Agriculture Canada, Soils and Crops Development Centre, Ste-Foy, QC, Canada
Microorganisms adapt to their environment by sensing signals and making necessary adjustments in their cellular processes. Much still remains to be learned about the nature of the signals, how they are sensed and transduced. The four presentations in this session dealt with chemical sensing in Azospirillum brasilense, rhizobial catabolic and chemoreceptor genes, biotin-dependent rhizobial carboxylases and the role of phosphate in the nodulation of actinorhizal plants.
Motile bacteria are able to detect and show taxis to agents involved in energy generation such as light, oxygen and oxidizable substrates. Energy taxis includes aerotaxis, phototaxis, redox taxis, taxis to alternative electron acceptors, and chemotaxis to oxidizable substrates (for review see Taylor et al. 1999). Bacteria utilize energy taxis to seek out environments most favorable for maintaining optimal cellular energy levels. Azospirillum brasilense colonizes the rhizosphere of agronomically important cereals and grasses, the roots of which exude significant amounts of organic acids, sugars and amino acids. Motility and chemotaxis are important factors for the bacterial colonization of the plant roots. A. brasilense shows a strong aerotaxis response which guides the bacteria to a low oxygen concentration that is optimal for energy generation and nitrogen fixation (Zhulin et al. 1996). The bacteria preferentially seek organic acids and sugars as carbon and energy sources thereby permitting the cells to achieve and maintain optimal energy levels in the plant rhizosphere. Zhulin and coworkers showed that energy taxis is the dominant behavior in A. brasilense with most chemoeffectors being processed by this method and that changes in the electron transport system govern most behavioral responses (Alexandre et al. 2000). The question as to whether the redox state of the electron transport system or an ion motive is the signal for chemotaxis remains to be established
Rhizobium and Sinorhizobium spp. contain plasmids on which symbiotic genes are located. These bacteria also contain a variable number of cryptic plasmids which can influence nitrogen fixation and nodulation as well as bacterial growth and survival. There is a need for data on the nature of the plasmid-encoded genes and their mechanism of action within the context of soil environment. The group of Michael Hynes has examined the function of many plasmid encoded genes of S. meliloti and R. leguminosarum with a view to elucidating their role in bacterial growth and survival as well as their impact on competition for nodulation. They have demonstrated that carbon catabolism genes are present on several plasmids of R. leguminosarum VF39. At least one of these genes, the malate synthase G gene, is required for the symbiotic nitrogen fixation of peas. Also of interest, was the finding of a number of genes that encode putative methyl-accepting chemotaxis proteins (MCPs) (Yost et al. 1998). Whether all of the MCP-like sequences encode genuine MCP proteins is not known. Mutation of mcpB resulted in impairment of chemotaxis towards a wide range of substrates. The mcpB and mcpC mutants were less competitive for the nodulation of peas than the wild type, while the mcpD mutant showed no decrease in competitiveness. The MCP-like receptors in S. meliloti differ from those of R. leguminosarum perhaps reflecting the observed differences in the chemotactic behavior of the two strains.
Biotin plays an important role in the growth and metabolism of diazotrophic bacteria via its participation in CO2 fixation reactions. The Cuernavaca group headed by Michel Dunn has been studying the properties and metabolic role of biotin-dependent carboxylases (BDCs). Pyruvate carboxylase (PYC) catalyzes the formation of oxalacetate from pyruvate. This enzyme was shown to be important in determining growth defects of an R. etli poly-P-hydroxybutyrate synthetase mutant, however, the mechanism controlling the interaction of these enzyme activities remains to be elucidated (Cevallos et al. 1996; Dunn et al. 1996). A bio tin independent phosphoenolpyruvate carboxylase (PCC) catalyzes the conversion of phosphoenolpyruvate to oxaloacetate. Dunn and colleagues have shown that some diazotrophs contain either PPC or PYC, while others have both enzymes, but it remains to be established what practical advantage accrues to the bacterium having a particular menu of these carboxylases. The Cuernavaca group has also characterized a propionyl CoA carboxylase (PCC) from R. etli. Future studies will be directed at integrating the roles of the BDCs in rhizobial metabolism and assessing the importance of these enzyme within the context of the bacterial environment.
The final presentation of the session related to the role of phosphate in the nodulation of actinorhizal plants by Huss-Danell's group. They showed that phosphate stimulated nodulation of Alnus by Frankia but that nitrogen inhibited both nodulation and nitrogen fixation (Ekblad, Huss-Danell 1995). Stimulation of nodulation by phosphate (1 mM) occurred even in the presence of relatively high nitrogen concentrations (7 mM N) and the stimulatory effect of phosphate was not due to a general stimulation of plant growth. High phosphate concentrations also stimulated nodule mass of clover roots suggesting a more generalized role for phosphate. Experiments using the split-root technique for Alnus and Hippohae indicated that the effects of N and P on nodulation were systemic. The mechanism whereby P stimulates nodulation is unknown. There are a number of questions that need to be investigated, these include whether there is a requirement for P to be present during the entire nodulation phase. This can be approached by pulsing in P for different time periods. Other questions relate to the role of other ions. With regard to the stimulation of nodulation, to what extent if any can phosphate be replaced by other ions?
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