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

Abstract

<i>Phyllobacterium brassicacearum</i> STM196, a plant growth-promoting rhizobacterium isolated from roots of oilseed rape, stimulates <i>Arabidopsis</i> growth. We have previously shown that the <i>NRT2.5</i> and <i>NRT2.6</i> genes are required for this growth promotion response. Since these genes are members of the <i>NRT2</i> 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 <i>NIA2</i> and G'4-3 altered in both <i>NIA1</i> and <i>NIA2</i> 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 <i>NIA1</i> transcript level was reduced in the shoots of <i>nrt2.5</i> and <i>nrt2.6</i> mutants suggesting a role for this nitrate reductase isoform independently from its role in nitrate assimilation.