Forschungsfelder
[Bradyrhizobium japonicum]
[Correlated physical and genetic map of the B. japonicum chromosome]
[Type III secretion systems in rhizobia]
Bradyrhizobium japonicum
Sustainable agricultural management combines the need for sufficient crop yields and the efficient use of available resources. Legumes belong to the most important crops.
They combine two advantages:
a) their seeds supply oil and proteins of high quality and
b) they are able to grow on combined nitrogen supplied by endosymbiotic bacteria, collectively called rhizobia.
One of the most important legume crops worldwide is soybean. According to the FAO soybeans are grown on 73 million hectares worldwide. Within the European Union the area harvested has grown from 180 hectares in 1961 to 363000 hectares in 2000 (Europe 1.199 million hectares). Its high oil content and protein quality as well as progress in plant breeding will lead to a further increase in acreage in Europe as well as worldwide.
The best characterized bacterial species that supplies up to 350 kg of fixed nitrogen to soybean (Glycine max) as well as cowpea (Vigna unguiculata), mungbean (Vigna radiata) and Macroptilium atropurpureum is Bradyrhizobium japonicum.
The rhizobia/legume root nodule symbiosis is the result of a complex interaction between the host plant and the bacteria. Within the rhizosphere, bacteria have access to carbon and nitrogen sources originating from the plant. In addition, rhizobia receive plant-derived signal molecules (flavonoids). In response, rhizobia themselves secrete molecules (Nod factors) that signal to the plant the presence of suitable symbiotic partners. The rhizobia are then able to enter the root in the region of emerging root hairs. They are guided via branched infection threads into the cortex of the root where a meristematic zone has formed. Finally, the bacteria are wrapped by a plant-derived membrane and enter the cytosol of plant cells. The bacteria are now referred to as bacteroids and are able to fix molecular nitrogen, which is released into the plant cells.
Our main research topics are:
- Structure, regulation and function of the type III secretion system of B. japonicum.
- Transcriptome and proteome analyses of B. japonicum. High-throughput methods
- e.g. the microarray technique are applied or will be adopted to our system.
Suggested books:
- Stacey, G., Burris, R. H., and H. J. Evans (ed.). 1992. Biological nitrogen fixation. Chapman & Hall, New York.
- Spaink, H., Kondorosi, A., and P. J. J. Hooykaas (ed.). 1998. The Rhizobiaceae. Kluwer Academic Publishers, Dordrecht.
- Triplett, E. W. (ed.). 2000. Prokaryotic nitrogen fixation: a model system for the analysis of a biological process. Horizon Scientific Press, Norfolk.
Plants infected with Bradyrhizobium japonicum:
- Macroptilium atropurpureum
- Vigna unguiculata
- Glycine max
Root nodules of the upper listed plant species. The infections with Bradyrhizobium japonicum are well expressed.
Cut of a soybean nodule that has a diameter of ca. 3 mm (coloured light micrograph).
The orange stained central zone consists of infected and uninfected cells (approximately in a 1:1 ratio).
The uncoloured electron micrograph enlargement shows a few plant cells.
Bacteroids are discernible within the infected cell as dark rods and have a length of ca. 2 µm.
Correlated physical and genetic map of the B. japonicum chromosome
The genome of B. japonicum consists of a single chromosome with a size of 8.7 Mb. The correlated physical and genetic map was created by a combination of site-specific integration of restriction sites for PacI, SwaI and PmeI, pulsed-field gel electrophoresis and Southern blot hybridization. As a major result we found that all known symbiosis-specific genes are clustered within a segment of the chromosome. To study this region in more detail, we determined its nucleotide sequence.
Type III secretion systems in rhizobia
Many human, animal and plant pathogenic bacteria provoke disease symptoms by secreting virulence proteins into the cytosol of the host cell. Central to this ability is a complex and conserved type III secretion system (TTSS) spanning the entire bacterial cell envelope.
Similar systems have been identified in the rhizobial strains Rhizobium sp. strain NGR234, Sinorhizobium fredii (the two strains are closely related), Mesorhizobium loti and Bradyrhizobium japonicum. The membrane spanning complex is in all cases highly conserved. Differences exist in the secreted proteins.
TTSS - for review see: Hueck CJ. 1998 [PubMed]
Rhizobium sp. strain NGR234 [PubMed]