Final Report

Legume−rhizobia (root bacteria) symbioses play a major role in food production for an ever growing human population. In this symbiosis, dinitrogen is reduced (“fixed“) to ammonia by the rhizobial nitrogenase enzyme complex and is secreted to the plant host cells, whereas dicarboxylic acids derived from photosynthetically produced sucrose are transported towards the bacterial symbionts and serve as respiratory substrates for the them. This enables plants to grow in nitrogen (N)-deficient soils as N is the most important nutrient for plant growth.
Early research found that sulfate availability of soils play a crucial role for symbiotic N-fixation (SNF) (Anderson and Spencer, 1950). Clover, grown under sulfate deficiency, showed drastically reduced SNF activity. Furthermore, Zhao et al. (1999) found that SNF was very sensitive to sulfate deficiency because the addition of sulfate to pea plants growing on sulfate–deficient soil doubled the amount of fixed N at all growth stages of the plant.
Whether this was caused by the general lack of sulfate in the host legume or specifically affecting N-fixation remained unclear.
Interestingly, the symbiosome membrane contains high levels of a sulfate transporter (Wienkoop and Saalbach 2003; Krusell et al. 2005). Sulfate is an essential nutrient for all living organisms, but its importance for SNF and nodule metabolism has long been underestimated.

In this project, the following questions could be answered:

Do the micro-symbionts take up the sulfate coming from the plant?
What do the mico-symbionts use the sulfate for?
Is sulfate directly influencing SNF efficiency?

The results of this study gave new insights into the important role of sulfate within this plant-microbe interaction. Using chemical imaging, we demonstrated that the microbial symbionts take up 20‐fold more sulfate than the plant nodule cells. Furthermore, we showed that the production of the bacterial key enzyme complex necessary for SNF, called nitrogenase, relies on high levels of imported sulfate from the plant, making sulfur as essential as carbon for the regulation and functioning of SNF. Our findings thus establish the importance of sulfate and its active transport for the plant–microbe interaction that is most relevant for agriculture and soil fertility.

The project was additionally leading to the analysis of another functionally still  uncharacterized symbiosome membrane protein, a Hypersensitive Induced Response protein (to be published soon).