Rhizobia as a Source of Plant Growth-Promoting Molecules: Potential Applications and Possible Operational Mechanisms

Abstract

The symbiotic interaction between rhizobia and legumes that leads to nodule formation is a complex chemical conversation involving plant release of nod-gene inducing signal molecules and bacterial secretion of lipo-chito-oligossacharide nodulation factors. During this process, the rhizobia and their legume hosts can synthesize and release various phytohormones, such as IAA, lumichrome, riboflavin, lipo-chito-oligossacharide Nod factors, rhizobitoxine, gibberellins, jasmonates, brassinosteroids, ethylene, cytokinins and the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase that can directly or indirectly stimulate plant growth. Whereas these attributes may promote plant adaptation to various edapho-climatic stresses including the limitations in nutrient elements required for plant growth promotion, tapping their full potential requires understanding of the mechanisms involved in their action. In this regard, several N2-fixing rhizobia have been cited for plant growth promotion by solubilizing soil-bound P in the rhizosphere via the synthesis of gluconic acid under the control of pyrroloquinoline quinone (PQQ) genes, just as others are known for the synthesis and release of siderophores for enhanced Fe nutrition in plants, the chelation of heavy metals in the reclamation of contaminated soils, and as biocontrol agents against diseases. Some of these metabolites can enhance plant growth via the suppression of the deleterious effects of other antagonistic molecules, as exemplified by the reduction in the deleterious effect of ethylene by ACC deaminase synthesized by rhizobia. Although symbiotic rhizobia are capable of triggering biological outcomes with direct and indirect effects on plant mineral nutrition, insect pest and disease resistance, a greater understanding of the mechanisms involved remains a challenge in tapping the maximum benefits of the molecules involved. Rather than the effects of individual rhizobial or plant metabolites however, a deeper understanding of their synergistic interactions may be useful in alleviating the effects of multiple plant stress factors for increased growth and productivity.

Current Perspectives on the Beneficial Effects of Soybean Isoflavones and Their Metabolites for Humans

“…In plants, phytoestrogens do not function as hormones, instead they have an important role as phytoalexins, small molecule compounds with antifungal, antibacterial, antiviral, and antioxidant properties that are produced in response to environmental stress and pathogenic attack [5]. Among these compounds, isoflavones play a crucial role in plant-microbe interactions, such as rhizobia-legume symbiosis and defense responses in legumes, including soybeans [42,43]. In plant defense, isoflavones act as a precursor of antibacterial and/or antiherbivore activities of phytoalexins that protect the plant from pathogen infection [44,45].…”

Effects of Abiotic Stress on Soil Microbiome

“…Exudates from the roots are believed to 'prime' the soil around the roots ecosystem, such that, they would recruit favorable bacteria to the rhizosphere, resulting in higher rhizosphere respiration rates and bacterial biomass percentage compared to surface soils [186]. Microorganism may respire between 64-86% of plant rhizodeposits [173,174,187,188]. Micallef et al, [189], also reported that different accessions of Arabidopsis have unique exudate profiles, and thus, their bacterial communities in rhizosphere are diverse among these accessions [189].…”

“…The main distinction between legumes and non-leguminous plant is that, leguminous plant associate symbiotically with N 2 -fixing bacteria from the genus Rhizobium (Alphaproteobacteria) [187,191], whereas non-leguminous plants interact in a symbiotic way with N 2 -fixing bacteria from the genus Frankia (Actinobacteria) [188,192]. The ability to colonize the rhizosphere and interact with plants is possessed by a large number of N 2 -fixing bacteria from various bacterial phyla [193].…”

Copper nitrite reductase from Sinorhizobium meliloti 2011: Crystal structure and interaction with the physiological versus a nonmetabolically related cupredoxin‐like mediator

“…Rhizobia are a group of microorganisms used for the formulation of bioinoculants of legumes such as Medicago sativa (alfalfa) and Glycine max (soya) seeds 1 . The symbiotic relationship between these microorganisms and alfalfa and soya crops relies on two finely tuned metabolic pathways: nitrogen fixation and denitrification 2,3 .…”

“…S. meliloti 2011 is one of the most exploited rhizobia for the formulation of bioinoculants for alfalfa seeds 1 . Genome mining revealed that this bacterium harbors two pseudoazurin genes, one located at the pSymA megaplasmid together with the Sm NirK gene ( azu1 ), and a second ( azu2 ) at the chromosome is clustered with sulfur‐related metabolism genes 19,20 .…”