Research article Plant-microorganism-soil interactions influence the Fe availability in the rhizosphere of cucumber plants Youry Pii a , Alexander Penn a , Roberto Terzano b , Carmine Crecchio b , Tanja Mimmo a, * , Stefano Cesco a a Faculty of Science and Technology, Free University of Bolzano, I-39100 Bolzano, Italy b Dipartimento di Scienze del Suolo, della Pianta e degli Alimenti, University of Bari “Aldo Moro”, I-70126 Bari, Italy article info Article history: Received 19 November 2014 Accepted 18 December 2014 Available online 19 December 2014 Keywords: Azospirillum brasilense Cucumis sativus L. Fe-deficiency Fe mobilization Rhizosphere Biofertilization Plant growth-promoting rhizobacteria abstract Iron (Fe) is a very important element for plants, since it is involved in many biochemical processes and, often, for the low solubility of the natural Fe sources in soil, plants suffer from Fe e deficiency, especially when grown on calcareous soils. Among the numerous plant growth-promoting rhizobacteria (PGPR) that colonize the rhizosphere of agronomically important crops, Azospirillum brasilense has been shown to exert strong stimulating activities on plants, by inducing alterations of the root architecture and an improvement of mineral nutrition, which could result from an enhancement of ion uptake mechanisms as well as by increased bioavailability of nutrients. Some studies have also established that A. brasilense can act as biocontrol agent, by preventing the growth and/or virulence of phytopathogens, most likely through the production of microbial siderophores that sequester Fe from the soil. Despite microbial siderophores complexed with Fe could be an easily accessible Fe source for plants, the possible involvement of A. brasilense in improving Fe nutrition in plants suffering from the micronutrient defi- ciency has not been investigated yet. Within the present research, the characterization of the physio- logical and biochemical effects induced by Fe starvation and PGPR inoculation in cucumber plants (Cucumis sativus L. cv. Chinese Long) was carried out. The analyses of root exudates released by hydro- ponically grown plants highlighted that cucumber plants respond differently depending on the nutri- tional status. In addition, following the cultivation period on calcareous soil, also the root exudates found in the extracts suggested a peculiar behaviour of plants as a function of the treatment. Interestingly, the presence of the inoculum in soil 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. These observations might suggest a feasible application of A. brasilense in alleviating symptoms gener- ated by Fe-limiting growth condition in cucumber plants. © 2014 Elsevier Masson SAS. All rights reserved. 1. Introduction Soil microorganisms play a central role in the nutrient miner- alization and transformation within the rhizosphere (Marschner et al., 2011). As well as plants, they are able to influence the avail- ability of nutrients by modifying the neighbouring environment through the secretion of molecules with solubilizing, chelating, reducing and/or oxidizing capacities. However, the activities of microorganisms in soil are mainly restricted by carbon sources availability (De Nobili et al., 2001; Demoling et al., 2007) and, in this context, plants root exudates might represent an easily accessible source of nourishment for soil bacteria. Therefore, their concen- tration is much higher in the close proximity of the root than in the bulk soil (Badri et al., 2009; Badri and Vivanco, 2009; Glick, 2012). Beside concentration, also the structure of the rhizosphere micro- bial community differs from that of the bulk soil. This feature, together with the physicalechemical characteristics of soil, is thought to be the base of plant ability to shape the rhizosphere microbiome (Philippot et al., 2013). It is interesting to note that plants experiencing Fe-deficiency, besides inducing morphological and physiological modifications of their root systems, enhance root exudation in response to the nutritional stress, not exclusively in terms of quantity but also increasing the complexity of the mo- lecular species released (Hinsinger et al., 2003). However, this * Corresponding author. E-mail address: tanja.mimmo@unibz.it (T. Mimmo). Contents lists available at ScienceDirect Plant Physiology and Biochemistry journal homepage: www.elsevier.com/locate/plaphy http://dx.doi.org/10.1016/j.plaphy.2014.12.014 0981-9428/© 2014 Elsevier Masson SAS. All rights reserved. Plant Physiology and Biochemistry 87 (2015) 45e52