Abstract
Nitrogen-fixing, plant-growth promoting bacteria are arguably the biotechnological tool of highest potential to improve agricultural productivity in short term. Nitrogen fixation and phytohormone production by these bacteria have been considered the most important factors for plant growth promotion. However, the underlying mechanisms responsible for productivity increases by associative bacteria are not clear. Moreover, the intensity of the plant growth promotion, including transfer of the fixed nitrogen from the bacteria to the plant, depends on an efficient interaction of the plant genotype and bacterial species. Azospirillum spp. are nitrogen-fixing, plant growth promoting bacteria that can associate with several cereals such as maize, rice and wheat, and also biofuel crops such as sugar cane and Pennisetum. Azospirillum brasilense is one of the most used plant growth promoting bacteria (PGPB), being used in Brazil, Argentina, Mexico, India and Europe. Analyses of field experiments have shown a success rate of inoculation with Azospirillum ranging from 60 to 70%, with statistically significant increases in yield varying from 5 to 30% [1]. The regulation of nitrogen metabolism in A. brasilense has been extensively studied. Here we will review nitrogen fixation regulation in this bacterium and advances in the understanding of aspects of interaction with cereal plants obtained by transcriptomic analyses.
Highlights
Nitrogen-fixing, plant-growth promoting bacteria are arguably the biotechnological tool of highest potential to improve agricultural productivity in short term
The nitrogenase activity of A. brasilense is reversibly regulated by ADP ribosylation of the dinitrogenase reductase (NifH) in response to increase in ammonium concentration or energy depletion
Upon consumption of added ammonium or restoration of energy levels, the ADP-ribose group is removed from NifH by the dinitrogenase reductase activating glycohydrolase (DraG) leading to nitrogenase reactivation
Summary
Nitrogen-fixing, plant-growth promoting bacteria are arguably the biotechnological tool of highest potential to improve agricultural productivity in short term. Regulation of nitrogen fixation in A. brasilense The transcription initiation at nif promoters is dependent on the NifA protein. The function of NifA can be regulated at two levels, nifA gene transcription and NifA protein activity, and regulation at both levels occurs in response to oxygen and/or fixed nitrogen. The expression of the nifA gene in A. brasilense is only partially repressed by ammonium and oxygen, being maximal under nitrogen fixation conditions.
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