Can stress response genes be used to improve the symbiotic performance of rhizobia?

Abstract

Rhizobia are soil bacteria able to form symbioses with legumes and fix atmospheric nitrogen, converting it into a form that can be assimilated by the plant. The biological nitrogen fixation is a possible strategy to reduce the environmental pollution caused by the use of chemical N-fertilizers in agricultural fields. Successful colonization of the host root by free-living rhizobia requires that these bacteria are able to deal with adverse conditions in the soil, in addition to stresses that may occur in their endosymbiotic life inside the root nodules. Stress response genes, such as otsAB , groEL , clpB , rpoH play an important role in tolerance of free-living rhizobia to different environmental conditions and some of these genes have been shown to be involved in the symbiosis. This review will focus on stress response genes that have been reported to improve the symbiotic performance of rhizobia with their host plants. For example, chickpea plants inoculated with a Mesorhizobium strain modified with extra copies of the groEL gene showed a symbiotic effectiveness approximately 1.5 fold higher than plants inoculated with the wild-type strain. Despite these promising results, more studies are required to obtain highly efficient and tolerant rhizobia strains, suitable for different edaphoclimatic conditions, to be used as field inoculants.

Rhizobial–Host Interactions and Symbiotic Nitrogen Fixation in Legume Crops Toward Agriculture Sustainability

“…Overall, these studies indicate that stress-induced impairment in the signaling mechanism leads to slower or inhibition of nodule formation (Lira et al, 2015). The molecular changes in response to temperature stress are discussed in detail elsewhere (Alexandre and Oliveira, 2013;da-Silva et al, 2017).…”

Exogenous ACC Deaminase Is Key to Improving the Performance of Pasture Legume-Rhizobial Symbioses in the Presence of a High Manganese Concentration

“…Although the supplementation of acidic soils with dolomitic limestone alleviates the problem [10], it is an expensive approach and is not universally available. The introduction of highly efficient native rhizobia or engineered rhizobia strains can be useful tools as legume inoculants, particularly under challenging soil and climate conditions [35]. However, the selection and characterization of native rhizobial strains is a slow and expensive process, and the use of genetically modified organisms (GMO) is limited or prohibited in many countries.…”

Molecular Biology in the Improvement of Biological Nitrogen Fixation by Rhizobia and Extending the Scope to Cereals

“…The higher nodulation was correlated with enhanced expression of nodulation genes nodA and nodC . Chaperone proteins and other stress-responsive genes that play an important role in symbiosis have been comprehensively reviewed by da-Silva et al [ 26 ]. Reduction in oxidative stress through enhanced expression of catalase, which effectively lowered the H 2 O 2 content, led to an approximately 2-fold increase in nitrogen fixation [ 52 ].…”

“…The effectiveness of other chaperone genes in heat stress tolerance has also been observed. Additional copies of groEL in a Mesorhizobium strain allowed enhanced recovery from heat shock of 48 °C for 15 min [ 26 ]. In addition, the modified strain displayed a greater symbiotic effectiveness.…”