RESUMEN
Drought stress has become one of the most uncontrolled and unpredictable constraints on crop production. The purpose of this study was to evaluate the impacts of two different Rhizobium leguminosarum strains on terminal drought tolerance induction in two faba bean genotypes cultivated in Algeria, Aquadulce and Maltais. To this end, we measured physiological parameters-osmoprotectants accumulation, oxidative stress markers and enzyme activities-to assess the effect of R. leguminosarum inoculation on V. faba under terminal water deficiency conditions in greenhouse trials. Upregulation of anti-oxidative mechanisms and production of compatible solutes were found differentially activated according to Rhizobium strain. Drought stress resilience of the Maltais variety was improved using the local Rhizobium strain OL13 compared to the common strain 3841. Symbiosis with OL13 strain leads in particular to a much better production of proline and soluble sugar in nodules but also in roots and leaves of Maltais plant. Even if additional work is still necessary to decipher the mechanism by which a Rhizobium strain can affect the accumulation of osmoprotectants or cellular redox status in all the plants, inoculation with selected Rhizobium could be a promising strategy for improving water stress management in the forthcoming era of climate change.
RESUMEN
Phyllobacterium brassicacearum STM196, a plant growth-promoting rhizobacterium isolated from roots of oilseed rape, stimulates Arabidopsis growth. We have previously shown that the NRT2.5 and NRT2.6 genes are required for this growth promotion response. Since these genes are members of the NRT2 family of nitrate transporters, the nitrogen assimilatory pathway could be involved in growth promotion by STM196. We address this hypothesis using two nitrate reductase mutants, G5 deleted in the major nitrate reductase gene NIA2 and G'4-3 altered in both NIA1 and NIA2 genes. Both mutants had a reduced growth rate and STM196 failed to increase their biomass production on a medium containing NO3- as the sole nitrogen source. However, they both displayed similar growth promotion by STM196 when grown on an NH4+ medium. STM196 was able to stimulate lateral roots development of the mutants under both nutrition conditions. Altogether, our results indicate that the nitrate assimilatory metabolism is not a primary target of STM196 interaction and is not involved in the root developmental response. The NIA1 transcript level was reduced in the shoots of nrt2.5 and nrt2.6 mutants suggesting a role for this nitrate reductase isoform independently from its role in nitrate assimilation.
RESUMEN
The Phyllobacterium brassicacearum STM196 strain stimulates Arabidopsis thaliana growth and antagonizes high nitrate inhibition of lateral root development. A previous study identified two STM196-responsive genes, NRT2.5 and NRT2.6 (Mantelin et al., 2006, Planta 223: 591-603). We investigated the role of NRT2.5 and NRT2.6 in the plant response to STM196 using single and double Arabidopsis mutants. The single mutants were also crossed with an nrt2.1 mutant, lacking the major nitrate root transporter, to distinguish the effects of NRT2.5 and NRT2.6 from potential indirect effects of nitrate pools. The nrt2.5 and nrt2.6 mutations abolished the plant growth and root system architecture responses to STM196. The determination of nitrate content revealed that NRT2.5 and NRT2.6 do not play an important role in nitrate distribution between plant organs. Conversely, NRT2.5 and NRT2.6 appeared to play a role in the plant response independent of nitrate uptake. Using a nitrate reductase mutant, it was confirmed that the NRT2.5/NRT2.6-dependent plant signalling pathway is independent of nitrate-dependent regulation of root development. Our findings demonstrate that NRT2.5 and NRT2.6, which are preferentially expressed in leaves, play an essential role in plant growth promotion by the rhizospheric bacterium STM196.
