Engineering Rhizobial Bioinoculants: A Strategy to Improve Iron Nutrition

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.

Rhizobia Isolated from Coal Mining Areas in the Nodulation and Growth of Leguminous Trees

“…This significantly increases the levels of this nutrient in the soil by the decomposition of deposited plant material, as well as by the breakdown of nodules following senescence (Geetha and Joshi, 2013).…”

“…Furthermore, symbiosis can lose effectiveness after transplanting of seedlings due to adaptation to local conditions and competition with the indigenous rhizobium population (Geetha and Joshi, 2013). Nevertheless, studies have shown the potential of N-fixing bacteria for the recovery of degraded areas (Melloni et al, 2004).…”

Molecular Studies for Putative Promoter Activity in mdh sucCDAB Operon in Sinorhizobium meliloti

“…While the intergenic region between sucD and sucA could contain active promoter working under glutamate and LBmc test conditions only. These results suggest that the upstream promoter of mdh gene is a constitutive promoter while the putative sucA upstream promoter could be an inducible one and is activated under certain conditions ( Dillewijn et al, 2001;Green et al, 2003;Zhou et al, 2006;Geetha and Joshi, 2013). Other suggestion proposed by Meek (2013), is that when S. meliloti has been differentiated into bacteroids, the putative sucA promoter could be functional in the Alfalafa plant, however more investigations are needed to discover it in details.…”

Biotechnology of siderophores in high-impact scientific fields

“…They noticed that pyoverdine from P. fluorescens has the potential to reduce uranium and decrease its water solubility. Iron solubility was also reduced but this could be also done in an abiotic approach by the formation of iron oxides [Fe(OH) 3 ]. Farkas and colleagues (22) determined that some bacterial isolates from a mine to be considered for a nuclear waste repository were siderophore producers and radiotolerant.…”

“…Other studies have suggested damages to adipose tissue and liver in obese persons (2). Additionally, iron is well known to be required for plant nutrition, development and health (3)(4)(5). Photosynthesis and chlorophyll biosynthesis are affected at low iron concentrations, hindering plant growth (6,7).…”