Abstract
Cereals such as maize, rice, wheat and sorghum are the most important crops for human nutrition. Like other plants, cereals associate with diverse bacteria (including nitrogen-fixing bacteria called diazotrophs) and fungi. As large amounts of chemical fertilizers are used in cereals, it has always been desirable to promote biological nitrogen fixation in such crops. The quest for nitrogen fixation in cereals started long ago with the isolation of nitrogen-fixing bacteria from different plants. The sources of diazotrophs in cereals may be seeds, soils, and even irrigation water and diazotrophs have been found on roots or as endophytes. Recently, culture-independent molecular approaches have revealed that some rhizobia are found in cereal plants and that bacterial nitrogenase genes are expressed in plants. Since the levels of nitrogen-fixation attained with nitrogen-fixing bacteria in cereals are not high enough to support the plant’s needs and never as good as those obtained with chemical fertilizers or with rhizobium in symbiosis with legumes, it has been the aim of different studies to increase nitrogen-fixation in cereals. In many cases, these efforts have not been successful. However, new diazotroph mutants with enhanced capabilities to excrete ammonium are being successfully used to promote plant growth as commensal bacteria. In addition, there are ambitious projects supported by different funding agencies that are trying to genetically modify maize and other cereals to enhance diazotroph colonization or to fix nitrogen or to form nodules with nitrogen-fixing symbiotic rhizobia.
Highlights
Cereals are grasses from the Poaceae family that were domesticated several thousand years ago in different geographical regions in order to take advantage of the edible components of their grain
By using a culture-independent approach, a better representation of existing diazotrophs may be obtained while the analysis of nifH transcripts has provided results on active diazotrophs. This approach based on nif gene amplification and sequencing has been used to identify nitrogen-fixing bacteria associated with rice, sorghum, wheat and maize
Nitrogen fixation is highly variable depending on the associated diazotroph and the plant variety, but the host plant exerts a determinant effect by supplying the carbon and energy source for bacterial growth and nitrogen fixation
Summary
Cereals are grasses from the Poaceae family that were domesticated several thousand years ago in different geographical regions in order to take advantage of the edible components of their grain. Diazotrophs are found among alphaproteobacteria, gammaproteobacteria, Firmicutes, betaproteobacteria, and cyanobacteria but do not seem to be the most abundant (dominant) bacteria in plant rhizospheres, so there are possibilities for increasing nitrogen-fixation by favoring their populations. There are many reports on the growth-promoting effects of Azospirillum inoculation in maize, wheat, rice, and sorghum but these will not be reviewed here because the main beneficial effects are not primarily attributed to nitrogen fixation.
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