J, Ong YT, Brechenmacher L, Berg RH, Hurley-Sommer A, Taylor CG, Stacey G. 2010. A member of the highly conserved FWL (tomato FW2.2-like) gene family is essential for soybean nodule organogenesis. The Plant Journal. Vol. 62: 852-864.
Libault M, Joshi T, Takahashi K, Hurley-Sommer A, Puricelli K, Blake S, Finger RE, Taylor CG, Xu D, Nguyen HT, Stacey G. 2009, Large-Scale Analysis of Putative Soybean Regulatory Gene Expression Identifies a Myb Gene Involved in Soybean Nodule Development. Plant Physiology 151:1207-1220.
Dalton DA, Boniface C, Turner Z, Lindahl A, Kim HJ, Jelinek L, Govindarajulu M,. Finger RE, Taylor CG. 2009. Physiological Roles of Glutathione S-transferases in Soybean Root Nodules. Plant Physiology 150:521-530.
Govindarajulu M, Kim S-Y, Libault M, Berg RH, Tanaka K, Stacey G, Taylor CG. 2009. GS52 ecto-apyrase plays a critical role during soybean nodulation. Plant Physiology 149:994-1004. Cover photo >
Govindarajulu M, Elmore JM, Taylor CG. 2008. Evaluation of viral promoters in transgenic soybean roots and nodules. MPMI 21:1027-1035 (Front Cover).