Feb. 4, 2005 In drought conditions, the capacity for retaining carbon in legume nodules is limited and this may be the reason why there is a drop in nitrogen-fixing in legumes under these conditions. This was the conclusion of María Dolores Gálvez in defending her PhD thesis at the Public University of Navarre. Her PhD was entitled, “Nodule metabolism in Pisum sativum L. in response to water stress: carbon/nitrogen interactions and the possible molecules involved in the modulation of the response”.
<b>Nitrogen-fixing and drought</b>
Biological nitrogen-fixing is a process of great agricultural and ecological interest, given that nitrogen, after water and carbon, is the nutrient that most limits vegetable growth and crop production. This process is particularly sensitive to adverse environmental conditions, such as water stress or drought. This is why María Dolores Gálvez’s PhD was aimed at investigating how the regulation of biological nitrogen-fixing is carried out under drought conditions.
The reduction of atmospheric nitrogen to ammonia - or nitrogen-fixing - can only be carried out by procariots. Amongst these, the ones generically known as rhizobes are capable of establishing symbiosis with legume plants giving rise to a new structure: the nodule.
On the one hand, the plant benefits from the micro-organism, that takes on the task of capturing nitrogen from the air and converting it into ammonia in such a way that the plant can use it. This ammonia is incorporated into carbon skeletons in order to form aminoacids and proteins. On the other hand, the micro-organism obtains nutrients necessary for its growth from the plant.
Under drought conditions, a reduction in nodule sacarose synthesis activity was observed. This drop occurred simultaneously with a decrease in nitrogen-fixing, enabling the establishment of a high correlation between both processes in adverse conditions. As a consequence of the inhibition of sacarose synthesis activity, a drop in the concentration of phosphate sugars and organic acids was also observed, indicating a decrease in carbon flow in the nodules, a drop which, in turn, limits the supply of carbon to the bacteroid and the capacity of the bacteroid to fix nitrogen thus affected.
<b>Perception of water stress</b>
Wishing to investigate the process of perception and transduction of water stress that leads to the inhibition of nitrogen-fixing, María Dolores Gálvez also studied abscisic acid and species of activated oxygen as possible molecules involved in the regulation of nitrogen-fixing.
The results obtained on the exogenous application of abscisic acid suggested the existence of at least two different mechanisms for the regulation of biological nitrogen-fixing depending on how the stress evolves. In situations in which the stress is produced gradually, a route that is independent of abscisic acid is activated, involving the inhibition of sacarose synthesis activity; in situations where the stress occurs rapidly and intensely, another route, dependent on abscisic acid, involving control through leghaemoglobine/oxygen, is activated.
In order to analyse the possible involvement of species of activated oxygen in the modulation of nitrogen-fixing, the exogenous application to plants exposed to water stress of abscisic acid - a natural antioxidant – was decided upon. Treatment with exogenous abscisic acid, carried out under conditions of water stress, played a beneficial role on the protein content of the plant. Likewise, a recovery in the total activity of the metabolism carbonand nitrogen enzymes in the nodules was observed.
Moreover, the application of abscisic acid did not reverse the negative effect of the water stress in and it is thus not possible to relate species of activated oxygen with the regulation of nitrogen-fixing. In this context, the decrease in of nitrogen-fixing happens in association with a limitation in the carbon flow in the nodules, caused by the inhibition of sacarose synthesis activity under these conditions.
Thus, the signs involved in the perception and the transduction route that lead to a decrease in biological nitrogen-fixing in water stress conditions are complex and need to be studied further in order to understand the mechanisms for the regulation of biological nitrogen-fixing
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