The aim of this project is to elucidate how the signal molecules determined by the Rhizobium nodulation (nod, nol and nfe) genes evoke responses in the plant host leading to root nodule development. Based on the these molecular genetic studies, methods for improving the efficiency and competitiveness of Rhizobium strains to be used as field inoculants will be developed.
Rhizobium bacteria in symbiosis with leguminous host plants are able to fix atmospheric N2 in the absence of combined nitrogen. The formation of nitrogen fixing root nodules requires specific interactions of certain combinations of plants and Rhizobium strains. The host specific recognition is established at multiple stages during the development of symbiosis. Signals are emitted by both partners and are recognized by the other. The aim of this project is to elucidate how the signal molecules determined by the Rhizobium nodulation (nod, nol) genes evoke responses in the plant host leading to root nodule development. The molecular basis for increased competitiveness conferred by the nod genes and by another class of bacterial genes, the rife genes are also being investigated. Based on these studies methods for improving the efficiecy and competitiveness of Rhizobium strains to be used as field inoculants will be developed.
Results of research to date include:
The knowledge gained about the regulation of nod gene expression has provided for the engineering of Rhizobium meliloti strains having an extended host range as well as strains producing several thousand fold higher amounts of Nod signals. This allows a one step purification scheme and characterization of Nod signal molecules which were previously undetectable;
The factors produced by the NodA and NodB protein alone or in combination are active in non-legumes and the expression of these genes affects the phytohormone balance in transgenic tobacco. Morphological abnormalities of the transgenic plants further indicate that tobacco must contain the necessary substrates allowing the nodA and nodB encoded proteins to synthesize growth controlling factors and also the necessary receptors to respond to the presence of these regulatory molecules; A gene (ocd) encoding the enzyme ornithine cyclodeaminase that converts ornithine into proline has been found in R meliloti GR4 as one of the nfe genes. This gene could provide bacteria with better abilities to nodulate Medicago.
The ability of this new class of oligosaccharide signals to control plant growth and development could increase our knowledge on cellular mechanisms of plant differentiation. This knowledge could also have a significant impact on plant breeding and agriculture yields.
The differentiation and development of plant cells are controlled by various signal molecules. For instance, rhizobia produce specific compounds eliciting the development of nitrogen-fixing root nodules on leguminous plant hosts. This signalling may provide an excellent model system for studying the mechanism and control of signal reception and transduction in plants. In addition, understanding How rhizobia induce nodules may help us to improve or extend Rhizobium-legume interactions which would be useful in the agricultural use of Rhizobium inoculants.
The specific aims of this project are as follows:
i. study the function and genetic control of nod and nfe gene expression in R. meliloti
ii. purify and determine the chemical structure of various signal molecules which are produced by the nod proteins and search for putative receptors on Medicago cells for the specific signals
iii. identify Medicago genes activated by the nod signals, study their expression and determine their RFLP map
iv. determine whether signals of rhizobia control the expression of genes implicated in the plant defense mechanism
v. based on i-iv, modification of R. meliloti to improve nodulation competitiveness.
Funding SchemeCSC - Cost-sharing contracts