Azospirillum brasilense modulates the Cucumis sativus root transcriptome under contrasting Fe regimes

Iron (Fe) deficiency limits crop productivity, particularly in alkaline soils where Fe availability is low. Plant growth-promoting rhizobacteria (PGPR), such as Azospirillum brasilense, may enhance plant nutrient acquisition. Here, we investigated the transcriptional modulations underlying the interaction between cucumber (Cucumis sativus) and A. brasilense under Fe sufficient and Fe deficient conditions through physiological and transcriptomic analyses. Bacterial inoculation significantly increased root ferric-chelate reductase activity, particularly under Fe deficiency. Transcriptomic profiling revealed that the bacterium modulated a similar number of genes under both Fe regimes, but with distinct regulatory patterns and only a small set of commonly regulated transcripts, suggesting the existence of a small group of shared transcripts responsive to bacterial inoculation irrespective of Fe status. Gene Ontology enrichment analysis highlighted distinct transcriptional responses in Fe sufficient and Fe deficient plants following A. brasilense inoculation. Under Fe sufficiency, A. brasilense attenuated stress-related while promoting growth- and metabolism-associated transcripts. Under Fe deficiency, the bacterium induced broader responses involving transcripts related to transport systems, transcriptional regulation, metabolic reorganization, and stress adaptation. In addition, inoculation modulated genes previously associated with Fe mobilization and homeostasis, indicating that the bacterial treatment affects Fe-related processes within this nutritional context. Overall, these findings show that A. brasilense stimulates root Fe(III)-reduction capacity and induces distinct transcriptional responses under Fe sufficient and Fe deficient conditions, involving processes beyond canonical Strategy I components and highlighting the importance of plant Fe status in shaping the molecular response to bacterial inoculation.