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4.4 - Symbiotic nitrogen fixation

Ulrike Mathesius, Research School of Biology, Australian National University

Nitrogen is an important nutrient for all plants. While there is an abundance of nitrogen in the atmosphere, plants are unable to convert N2 into a usable form. Fixation of nitrogen gas into ammonia is an ability restricted to nitrogen-fixing bacteria, which contribute most of the inorganic nitrogen to the Earth’s nitrogen cycle.

This chapter explores the importance, evolution and regulation of biological nitrogen fixation, especially of bacteria that have evolved symbiotic associations with higher plant plants. The symbiosis of legumes with nitrogen-fixing soil bacteria called rhizobia has become a model for our understanding of plant-microbe interactions.


Figure 4.38 An infected nodule of Medicago truncatula. Rhizobia are shown as green inside the large nodule, which is outlined in blue. The root is outlined in red. (Photograph courtesy U. Mathesius)

Research over the last decade and beyond has revealed major principles and molecular mechanisms of how plants have evolved to recognise their symbiotic partners, how they allow them entry into their root systems, how nutrients are exchanged between the partners and how the symbiosis is controlled systemically to balance demand and supply.