Arabidopsis Growth-Promotion and Root Architecture Responses to the Beneficial Rhizobacterium Phyllobacterium brassicacearum Strain STM196 Are Independent of the Nitrate Assimilatory Pathway.

Maya Kechid,Lydia Gamet,Guilhem Desbrosses,Fabrice Varoquaux,Abdelhamid Djekoun,Bruno Touraine,Loren Castaings

doi:10.3390/plants11010128

Abstract

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.

Highlights

Introduction published maps and institutional affilPlant roots develop associative symbioses with bacteria that colonize the rhizosphere with beneficial effects on plant growth and health which are, collectively called plant growth-promoting rhizobacteria (PGPR)
Our results demonstrate that the lateral root development response to inoculation with STM196 is independent of nitrate reductase activity, and, from NO3 − pools changes triggered by nitrate reductase mutations
Nitrate reductase activity (NRA) was hardly detectable in the roots of our young Col-0 seedlings while it was measurable in their shoots, indicating that

Summary

Introduction

Plant roots develop associative symbioses with bacteria that colonize the rhizosphere with beneficial effects on plant growth and health which are, collectively called plant growth-promoting rhizobacteria (PGPR). From various genera, can interact with roots of a given plant species and, each PGPR strain can interact with numerous host plants, indicating a very weak host-specificity [1,2,3]. The beneficial effects of PGPR on plants are likely to be a complex response of multifactorial origin, including biofertilization, phytostimulation, and biocontrol [4,5]. A first hypothesis to explain growth promotion by PGPR consider that they enhance plant nutrition. In experiments performed in vitro, these bacterial activities cannot, account for their plant growth-promoting effect.

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