The regulation of symbiotic N2 fixation: a conceptual model of N feedback from the ecosystem to the gene expression level

Abstract Symbiotic nitrogen (N2) fixation in legumes may give the host plant a distinct competitive advantage; at the same time it is mainly responsible for introducing N into terrestrial ecosystems which may ultimately benefit all organisms. Depending on environmental conditions, symbiotic N2 fixation may be tuned to the plant's N demand or specifically inhibited (a disadvantage for plants which depend mainly on symbiotic N2 fixation), or even prevented. Thus, the ecological range for symbiotic N2 fixation can be narrower than that of the host plants. A shortage of mineral N is the only case in which adverse environmental conditions clearly favour symbiotic N2 fixation. Variations in number or mass of nodules or nodule morphology are persistent features, that may represent one kind of regulation of N2 fixation. In addition, varying O2 permeability of nodules functions as a rapid and reversible control of N2 fixation which may compensate partially or fully for poor nodulation. The plant's demand for symbiotically fixed N is thought to play a central role in modulating both nodulation and N2 fixation activity; an N feedback mechanism is assumed. The control of symbiotic N2 fixation operates through a series of ecophysiological triggers which are also influenced by complex interactions between legume plants and other organisms in the ecosystem. The proportion of legume biomass and the performance of symbiotic N2 fixation in each individual legume are the main parameters which determine the amount of symbiotically fixed N introduced into a terrestrial ecosystem. The various triggers and N feedback mechanisms from the whole ecosystem to the gene expression level which regulate symbiotic N2 fixation in terrestrial ecosystems are reviewed and discussed in terms of a conceptual model. Although the presented model is based primarily on our knowledge about the physiology of a few leguminous crop species and of ecosystem processes in managed, perennial grassland in temperate climatic conditions, it may stimulate thinking about functional relationships between symbiotic N2 fixation and terrestrial ecosystems at various system levels.