Abstract
Under field conditions, inoculated rhizobial strains are at a survival disadvantage as compared to indigenous strains. In order to out-compete native rhizobia it is not only important to develop strong nodulation efficiency but also increase their competence in the soil and rhizosphere. Competitive survival of the inoculated strain may be improved by employing strain selection and by genetic engineering of superior nitrogen fixing strains. Iron sufficiency is an important factor determining the survival and nodulation by rhizobia in soil. Siderophores, a class of ferric specific ligands that are involved in receptor specific iron transport into bacteria, constitute an important part of iron acquisition systems in rhizobia and have been shown to play a role in symbiosis as well as in saprophytic survival. Soils predominantly have iron bound to hydroxamate siderophores, a pool that is largely unavailable to catecholate-utilizing rhizobia. Outer membrane receptors for uptake of ferric hydroxamates include FhuA and FegA which are specific for ferrichrome siderophore. Increase in nodule occupancy and enhanced plant growth of the fegA and fhuA expressing engineered bioinoculants rhizobial strain have been reported. Engineering rhizobia for developing effective bioinoculants with improved ability to utilize heterologous siderophores could provide them with better iron acquisition ability and consequently, rhizospheric stability.
Highlights
The best known and most exploited symbiotic N2-fixing bacteria are those belonging to the family Rhizobiaceae (Rhizobia) and include the following genera: Rhizobium, Bradyrhizobium, Sinorhizobium, Azorhizobium, Mesorhizobium, and Allorhizobium [1, 2]
Studies with the native strain S. meliloti 242 demonstrated that the siderophore-mediated iron uptake systems are not essential for an efficient biological nitrogen fixation but they are involved in early steps of nodulation and in rhizobial competitiveness [117]
These findings provide evidence that engineering rhizobial strains with ferrichrome utilization ability provides them with a competitive edge in an environment where Fe-ferrichrome is the only available source of iron
Summary
The best known and most exploited symbiotic N2-fixing bacteria are those belonging to the family Rhizobiaceae (Rhizobia) and include the following genera: Rhizobium, Bradyrhizobium, Sinorhizobium, Azorhizobium, Mesorhizobium, and Allorhizobium [1, 2] Bacteria of these genera have the ability to infect the roots of leguminous plants, causing the formation of a new organ called nodule and establishing a nitrogen-fixing symbiosis. Rhizobial bacteroids fix atmospheric nitrogen in a form that the plants can utilize and in turn the bacteroids are supplied with an environment rich in carbon as an energy source These bacteria infect legumes and are distributed globally [5]. Competitiveness between two rhizobia is measured by the relative ability of the individual rhizobial strain to occupy the nodule [9] These rhizobia may nodulate the host plant but fix little or no nitrogen, reducing the nitrogen-fixing process. The objectives of our review was to consider the limitations that are imposed by the lack of iron as nutrition to rhizobial bioinoculant strains and to focus on ways to determine possible avenues for improving the competitive survivability of rhizobial bioinoculant strain by engineering the iron uptake machinery
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