Physiological regulation of symbiotic N2 fixation in the model legume

Beschreibung

Symbiotic N2 fixation in legume root nodules is known to be very sensitive to a wide range of environmental perturbations. However, the regulatory mechanisms that allow nodules to respond to such adverse environmental conditions are still poorly understood. For this reason, the study was carried out to investigate the regulation aspects of N2 fixation for the model legume Medicago truncatula in response to P-deficiency and combined N-supply. The thesis reports the results in N metabolism and nitrogenase activity after growing the plants under P shortage conditions. The response of short- and long-term exogenous N-supply on the physiology of nodulated roots was also investigated. The concentrations of selected C and N metabolites were measured in root nodules as well as in the plant phloem sap. It was shown that both treatments caused an increase in phloem-N which was mainly due to an increase in the N asparagine. These findings were combined with those of previous studies to establish a model to explain the role of specific phloem N-compounds in the regulation of nitrogenase activity. The use of the non-invasive, real-time gas exchange measurements (i.e. open-flow gas exchange system) in addition to 15N measurements provided substantial evidences for such explanations. This work also showed that Medicago truncatula is a useful model to study the physiological events occurring in nodule metabolism in response to environmental and physiological perturbations. In the future, these findings could be used in parallel with molecular studies, thus, it can be successfully employed in interdisciplinary approaches. The thesis also covers some investigations assessing the efficiency of the nitrogen fixing system of the Medicago truncatula cultivar ´Jemalong A17` and compares it with the most relevant forage legume alfalfa (Medicago sativa L.) in combination with the model strain Sinorhizobium meliloti 2011. The response of both plant species to combined-N revealed that Medicago truncatula reacts more positively to nitrate application which might indicate a less efficiency of N2 fixing system. The inefficient fixing system of M. truncatula cv. Jemalong A17 S. meliloti 2011 was mainly connected with the low carbohydrate concentration (i.e. malate) in nodules as well as slow N transport system to the shoot.
Symbiotic N2 fixation in legume root nodules is known to be very sensitive to a wide range of environmental perturbations. However, the regulatory mechanisms that allow nodules to respond to such adverse environmental conditions are still poorly understood. For this reason, the study was carried out to investigate the regulation aspects of N2 fixation for the model legume Medicago truncatula in response to P-deficiency and combined N-supply. The thesis reports the results in N metabolism and nitrogenase activity after growing the plants under P shortage conditions. The response of short- and long-term exogenous N-supply on the physiology of nodulated roots was also investigated. The concentrations of selected C and N metabolites were measured in root nodules as well as in the plant phloem sap. It was shown that both treatments caused an increase in phloem-N which was mainly due to an increase in the N asparagine. These findings were combined with those of previous studies to establish a model to explain the role of specific phloem N-compounds in the regulation of nitrogenase activity. The use of the non-invasive, real-time gas exchange measurements (i.e. open-flow gas exchange system) in addition to 15N measurements provided substantial evidences for such explanations. This work also showed that Medicago truncatula is a useful model to study the physiological events occurring in nodule metabolism in response to environmental and physiological perturbations. In the future, these findings could be used in parallel with molecular studies, thus, it can be successfully employed in interdisciplinary approaches. The thesis also covers some investigations assessing the efficiency of the nitrogen fixing system of the Medicago truncatula cultivar ´Jemalong A17` and compares it with the most relevant forage legume alfalfa (Medicago sativa L.) in combination with the model strain Sinorhizobium meliloti 2011. The response of both plant species to combined-N revealed that Medicago truncatula reacts more positively to nitrate application which might indicate a less efficiency of N2 fixing system. The inefficient fixing system of M. truncatula cv. Jemalong A17 S. meliloti 2011 was mainly connected with the low carbohydrate concentration (i.e. malate) in nodules as well as slow N transport system to the shoot.

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Saad Abdel Rahman Sulieman

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