Chair's comments: signals in the soil

摘要

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 arnino 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 toa 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