Influence of Azospirillum brasilense on the Fe acquisition process and the Fe availability in the rhizosphere of cucumber plants

Even though iron (Fe) is one of the most abundant elements in soils, its availability represents one of the major constraints for plants growth and productivity. Plant Growth-Promoting Rhizobacteria (PGPR) affect the biogeochemical cycles of mineral elements and might increase the metal bioavailable fraction. In addition, they might also induce and/or regulate molecular and biochemical responses in plants. Azospirillum brasilense has been shown to alter root architecture and enhance mineral nutrition. In the present study, we characterized the physiological and biochemical effects induced by Fe starvation and PGPR inoculation in cucumber plants (Cucumis sativusL. cv. Chinese Long) analyzing the exudation pattern in hydroponic and soil conditions. In addition, we also investigated the molecular responses of cucumber plants induced by the inoculation with the PGPR assessing the mechanisms activated under Fe starvation (Fe - chelate reductase - CsFRO and PM H+-ATPase CsHA1). A. brasilense induced and anticipated Fe reduction and rhizosphere acidification in both Fe-sufficient and Fedeficient plants. These biochemical mechanisms were supported by an increased expression of the genes encoding CsFRO and CsHA1 and resulted in a higher Fe uptake rate (measured as 57Fe). The analyses of root exudates released by hydroponically grown plants highlighted that cucumber plants respond differently depending on the nutritional status. In addition, following the cultivation period on calcareous soil, also the root exudates suggested a peculiar behaviour of plants as a function of the treatment (either Fe deficient or sufficient). In soil the inoculum allowed a faster recovery of cucumber plants from Fe-deficiency symptoms, i.e. increase in the chlorophyll content, in the biomass and in the Fe content of leaves. In addition, this work highlighted for the first time on two amino acids, namely Glycine and Glutamate, which could be involved in the plant-microorganism-soil interaction for the retrieval of Fe within a calcareous soil. In conclusion, our findings highlighted that the molecular and physiological responses of cucumber plants to the inoculation with A. brasilenseare strictly related to the Fe nutritional status, suggesting the possible co-existence of multiple regulation mechanisms. These results further strengthen the capability of PGPRs to modulate Fe acquisition in plants by differently triggering genes transcription.